Chirality of the right and left hand. chiral molecules. tRNAs chose the correct enantiomers
Stereoisomers, their types
Definition 1
Stereoisomers are substances in which the atoms are related to each other in the same way, but their arrangement in space is different.
Stereoisomers are divided into:
- Enantiomers (optical isomers). They have the same physical and chemical properties (density, boiling and melting points, solubility, spectral properties) in an achiral environment, but different optical activity.
- Diasteromers are compounds that may contain two or more chiral centers.
Chirality is the ability of an object not to match its mirror image. That is, molecules that do not have mirror-rotational symmetry are chiral.
Definition 2
A prochiral molecule is a molecule that can be made chiral by a single change in any of its fragments.
In chiral and prochiral molecules, some groups of nuclei, which at first glance are chemically equivalent, are magnetically nonequivalent, which is confirmed by nuclear magnetic resonance spectra. This phenomenon is called nuclear diastereotopia, and can be observed in the spectra of nuclear magnetic resonance in the presence of prochiral and chiral fragments in one molecule.
For example, in a prochiral molecule, two OPF2 groups are equivalent, but in each $PF_2$ group of atoms, the fluorine atoms are not equivalent.
This is manifested in the spin-spin interaction constant 2/$FF$.
If the molecule is optically active, then the non-equivalence of X nuclei in tetrahedral groups –$MX_2Y$ (for example, -$CH_2R$, -$SiH_2R$, etc.) or pyramidal groups –$MX_2$ (for example, -$PF_2$, -$NH_2 $, etc.) does not depend on the height of the barrier of internal rotation of these groups. During the rotation of flat groups –$MX_2$ and tetrahedral –$MX_3$ the potential barrier is very low, as a result of which the nuclei $X$ become equivalent.
Construction of the names of chiral molecules
The modern naming system for chiral molecules was proposed by Ingold, Kahn, and Prelog. According to this system, for all possible groups $A$, $B$, $C$, $D$ with an asymmetric carbon atom, the order of precedence is determined. The larger the atomic number, the older it is:
If the atoms are the same, then compare the second environment:
Assume that the groups are arranged in descending order of precedence: $A → B → C → D$. Let's turn the molecule in such a way that the junior substituent $D$ is directed beyond the plane of the figure, away from us. Then the decrease in seniority in the remaining groups can occur either clockwise or counterclockwise.
Remark 1
If the decrease in precedence occurs clockwise, the symbol $R$ (right) is used in the designation of the isomer, if counterclockwise - $S$ (left). The concepts of "left" and "right" do not reflect the real direction of rotation of linearly polarized light.
Emil Fischer proposed the $DL$ nomenclature, according to which the dextrorotatory enantiomer is denoted by the letter $D$, and the left-handed enantiomer by $L$. This nomenclature is widely used for amino acids and carbohydrates.
Stereospecificity of physiological activity of optical isomers
Optical isomers exhibit different physiological activities. The active sites of enzymes and receptors consist of amino acid residues, which are optically active elements.
The receptor recognizes a physiologically active molecule according to the "key in the lock" principle. When a substrate molecule is attached, the active center changes its geometry.
For example, the nicotinic alkaloid contains one center of optical isomerism and can exist as two enantiomers. $S$ - the isomer is located on the right and is poisonous to humans (lethal dose is 20 mg), $R$ - the isomer is less poisonous:
$L$ - glutamic acid
widely used as a meat flavor enhancer in the preparation of canned food. $D$ - glutamic acid does not have such properties.
In conjunction
there are two asymmetric carbon atoms, therefore, the existence of 4 isomers ($2^n$) is possible. But only one ($R,R$)-isomer - chloromycetin - exhibits antibiotic properties
Obtaining pure optical isomers is an important chemical-technological problem.
Ways to obtain pure enantiomers.
) — the geometric property of a rigid object (spatial structure) to be incompatible with its mirror image in an ideal flat mirror.
Description
A chiral object does not have elements of symmetry of the 2nd kind, such as planes of symmetry, centers of symmetry, and mirror-rotation axes. If at least one of these symmetry elements is present, the object is achiral. Chiral are molecules, crystals, (for example,).
Chiral molecules can exist as two optical isomers (enantiomers) that are mirror images of each other and differ in their ability to rotate the plane of polarization of light clockwise (D-isomers) or counterclockwise (L-isomers) (Fig.). Enantiomers are characterized by the same physical properties, as well as the same chemical properties when interacting with achiral substances. At the same time, the separation of enantiomers, for example, the chiral method, can be based on differences in the interaction of enantiomers of a given substance with a specific optical isomer of another substance. In chemistry, chirality is most often associated with the presence of an asymmetric carbon center bearing four different substituents.
In the presence of several asymmetric centers in a molecule, one speaks of diastereoisomerism. In this case, several pairs of enantiomers may exist (a pair of enantiomers must be characterized by a mutually opposite configuration of all asymmetric centers), and the properties of diastereomers from different enantiomeric pairs may differ greatly.
Almost all biomolecules are chiral, including naturally occurring amino acids and sugars. In nature, most of these substances have a certain spatial configuration: for example, most amino acids belong to the spatial configuration L, and sugars - to D. In this regard, enantiomeric purity is a necessary requirement for biologically active drugs.
Illustrations
Author
- Eremin Vadim Vladimirovich
Sources
- Chemical encyclopedia. T. 5. - M.: Great Russian Encyclopedia, 1998. S. 538.
- Compendium of Chemical Technology. IUPAC Recommendations. — Blackwell, 1997.
concept chirality- one of the most important in modern stereochemistry. A model is chiral if it does not have any symmetry elements (plane, center, mirror-rotation axes), except for simple rotation axes. We call a molecule that is described by such a model chiral (meaning "like a hand", from the Greek . hero- hand) for the reason that, like hands, molecules are not compatible with their mirror images. In fig. 1 shows a number of simple chiral molecules. Two facts are absolutely obvious: firstly, the pairs of the above molecules are mirror images of each other, and secondly, these mirror images cannot be combined with each other. It can be seen that in each case the molecule contains a carbon atom with four different substituents. Such atoms are called asymmetric. The asymmetric carbon atom is a chiral or stereogenic center. This is the most common type of chirality. If a molecule is chiral, then it can exist in two isomeric forms, related as an object and its mirror image and incompatible in space. Such isomers (pair) are called enantiomers.
