Share Preface:Customers often ask about the configuration of compounds. This article makes it clear that the configuration of natural products is particularly complex. A lot of work needs to be done to determine the configuration of compounds from scratch, which is generally determined by references. The compound configuration is the characteristic of the compound itself, and the separation process generally does not lead to the configuration change, so the compounds we provide are basically their original configurations.
Arrange the four groups (a, B, C, d) connected by chiral carbon atoms in order, such as: a> b> c> d， Then the group D with the smallest order is placed farthest from the observer. The other three groups a, B and C point to the observer. If a → B → C (clockwise from large to small, the configuration is R; otherwise, it is S.)
In organic chemistry, we often use Fischer projection to write. How can we directly judge the R and s configurations from the Fischer projection? Remember the horizontal front and vertical back?
● When the group with the smallest order is on the vertical line, the R and s configurations can be judged directly from the arrangement direction of the other three groups.
● when the group with the smallest order is on the horizontal line, it is in front of the paper. The observer should look forward from the back of the paper. Therefore, judging from the other three groups, if R should be changed to s, if s should be changed to R.
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● R, s configuration refers to that the configuration of chiral atoms is not directly related to the optical rotation direction of enantiomers. The optical rotation direction is measured by the polarimeter. ; The racemate is written as (±) - （RS）-、dl-。
. When one molecule is converted from R-type to S-type, it cannot be considered that the configuration must be reversed.
. Chiral compounds are called chiral compounds. In addition to the commonly mentioned compounds containing chiral centers, chiral compounds also include compounds containing chiral axis, chiral plane, chiral helix and other factors. Generally speaking, a molecule has chirality if it has neither symmetry plane nor symmetry center.
The determination of the absolute configuration of chiral molecules is an extremely important and long-standing problem. At present, there are four main methods to determine the absolute configuration of chiral molecules: (1) organic chemistry; (2) NMR method; (3) X-ray diffraction; (4) Spectroscopic methods, such as optical rotation spectroscopy, circular dichroism, vibrational circular dichroism, etc.
1. organic chemical method
Organic synthesis is the earliest method to determine molecular chirality, mainly chemical correlation method. . The synthesis of many challenging complex chiral compounds has been overcome by organic chemists, but organic synthesis is always a cumbersome and laborious choice.
2. nuclear magnetic resonance (NMR)
NMR technology is the preferred method to obtain the structure of compounds, and its coupling constant and noe spectrum are important means to obtain the relative configuration of compounds, which is suitable for the configuration confirmation of diastereomers with rigid structures. . In recent years, some indirect methods have been developed to determine the absolute configuration of enantiomers through derivatives of chiral