The term "chiral" does not allow free interpretation. When a molecule is chiral, it, by analogy with a hand, must be either left or right. When we call a substance or some sample of it chiral, it simply means that it (it) consists of chiral molecules; in this case, it is not at all necessary that all molecules are the same in terms of chirality (left or right, R or S, see section 1.3). Two limiting cases can be distinguished. In the first, the sample consists of molecules that are identical in terms of chirality (homochiral, only R or only S); such a pattern is called enantiomerically pure. In the second (opposite) case, the sample consists of the same number of molecules that are different in terms of chirality (heterochiral, the molar ratio R: S=1:1); such a sample is also chiral, but racemic. There is also an intermediate case - a non-equimolar mixture of enantiomers. Such a mixture is called scalemic or non-racemic. Thus, the assertion that a macroscopic sample (unlike an individual molecule) is chiral should be considered not quite clear and, therefore, insufficient in some cases. Additional indication may be required as to whether the sample is racemic or non-racemic. The lack of accuracy in understanding this leads to a certain kind of misconception, for example, in the headings of articles, when the synthesis of some chiral compound is proclaimed, but it remains unclear whether the author simply wants to draw attention to the very fact of the chirality of the structure discussed in the article, or whether the product was actually obtained in the form a single enantiomer (i.e., an ensemble of homochiral molecules; this ensemble, however, should not be called a homochiral sample). Thus, in the case of a chiral non-racemic sample, it is more correct to say "enantiomerically enriched" or " enantiomerically pure".
Methods for displaying optical isomers
The image method is chosen by the author solely for reasons of ease of information transfer. In Figure 1, images of enantiomers are given using perspective pictures. In this case, it is customary to draw connections lying in the image plane with a solid line; connections that go beyond the plane - dotted line; and the connections directed to the observer are marked with a thick line. This method of representation is quite informative for structures with one chiral center. The same molecules can be depicted as a Fischer projection. This method was proposed by E. Fisher for more complex structures (in particular, carbohydrates) having two or more chiral centers.
Mirror plane
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To construct Fisher's projection formulas, the tetrahedron is rotated so that two bonds lying in the horizontal plane are directed towards the observer, and two bonds lying in the vertical plane are directed away from the observer. Only an asymmetric atom falls on the image plane. In this case, the asymmetric atom itself, as a rule, is omitted, retaining only the intersecting lines and substituent symbols. To keep in mind the spatial arrangement of substituents, a broken vertical line is often kept in the projection formulas (the upper and lower substituents are removed beyond the plane of the drawing), but this is often not done. Below are examples of different ways to image the same structure with a certain configuration (Fig. 2)
Fisher projection
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Let's give some examples of Fisher's projection formulas (Fig. 3)
(+)-(L)-alanine(-)-2-butanol (+)-( D)-glyceraldehyde
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Since the tetrahedron can be viewed from different angles, each stereoisomer can be represented by twelve (!) different projection formulas. To standardize projection formulas, certain rules for writing them have been introduced. So, the main (nomenclature) function, if it is at the end of the chain, is usually placed at the top, the main chain is depicted vertically.
In order to compare "non-standard" written projection formulas, you need to know the following rules for transforming projection formulas.
1. The formula cannot be derived from the plane of the drawing and cannot be rotated by 90 o, although it can be rotated in the plane of the drawing by 180 o without changing their stereochemical meaning (Fig. 4)
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2. Two (or any even number) permutations of substituents on one asymmetric atom do not change the stereochemical meaning of the formula (Fig. 5)
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3. One (or any odd number) permutation of substituents at the asymmetric center leads to the optical antipode formula (Fig. 6)
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4. A rotation in the plane of the drawing by 90 0 turns the formula into an antipode, unless at the same time the condition for the location of the substituents relative to the plane of the drawing is changed, i.e. consider that now the side deputies are behind the plane of the drawing, and the top and bottom ones are in front of it. If you use the formula with a dotted line, then the changed orientation of the dotted line will directly remind you of this (Fig. 7)
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5. Instead of permutations, projection formulas can be transformed by rotating any three substituents clockwise or counterclockwise (Fig. 8); the fourth substituent does not change the position (such an operation is equivalent to two permutations):
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Fischer projections cannot be applied to molecules whose chirality is associated not with the chiral center, but with other elements (axis, plane). In these cases, 3D images are needed.
D , L - Fisher nomenclature
One problem we discussed was how to represent a three-dimensional structure on a plane. The choice of method is dictated solely by the convenience of presentation and perception of stereoinformation. The next problem is related to the naming of each individual stereoisomer. The name should contain information about the configuration of the stereogenic center. Historically, the first nomenclature for optical isomers was D, L- the nomenclature proposed by Fischer. Until the 1960s, it was more common to designate the configuration of chiral centers based on planar projections (Fischer) rather than on the basis of three-dimensional 3D formulas, using descriptors DAndL. Currently D, L- the system is used to a limited extent - mainly for such natural compounds as amino acids, hydroxy acids and carbohydrates. Examples illustrating its application are shown in Figure 10.
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For α-amino acids, the configuration is denoted by the symbol L, if in the Fisher projection formula the amino - (or ammonium) group is located on the left,; symbol D used for the opposite enantiomer. For sugars, the configuration designation is based on the orientation of the highest numbered OH group (farthest from the carbonyl end). If OH - the group is directed to the right, then this is the configuration D; if OH is on the left - configuration L.
Fischer's system at one time made it possible to create a logical and consistent stereochemical systematics of a large number of natural compounds originating from amino acids and sugars. However, the limitations of the Fisher system, as well as the fact that in 1951 an X-ray diffraction method for determining the true arrangement of groups around a chiral center appeared, led to the creation in 1966 of a new, more rigorous and consistent system for describing stereoisomers, known as R, S - Cahn-Ingold-Prelog (KIP) nomenclature. In the CIP system, special descriptors are added to the usual chemical name R or S(marked in italics in the text) that strictly and unambiguously define the absolute configuration.
NomenclatureCana-Ingold-Preloga
To define a descriptor R or S for a given chiral center, the so-called chirality rule. Consider four substituents associated with a chiral center. They should be arranged in a uniform sequence of stereochemical seniority; for convenience, let's denote these substituents by the symbols A, B, D and E and agree that in the general sequence of precedence (in other words, by priority) A is older than B, B is older than D, D is older than E (A> B> D> E) . The CIA chirality rule requires that the model be viewed from the opposite side of that occupied by the lowest priority substituent E or the stereochemically junior substituent (Fig. 11). Then the remaining three deputies form something like a tripod, the legs of which are directed towards the viewer.
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If the fall in the precedence of deputies in the row A>B>D is clockwise (as in Figure 11), then the configuration descriptor is assigned to the center R ( from Latin word rectus - right). In another arrangement, when the stereochemical seniority of the substituents falls counterclockwise, the configuration descriptor is assigned to the center S (from Latin sinister - left).
When depicting connections using Fisher projections, you can easily determine the configuration without building spatial models. The formula must be written in such a way that the junior substituent is at the bottom or at the top, since according to the rules for the representation of Fisher projections, vertical connections are directed away from the observer (Fig. 12). If the remaining substituents are arranged clockwise in descending order of precedence, the compound is assigned to ( R)-series, and if counterclockwise, then to ( S)-series, for example:
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If the junior group is not on vertical links, then you should swap it with the bottom group, but you should remember that in this case the configuration is reversed. You can make any two permutations - the configuration will not change.
Thus, the determining factor is stereochemical seniority . Let's discuss now precedence sequence rules, i.e. the rules by which groups A, B, D and E are arranged in order of priority.
Preference for seniority is given to atoms with a large atomic number. If the numbers are the same (in the case of isotopes), then the atom with the highest atomic mass becomes more senior (for example, D>H). The youngest "substituent" is an unshared electron pair (for example, in nitrogen). Thus, seniority increases in the series: lone pair
Consider a simple example: in bromochlorofluoromethane CHBrCIF (Fig. 13) there is one stereogenic center, and two enantiomers can be distinguished as follows. First, the substituents are ranked according to their stereochemical seniority: the higher the atomic number, the older the substituent. Therefore, in this example, Br > C1 > F > H, where ">" means "more preferred" (or "older"). The next step is to look at the molecule from the side opposite the youngest substituent, in this case hydrogen. It can be seen that the other three substituents are located at the corners of the triangle and directed towards the observer. If the seniority in this triple of substituents decreases clockwise, then this enantiomer is designated as R. In another arrangement, when the seniority of the substituents falls counterclockwise, the enantiomer is designated as S. Notation R And S write in italics and placed in parentheses before the name of the structure. Thus, the two considered enantiomers have names ( S)-bromochlorofluoromethane and ( R)-bromochlorofluoromethane.
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2. If two, three or all four identical atoms are directly connected to an asymmetric atom, the seniority is established by the atoms of the second belt, which are no longer connected to the chiral center, but to those atoms that had the same seniority.
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For example, in the molecule of 2-bromo-3-methyl-1-butanol (Fig. 14), the oldest and smallest substituents are easily determined by the first belt - these are bromine and hydrogen, respectively. But the first atom of the CH 2 OH and CH (CH 3) 2 groups cannot be established as seniority, since in both cases it is a carbon atom. In order to determine which of the groups is older, the sequence rule is again applied, but now the atoms of the next belt are considered. Compare two sets of atoms (two triplets), written in descending order of precedence. Seniority is now determined by the first point where a difference is found. Group WITH H 2 OH - oxygen, hydrogen, hydrogen WITH(ABOUT HH) or in numbers 6( 8 eleven). Group WITH H (CH 3) 2 - carbon, carbon, hydrogen WITH(WITH CH) or 6( 6 61). The first difference point is underlined: oxygen is older than carbon (by atomic number), so the CH 2 OH group is older than CH (CH 3) 2 . Now you can designate the configuration of the enantiomer depicted in Figure 14 as ( R).
If such a procedure does not lead to the construction of an unambiguous hierarchy, it is continued at ever increasing distances from the central atom, until, finally, differences are encountered, and all four deputies receive their seniority. At the same time, any preference acquired by one or another deputy at one of the stages of seniority agreement is considered final and is not subject to reassessment at subsequent stages.
3. If branching points occur in the molecule, the procedure for establishing the seniority of atoms should be continued along the molecular chain of the highest seniority. Let's assume, it is necessary to determine the sequence of precedence of the two deputies shown in Fig.15. Obviously, the solution will not be reached either in the first (C), or in the second (C, C, H) or in the third (C, H, F, C, H, Br) layers. In this case, you will have to go to the fourth layer, but this should be done along the path, the advantage of which is established in the third layer (Br>F). Therefore, the decision on the priority of the substitute IN over deputy A is done on the basis of the fact that in the fourth layer Br > CI for that branch, the transition to which is dictated by seniority in the third layer, and not on the basis of the fact that the highest atomic number in the fourth layer has atom I (which is located on the less preferred and therefore not branch under study).
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4. Multiple bonds are presented as the sum of the corresponding simple bonds. In accordance with this rule, each atom connected by a multiple bond is assigned an additional “phantom” atom (or atoms) of the same kind, located at the other end of the multiple bond. Complementary (additional or phantom) atoms are enclosed in brackets, and it is considered that they do not carry any substituents in the next layer. As an example, consider the representations of the following groups (Fig. 16).
Group Representation
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5. An artificial increase in the number of substituents is also required when the substituent (ligand) is bidentate (or tri- or tetradentate), and also when the substituent contains a cyclic or bicyclic fragment. In such cases, each branch of the cyclic structure is cut after the branch point [where it bifurcates on its own], and the atom that is the branch point is placed (in brackets) at the end of the chain resulting from the cut. In Fig. 17, using the example of a tetrahydrofuran (THF) derivative, the case of a bidentate (cyclic) substituent is considered. The two branches of the five-membered ring (separately) are cut through bonds to a chiral atom, which is then added to the end of each of the two newly formed chains. It can be seen that as a result of cutting A a hypothetical substituent –CH 2 OCH 2 CH 2 -(C) is obtained, which turns out to be older than the real acyclic substituent -CH 2 OCH 2 CH 3 due to the advantage of the phantom (C) at the end of the first substituent. On the contrary, formed as a result of dissection IN the hypothetical ligand –CH 2 CH 2 OCH 2 –(C) turns out to be lower in seniority than the real substituent –CH 2 CH 2 OCH 2 CH 3, since the latter has three hydrogen atoms attached to the terminal carbon, while the former has none in this layer. Therefore, taking into account the established order of substituent precedence, the configuration symbol for this enantiomer is S.
Determine seniority Deputy A
IN>A
Deputy A
Fig.17
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A similar case of dissection of a cyclic substituent is illustrated by the example of the compound in Fig. 18 where structure IN illustrates the interpretation of the cyclohexyl ring (in the structure A). In this case, the correct sequence of precedence is di- n-gesylmethyl > cyclohexyl > di- n-pentylmethyl > H.
Now we are sufficiently prepared to consider such a substituent as phenyl (Fig. 19 structure A). We discussed the scheme for opening each multiple bond above. Since (in any Kekule structure) each of the six carbon atoms is double-bonded to another carbon atom, then (in the CIA system) each carbon atom of the ring carries an additional carbon as a "substituent". The ring supplemented in this way (Fig. 19, structure IN) is then expanded according to the rules for cyclic systems. As a result, the dissection is described by the diagram shown in Fig. 19, the structure WITH.
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6. Now we will consider chiral compounds in which the differences between the substituents are not of a material or constitutional nature, but are reduced to differences in configuration. Compounds containing more than one chiral center will be discussed below (see section 1.4) Here we will also touch on substituents that differ cis-trans- isomerism (olefin type). According to Prelog and Helmchen, the olefin ligand in which the senior substituent is located on the same side from the double bond of the olefin, which is the chiral center, has an advantage over the ligand in which the senior substituent is in trance-position to the chiral center. This position has nothing to do with classical cis-trans-, nor to E-Z - nomenclature for double bond configuration. Examples are shown in Figure 20.
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Compounds with multiple chiral centers
If there are two chiral centers in a molecule, then since each center can have (R)- or ( S)-configuration, the existence of four isomers is possible - RR, SS, RS And SR:
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Since the molecule has only one mirror image, the enantiomer of the compound (RR) can only be an isomer (SS). Similarly, another pair of enantiomers form isomers (RS) And (SR). If the configuration of only one asymmetric center changes, then such isomers are called diastereomers. Diastereomers are stereoisomers that are not enantiomers. So, diastereomeric pairs (RR)/(RS), (RR)/(SR), (SS)/(RS) And (SS)/(SR). Although, in general, the combination of two chiral centers produces four isomers, the combination of centers of the same chemical structure gives only three isomers: (RR) And (SS), which are enantiomers, and (RS), diastereomeric to both enantiomers (RR) And (SS). A typical example is tartaric acid (Fig. 22), which has only three isomers: a pair of enantiomers and meso form.
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Meso-Vinnaya acid is (R, S)-isomer, which is optically inactive, since the union of two mirror-symmetric fragments leads to the appearance of a symmetry plane (a). Meso-Vinnaya an acid is an example of an achiral meso-configuration compound, which is built from an equal number of chiral elements identical in structure but different in absolute configuration.
If the molecule has P chiral centers, the maximum number of stereoisomers can be calculated using formula 2 n; however, sometimes the number of isomers will be less due to the presence of meso forms.
For the names of stereoisomers of molecules containing two asymmetric carbon atoms, two substituents for each of which are the same, and the third are different, prefixes are often used erythro- And treo- from the names of sugars erythrose and threose. These prefixes characterize the system as a whole, and not each chiral center separately. When depicting such compounds using Fischer projections in a pair erythro- isomers, the same groups are located on one side, and if the different groups (C1 and Br in the example below) were the same, the meso form would be obtained. Paired with treo- isomers, the same groups are located on different sides, and if the different groups were the same, the new pair would remain an enantiomeric pair.
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All examples of compounds considered above have a center of chirality. Such a center is an asymmetric carbon atom. However, other atoms (silicon, phosphorus, sulfur) can also be the center of chirality, as, for example, in methylnaphthylphenylsilane, o-anisylmethylphenylphosphine, methyl-p-tolyl sulfoxide (Fig. 24)
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Chirality of molecules devoid of chiral centers
A necessary and sufficient condition for the chirality of a molecule is its incompatibility with its mirror image. The presence of a single (configurationally stable) chiral center in a molecule is a sufficient, but by no means necessary, condition for the existence of chirality. Consider chiral molecules lacking chiral centers. Some examples are shown in figures 25 and 26.
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These are compounds with axes of chirality ( axial chirality type): allenes; alkylidenecycloalkanes; spiranes; the so-called atropisomers (biphenyls and similar compounds whose chirality arises due to hindered rotation around a single bond). Another element of chirality is the chirality plane ( planar chirality type). Examples of such compounds are ansa compounds (in which the alicyclic ring is too small for the aromatic ring to pass through); paracyclophanes; metallocenes. Finally, the chirality of a molecule can be related to the helical organization of the molecular structure. The molecule can wrap either in the left or in the right helix. In this case, one speaks of helicity (helical type of chirality).
In order to determine the configuration of a molecule that has axis of chirality, it is necessary to introduce an additional clause in the sequence rule: the groups closest to the observer are considered older than the groups remote from the observer. This addition must be made, since for molecules with axial chirality, the presence of identical substituents at opposite ends of the axis is permissible. Applying this rule to the molecules shown in Fig. 25 shown in fig. 27.
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In all cases, the molecules are considered along the chiral axis on the left. In this case, it should be understood that if the molecules are considered from the right, then the configuration descriptor will remain the same. Thus, the spatial arrangement of the four support groups corresponds to the vertices of the virtual tetrahedron and can be represented using the corresponding projections (Fig. 27). To determine the appropriate descriptor, we use the standard rules R, S- nomenclature. In the case of biphenyls, it is important to note that ring substituents are considered from the center (through which the chirality axis passes) to the periphery, in violation of the standard sequence rules. Thus, for biphenyl in Fig. 25 correct sequence of substituents in the right ring C-OCH 3 >C-H; the chlorine atom is too far away to be taken into account. The reference atoms (those by which the configuration symbol is determined) are the same when the molecule is viewed from the right. Sometimes descriptors are used to distinguish axial chirality from other types. aR And aS (or R a And S a), but the use of the prefix " a' is not mandatory.
Alternatively, molecules with axes of chirality can be thought of as helical, and their configuration can be denoted by the symbols R And M. In this case, to determine the configuration, only substituents with the highest priority are considered both in the front and back (remote from the observer) parts of the structure (substituents 1 and 3 in Fig. 27). If the transition from the highest priority front substituent 1 to the priority rear substituent 3 is clockwise, then this is the configuration R; if counterclockwise, is the configuration M.
On fig. 26 shows molecules with chirality planes. It is not so easy to give a definition of the plane of chirality, and it is not as unambiguous as the definition of the center and axis of chirality. This is a plane that contains as many atoms of a molecule as possible, but not all. In fact, chirality is because (and only because) that at least one substituent (often more) does not lie in the chirality plane. Thus, the chiral plane of the ansa compound A is the plane of the benzene ring. In paracyclophane IN the most substituted (lower) ring is considered as the chiral plane. In order to determine the descriptor for planar-chiral molecules, the plane is viewed from the side of the atom closest to the plane, but not lying in this plane (if there are two or more candidates, then the one closest to the atom with the highest priority is chosen according to the rules of sequence ). This atom, sometimes called a test or pilot atom, is marked with an arrow in Fig. 26. Then, if three consecutive atoms (a, b, c) with the highest priority form a broken line in the chiral plane, curving clockwise, then the compound configuration pR (or R p), and if the polyline curves counterclockwise, then the configuration descriptor PS(or S p). Planar chirality, like axial chirality, can alternatively be viewed as a kind of chirality. In order to determine the direction (configuration) of the helix, one must consider the pilot atom together with the atoms a,b, and c, as defined above. From here it is clear that pR- connections corresponds R-, A PS- connections - M– helicity.
concept chirality- one of the most important in modern stereochemistry. A model is chiral if it does not have any symmetry elements (plane, center, mirror-rotation axes), except for simple rotation axes. We call a molecule that is described by such a model chiral (meaning "like a hand", from the Greek . hero- hand) for the reason that, like hands, molecules are not compatible with their mirror images. In fig. 1 shows a number of simple chiral molecules. Two facts are absolutely obvious: firstly, the pairs of the above molecules are mirror images of each other, and secondly, these mirror images cannot be combined with each other. It can be seen that in each case the molecule contains a carbon atom with four different substituents. Such atoms are called asymmetric. The asymmetric carbon atom is a chiral or stereogenic center. This is the most common type of chirality. If a molecule is chiral, then it can exist in two isomeric forms, related as an object and its mirror image and incompatible in space. Such isomers (pair) are called enantiomers.
The term "chiral" does not allow free interpretation. When a molecule is chiral, it, by analogy with a hand, must be either left or right. When we call a substance or some sample of it chiral, it simply means that it (it) consists of chiral molecules; in this case, it is not at all necessary that all molecules are the same in terms of chirality (left or right, R or S, see section 1.3). Two limiting cases can be distinguished. In the first, the sample consists of molecules that are identical in terms of chirality (homochiral, only R or only S); such a pattern is called enantiomerically pure. In the second (opposite) case, the sample consists of the same number of molecules that are different in terms of chirality (heterochiral, the molar ratio R: S=1:1); such a sample is also chiral, but racemic. There is also an intermediate case - a non-equimolar mixture of enantiomers. Such a mixture is called scalemic or non-racemic. Thus, the assertion that a macroscopic sample (unlike an individual molecule) is chiral should be considered not quite clear and, therefore, insufficient in some cases. Additional indication may be required as to whether the sample is racemic or non-racemic. The lack of accuracy in understanding this leads to a certain kind of misconception, for example, in the headings of articles, when the synthesis of some chiral compound is proclaimed, but it remains unclear whether the author simply wants to draw attention to the very fact of the chirality of the structure discussed in the article, or whether the product was actually obtained in the form a single enantiomer (i.e., an ensemble of homochiral molecules; this ensemble, however, should not be called a homochiral sample). Thus, in the case of a chiral non-racemic sample, it is more correct to say "enantiomerically enriched" or " enantiomerically pure".
Methods for displaying optical isomers
The image method is chosen by the author solely for reasons of ease of information transfer. In Figure 1, images of enantiomers are given using perspective pictures. In this case, it is customary to draw connections lying in the image plane with a solid line; connections that go beyond the plane - dotted line; and the connections directed to the observer are marked with a thick line. This method of representation is quite informative for structures with one chiral center. The same molecules can be depicted as a Fischer projection. This method was proposed by E. Fisher for more complex structures (in particular, carbohydrates) having two or more chiral centers.
Mirror plane
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To construct Fisher's projection formulas, the tetrahedron is rotated so that two bonds lying in the horizontal plane are directed towards the observer, and two bonds lying in the vertical plane are directed away from the observer. Only an asymmetric atom falls on the image plane. In this case, the asymmetric atom itself, as a rule, is omitted, retaining only the intersecting lines and substituent symbols. To keep in mind the spatial arrangement of substituents, a broken vertical line is often kept in the projection formulas (the upper and lower substituents are removed beyond the plane of the drawing), but this is often not done. Below are examples of different ways to image the same structure with a certain configuration (Fig. 2)
Fisher projection
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Let's give some examples of Fisher's projection formulas (Fig. 3)
(+)-(L)-alanine(-)-2-butanol (+)-( D)-glyceraldehyde
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Since the tetrahedron can be viewed from different angles, each stereoisomer can be represented by twelve (!) different projection formulas. To standardize projection formulas, certain rules for writing them have been introduced. So, the main (nomenclature) function, if it is at the end of the chain, is usually placed at the top, the main chain is depicted vertically.
In order to compare "non-standard" written projection formulas, you need to know the following rules for transforming projection formulas.
1. The formula cannot be derived from the plane of the drawing and cannot be rotated by 90 o, although it can be rotated in the plane of the drawing by 180 o without changing their stereochemical meaning (Fig. 4)
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2. Two (or any even number) permutations of substituents on one asymmetric atom do not change the stereochemical meaning of the formula (Fig. 5)
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3. One (or any odd number) permutation of substituents at the asymmetric center leads to the optical antipode formula (Fig. 6)
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4. A rotation in the plane of the drawing by 90 0 turns the formula into an antipode, unless at the same time the condition for the location of the substituents relative to the plane of the drawing is changed, i.e. consider that now the side deputies are behind the plane of the drawing, and the top and bottom ones are in front of it. If you use the formula with a dotted line, then the changed orientation of the dotted line will directly remind you of this (Fig. 7)
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5. Instead of permutations, projection formulas can be transformed by rotating any three substituents clockwise or counterclockwise (Fig. 8); the fourth substituent does not change the position (such an operation is equivalent to two permutations):
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Fischer projections cannot be applied to molecules whose chirality is associated not with the chiral center, but with other elements (axis, plane). In these cases, 3D images are needed.
D , L - Fisher nomenclature
One problem we discussed was how to represent a three-dimensional structure on a plane. The choice of method is dictated solely by the convenience of presentation and perception of stereoinformation. The next problem is related to the naming of each individual stereoisomer. The name should contain information about the configuration of the stereogenic center. Historically, the first nomenclature for optical isomers was D, L- the nomenclature proposed by Fischer. Until the 1960s, it was more common to designate the configuration of chiral centers based on planar projections (Fischer) rather than on the basis of three-dimensional 3D formulas, using descriptors DAndL. Currently D, L- the system is used to a limited extent - mainly for such natural compounds as amino acids, hydroxy acids and carbohydrates. Examples illustrating its application are shown in Figure 10.
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For α-amino acids, the configuration is denoted by the symbol L, if in the Fisher projection formula the amino - (or ammonium) group is located on the left,; symbol D used for the opposite enantiomer. For sugars, the configuration designation is based on the orientation of the highest numbered OH group (farthest from the carbonyl end). If OH - the group is directed to the right, then this is the configuration D; if OH is on the left - configuration L.
Fischer's system at one time made it possible to create a logical and consistent stereochemical systematics of a large number of natural compounds originating from amino acids and sugars. However, the limitations of the Fisher system, as well as the fact that in 1951 an X-ray diffraction method for determining the true arrangement of groups around a chiral center appeared, led to the creation in 1966 of a new, more rigorous and consistent system for describing stereoisomers, known as R, S - Cahn-Ingold-Prelog (KIP) nomenclature. In the CIP system, special descriptors are added to the usual chemical name R or S(marked in italics in the text) that strictly and unambiguously define the absolute configuration.
NomenclatureCana-Ingold-Preloga
To define a descriptor R or S for a given chiral center, the so-called chirality rule. Consider four substituents associated with a chiral center. They should be arranged in a uniform sequence of stereochemical seniority; for convenience, let's denote these substituents by the symbols A, B, D and E and agree that in the general sequence of precedence (in other words, by priority) A is older than B, B is older than D, D is older than E (A> B> D> E) . The CIA chirality rule requires that the model be viewed from the opposite side of that occupied by the lowest priority substituent E or the stereochemically junior substituent (Fig. 11). Then the remaining three deputies form something like a tripod, the legs of which are directed towards the viewer.
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If the fall in the precedence of deputies in the row A>B>D is clockwise (as in Figure 11), then the configuration descriptor is assigned to the center R ( from Latin word rectus - right). In another arrangement, when the stereochemical seniority of the substituents falls counterclockwise, the configuration descriptor is assigned to the center S (from Latin sinister - left).
When depicting connections using Fisher projections, you can easily determine the configuration without building spatial models. The formula must be written in such a way that the junior substituent is at the bottom or at the top, since according to the rules for the representation of Fisher projections, vertical connections are directed away from the observer (Fig. 12). If the remaining substituents are arranged clockwise in descending order of precedence, the compound is assigned to ( R)-series, and if counterclockwise, then to ( S)-series, for example:
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If the junior group is not on vertical links, then you should swap it with the bottom group, but you should remember that in this case the configuration is reversed. You can make any two permutations - the configuration will not change.
Thus, the determining factor is stereochemical seniority . Let's discuss now precedence sequence rules, i.e. the rules by which groups A, B, D and E are arranged in order of priority.
Preference for seniority is given to atoms with a large atomic number. If the numbers are the same (in the case of isotopes), then the atom with the highest atomic mass becomes more senior (for example, D>H). The youngest "substituent" is an unshared electron pair (for example, in nitrogen). Thus, seniority increases in the series: lone pair
Consider a simple example: in bromochlorofluoromethane CHBrCIF (Fig. 13) there is one stereogenic center, and two enantiomers can be distinguished as follows. First, the substituents are ranked according to their stereochemical seniority: the higher the atomic number, the older the substituent. Therefore, in this example, Br > C1 > F > H, where ">" means "more preferred" (or "older"). The next step is to look at the molecule from the side opposite the youngest substituent, in this case hydrogen. It can be seen that the other three substituents are located at the corners of the triangle and directed towards the observer. If the seniority in this triple of substituents decreases clockwise, then this enantiomer is designated as R. In another arrangement, when the seniority of the substituents falls counterclockwise, the enantiomer is designated as S. Notation R And S write in italics and placed in parentheses before the name of the structure. Thus, the two considered enantiomers have names ( S)-bromochlorofluoromethane and ( R)-bromochlorofluoromethane.
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2. If two, three or all four identical atoms are directly connected to an asymmetric atom, the seniority is established by the atoms of the second belt, which are no longer connected to the chiral center, but to those atoms that had the same seniority.
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For example, in the molecule of 2-bromo-3-methyl-1-butanol (Fig. 14), the oldest and smallest substituents are easily determined by the first belt - these are bromine and hydrogen, respectively. But the first atom of the CH 2 OH and CH (CH 3) 2 groups cannot be established as seniority, since in both cases it is a carbon atom. In order to determine which of the groups is older, the sequence rule is again applied, but now the atoms of the next belt are considered. Compare two sets of atoms (two triplets), written in descending order of precedence. Seniority is now determined by the first point where a difference is found. Group WITH H 2 OH - oxygen, hydrogen, hydrogen WITH(ABOUT HH) or in numbers 6( 8 eleven). Group WITH H (CH 3) 2 - carbon, carbon, hydrogen WITH(WITH CH) or 6( 6 61). The first difference point is underlined: oxygen is older than carbon (by atomic number), so the CH 2 OH group is older than CH (CH 3) 2 . Now you can designate the configuration of the enantiomer depicted in Figure 14 as ( R).
If such a procedure does not lead to the construction of an unambiguous hierarchy, it is continued at ever increasing distances from the central atom, until, finally, differences are encountered, and all four deputies receive their seniority. At the same time, any preference acquired by one or another deputy at one of the stages of seniority agreement is considered final and is not subject to reassessment at subsequent stages.
3. If branching points occur in the molecule, the procedure for establishing the seniority of atoms should be continued along the molecular chain of the highest seniority. Let's assume, it is necessary to determine the sequence of precedence of the two deputies shown in Fig.15. Obviously, the solution will not be reached either in the first (C), or in the second (C, C, H) or in the third (C, H, F, C, H, Br) layers. In this case, you will have to go to the fourth layer, but this should be done along the path, the advantage of which is established in the third layer (Br>F). Therefore, the decision on the priority of the substitute IN over deputy A is done on the basis of the fact that in the fourth layer Br > CI for that branch, the transition to which is dictated by seniority in the third layer, and not on the basis of the fact that the highest atomic number in the fourth layer has atom I (which is located on the less preferred and therefore not branch under study).
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4. Multiple bonds are presented as the sum of the corresponding simple bonds. In accordance with this rule, each atom connected by a multiple bond is assigned an additional “phantom” atom (or atoms) of the same kind, located at the other end of the multiple bond. Complementary (additional or phantom) atoms are enclosed in brackets, and it is considered that they do not carry any substituents in the next layer. As an example, consider the representations of the following groups (Fig. 16).
Group Representation
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5. An artificial increase in the number of substituents is also required when the substituent (ligand) is bidentate (or tri- or tetradentate), and also when the substituent contains a cyclic or bicyclic fragment. In such cases, each branch of the cyclic structure is cut after the branch point [where it bifurcates on its own], and the atom that is the branch point is placed (in brackets) at the end of the chain resulting from the cut. In Fig. 17, using the example of a tetrahydrofuran (THF) derivative, the case of a bidentate (cyclic) substituent is considered. The two branches of the five-membered ring (separately) are cut through bonds to a chiral atom, which is then added to the end of each of the two newly formed chains. It can be seen that as a result of cutting A a hypothetical substituent –CH 2 OCH 2 CH 2 -(C) is obtained, which turns out to be older than the real acyclic substituent -CH 2 OCH 2 CH 3 due to the advantage of the phantom (C) at the end of the first substituent. On the contrary, formed as a result of dissection IN the hypothetical ligand –CH 2 CH 2 OCH 2 –(C) turns out to be lower in seniority than the real substituent –CH 2 CH 2 OCH 2 CH 3, since the latter has three hydrogen atoms attached to the terminal carbon, while the former has none in this layer. Therefore, taking into account the established order of substituent precedence, the configuration symbol for this enantiomer is S.
Determine seniority Deputy A
IN>A
Deputy A
Fig.17
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A similar case of dissection of a cyclic substituent is illustrated by the example of the compound in Fig. 18 where structure IN illustrates the interpretation of the cyclohexyl ring (in the structure A). In this case, the correct sequence of precedence is di- n-gesylmethyl > cyclohexyl > di- n-pentylmethyl > H.
Now we are sufficiently prepared to consider such a substituent as phenyl (Fig. 19 structure A). We discussed the scheme for opening each multiple bond above. Since (in any Kekule structure) each of the six carbon atoms is double-bonded to another carbon atom, then (in the CIA system) each carbon atom of the ring carries an additional carbon as a "substituent". The ring supplemented in this way (Fig. 19, structure IN) is then expanded according to the rules for cyclic systems. As a result, the dissection is described by the diagram shown in Fig. 19, the structure WITH.
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6. Now we will consider chiral compounds in which the differences between the substituents are not of a material or constitutional nature, but are reduced to differences in configuration. Compounds containing more than one chiral center will be discussed below (see section 1.4) Here we will also touch on substituents that differ cis-trans- isomerism (olefin type). According to Prelog and Helmchen, the olefin ligand in which the senior substituent is located on the same side from the double bond of the olefin, which is the chiral center, has an advantage over the ligand in which the senior substituent is in trance-position to the chiral center. This position has nothing to do with classical cis-trans-, nor to E-Z - nomenclature for double bond configuration. Examples are shown in Figure 20.
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Compounds with multiple chiral centers
If there are two chiral centers in a molecule, then since each center can have (R)- or ( S)-configuration, the existence of four isomers is possible - RR, SS, RS And SR:
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Since the molecule has only one mirror image, the enantiomer of the compound (RR) can only be an isomer (SS). Similarly, another pair of enantiomers form isomers (RS) And (SR). If the configuration of only one asymmetric center changes, then such isomers are called diastereomers. Diastereomers are stereoisomers that are not enantiomers. So, diastereomeric pairs (RR)/(RS), (RR)/(SR), (SS)/(RS) And (SS)/(SR). Although, in general, the combination of two chiral centers produces four isomers, the combination of centers of the same chemical structure gives only three isomers: (RR) And (SS), which are enantiomers, and (RS), diastereomeric to both enantiomers (RR) And (SS). A typical example is tartaric acid (Fig. 22), which has only three isomers: a pair of enantiomers and meso form.
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Meso-Vinnaya acid is (R, S)-isomer, which is optically inactive, since the union of two mirror-symmetric fragments leads to the appearance of a symmetry plane (a). Meso-Vinnaya an acid is an example of an achiral meso-configuration compound, which is built from an equal number of chiral elements identical in structure but different in absolute configuration.
If the molecule has P chiral centers, the maximum number of stereoisomers can be calculated using formula 2 n; however, sometimes the number of isomers will be less due to the presence of meso forms.
For the names of stereoisomers of molecules containing two asymmetric carbon atoms, two substituents for each of which are the same, and the third are different, prefixes are often used erythro- And treo- from the names of sugars erythrose and threose. These prefixes characterize the system as a whole, and not each chiral center separately. When depicting such compounds using Fischer projections in a pair erythro- isomers, the same groups are located on one side, and if the different groups (C1 and Br in the example below) were the same, the meso form would be obtained. Paired with treo- isomers, the same groups are located on different sides, and if the different groups were the same, the new pair would remain an enantiomeric pair.
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All examples of compounds considered above have a center of chirality. Such a center is an asymmetric carbon atom. However, other atoms (silicon, phosphorus, sulfur) can also be the center of chirality, as, for example, in methylnaphthylphenylsilane, o-anisylmethylphenylphosphine, methyl-p-tolyl sulfoxide (Fig. 24)
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Chirality of molecules devoid of chiral centers
A necessary and sufficient condition for the chirality of a molecule is its incompatibility with its mirror image. The presence of a single (configurationally stable) chiral center in a molecule is a sufficient, but by no means necessary, condition for the existence of chirality. Consider chiral molecules lacking chiral centers. Some examples are shown in figures 25 and 26.
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These are compounds with axes of chirality ( axial chirality type): allenes; alkylidenecycloalkanes; spiranes; the so-called atropisomers (biphenyls and similar compounds whose chirality arises due to hindered rotation around a single bond). Another element of chirality is the chirality plane ( planar chirality type). Examples of such compounds are ansa compounds (in which the alicyclic ring is too small for the aromatic ring to pass through); paracyclophanes; metallocenes. Finally, the chirality of a molecule can be related to the helical organization of the molecular structure. The molecule can wrap either in the left or in the right helix. In this case, one speaks of helicity (helical type of chirality).
In order to determine the configuration of a molecule that has axis of chirality, it is necessary to introduce an additional clause in the sequence rule: the groups closest to the observer are considered older than the groups remote from the observer. This addition must be made, since for molecules with axial chirality, the presence of identical substituents at opposite ends of the axis is permissible. Applying this rule to the molecules shown in Fig. 25 shown in fig. 27.
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In all cases, the molecules are considered along the chiral axis on the left. In this case, it should be understood that if the molecules are considered from the right, then the configuration descriptor will remain the same. Thus, the spatial arrangement of the four support groups corresponds to the vertices of the virtual tetrahedron and can be represented using the corresponding projections (Fig. 27). To determine the appropriate descriptor, we use the standard rules R, S- nomenclature. In the case of biphenyls, it is important to note that ring substituents are considered from the center (through which the chirality axis passes) to the periphery, in violation of the standard sequence rules. Thus, for biphenyl in Fig. 25 correct sequence of substituents in the right ring C-OCH 3 >C-H; the chlorine atom is too far away to be taken into account. The reference atoms (those by which the configuration symbol is determined) are the same when the molecule is viewed from the right. Sometimes descriptors are used to distinguish axial chirality from other types. aR And aS (or R a And S a), but the use of the prefix " a' is not mandatory.
Alternatively, molecules with axes of chirality can be thought of as helical, and their configuration can be denoted by the symbols R And M. In this case, to determine the configuration, only substituents with the highest priority are considered both in the front and back (remote from the observer) parts of the structure (substituents 1 and 3 in Fig. 27). If the transition from the highest priority front substituent 1 to the priority rear substituent 3 is clockwise, then this is the configuration R; if counterclockwise, is the configuration M.
On fig. 26 shows molecules with chirality planes. It is not so easy to give a definition of the plane of chirality, and it is not as unambiguous as the definition of the center and axis of chirality. This is a plane that contains as many atoms of a molecule as possible, but not all. In fact, chirality is because (and only because) that at least one substituent (often more) does not lie in the chirality plane. Thus, the chiral plane of the ansa compound A is the plane of the benzene ring. In paracyclophane IN the most substituted (lower) ring is considered as the chiral plane. In order to determine the descriptor for planar-chiral molecules, the plane is viewed from the side of the atom closest to the plane, but not lying in this plane (if there are two or more candidates, then the one closest to the atom with the highest priority is chosen according to the rules of sequence ). This atom, sometimes called a test or pilot atom, is marked with an arrow in Fig. 26. Then, if three consecutive atoms (a, b, c) with the highest priority form a broken line in the chiral plane, curving clockwise, then the compound configuration pR (or R p), and if the polyline curves counterclockwise, then the configuration descriptor PS(or S p). Planar chirality, like axial chirality, can alternatively be viewed as a kind of chirality. In order to determine the direction (configuration) of the helix, one must consider the pilot atom together with the atoms a,b, and c, as defined above. From here it is clear that pR- connections corresponds R-, A PS- connections - M– helicity.
Chirality (chemistry)
Chirality(molecular chirality) - in chemistry, the property of a molecule to be incompatible with its mirror image by any combination of rotations and displacements in three-dimensional space.
Enzymes (and they are chiral) often differ between the two enantiomers of a chiral substrate. Imagine that the enzyme has a glove-shaped depression that binds the substrate. If the glove is right handed, then one enantiomer will go in and bind, while the other enantiomer will not fit well and there is little chance of binding. The D-form of amino acids is usually sweet in taste, while the L-form is usually tasteless. Peppermint leaves and cumin seeds contain L-carvone and D-carvone, respectively, enantiomers of carvone. They smell differently because most people's olfactory receptors also contain chiral molecules that behave differently in the presence of different enantiomers.
Chirality in pharmacology
Many chiral drugs are made in high enantiometric purity due to the side effects of the other enantiomer (which may even be therapeutically inactive).
- Thalidomide: Thalidomide is racemic. One enantiomer is effective against nausea and the other is teratogenic. In this case, the administration of one of the enantiomers to a pregnant patient will not help, since both enantiomers are easily converted into each other in the body. And if you give a person a different enantiomer, then both D- and L-isomers will be present in the patient's plasma.
- Ethambutol: one enantiomer is used in the treatment of tuberculosis, the other causes blindness.
- Naproxen: One enantiomer treats arthritis, but the other causes liver poisoning without analgesic effect.
- The location of steroid receptors also shows the specificity of the stereoisomers.
- The activity of penicillin is stereodependent. The antibiotic must mimic the D-alanine chains that are present in bacterial cell walls in order to react with the transpeptidase enzyme and gradually engulf it.
- Only L-Inderal is a powerful adrenergic receptor, while D-Inderal is not. However, both isomers of anaprilin have a local anesthetic effect.
- L-methorphan (levomethorphan) is a powerful opioid analgesic, while the D-isomer, dextromethorphan, is a dissociative cough reliever.
- S-carvedilol, an adrenoceptor-reactive agent, is 100 times more β-blocking than the R(+) isomer. But both isomers approximately equally block α-receptors.
- The D-isomers of pervitin and amphetamine are strong CNS stimulants, and the L-isomers of both lack the major CNS stimulant properties but instead stimulate the PNS (peripheral nervous system). Therefore, the L-isomer of Pervitin is available as a nasal agent, and the dextroisomer is banned for medical use in almost all (with rare exceptions) countries of the world and is strictly controlled where it is allowed.
- S-amlodipine, the pure optically active isomer of amlodipine, is responsible for calcium channel blockade and vasodilation.
- levocetirizine, RR-cytirizine, an antihistamine drug, an active blocker of histamine receptors in the composition of cytirizine.
- S-pantaprozole, a pure optically active isomer of pantaprozole, selectively blocks the proton pump of the parietal cells of the gastric mucosa.
- R-rabeprozole, a pure optically active isomer of rabeprozole, selectively blocks the proton pump of the parietal cells of the gastric mucosa.
- dexibuprofen, a pure optically active isomer of ibuprofen, selectively blocking cyclooxygenase.
- dexketoprofen, a pure optically active isomer of ketoprofen, selectively blocking cyclooxygenase.
- esetodolac, a pure optically active isomer of ietodolac, selectively and selectively blocking cyclooxygenase.
- Esomeprazole, a pure optically active isomer of omeprozole, selectively blocks the proton pump of the parietal cells of the gastric mucosa.
- S-metoprolol, a selective blocker of beta-adrenergic receptors of the heart and blood vessels, isolated from racemic metoprolol
- Levomycetin.
- quinine.
- quinidine.
- L-lysine.
- L-thyroxine.
- L-dopa.
- levotiracetam.
- R-sibutramine. Not widely used (probably only in India), due to the FDA ban on the use of racemic sibutramine for the treatment of obesity due to side effects. According to Indian researchers, R-sibutramine is maximally devoid of these side effects, however, the effectiveness of R-sibutramine for safe weight loss has not been proven.
- L-carnitine. Used in food supplements.
Chirality in inorganic chemistry
Many complex compounds are chiral, such as the well-known 2+ complex, in which three bipyridine ligands assume a chiral propeller arrangement. In this case, the ruthenium atom can be considered a stereogenic center in a complex with point chirality. The two enantiomers of complexes, such as 2+ , can be referred to as Λ (left turn of the propeller described by ligands) and Δ (right turn). Hexol is a chiral cobalt-containing complex, first discovered by Alfred Werner. Solid hexol is important as the first carbon-free substance to reflect optical activity.
Chirality of amines
Tertiary amines are chiral in a manner similar to carbon-containing compounds: the nitrogen atom carries four different substituent groups, including a lone pair. However, the energy barrier to stereocenter inversion is generally around 30 kJ/mol, which means that the two stereoisomers quickly convert into each other at room temperature. As a result, amines such as NHRR' cannot be recognized by sight, but NRR'R'' can be recognized when R, R' and R'' are enclosed in cyclic structures.
Chirality in literature
Although little was known about chirality in Lewis Carroll's day, his work Alice Through the Looking-Glass contains a prescient reference to the various biological activities of enantiometric drugs: "Maybe the looking-glass milk is not drinkable," Alice said to her cat. In James Blish's novel Spock Must Die! from the Star Trek series, Mr. Spock's mirror tachyon is revealed to steal chemicals from the medical bay and use them to convert certain amino acids and the opposite chiral isomers.
Achirality and prochirality
The absence of chirality is denoted by the term "achirality". Achiral molecules can exhibit induced optical activity. A molecule is said to be prochiral if it can be made chiral by replacing a single atom, such as the hydrogen atom in CH 2 BrCl, with fluorine. When the chiral and prochiral fragments are combined in one molecule, the phenomenon of nuclear diastereotopy occurs, which is observed in the spectra of nuclear magnetic resonance. One of the methods for detecting the chirality of molecules is based on this.
see also
Notes
Links
- A. Borisova Chemists have flattened carbon. Gazeta.ru (30.07.2010). - Contains a description of some features of chirality. Archived from the original on August 22, 2011. Retrieved August 22, 2010.
| Structural chemistry | |
|---|---|
| Chemical bond: | Aromaticity | Covalent bond | Ionic bond | Metal connection | Hydrogen bond | Donor-acceptor bond | Tautomerism | Van der Waals connection |
| Structure display: | Functional group | Structural formula | Skeletal formula of organic compounds | Chemical formula | Ligand | Coordination geometry | Coordination sphere |
| Electronic properties: | Electronegativity | Electron affinity | Ionization energy | Polarity of chemical bonds | Octet rule |
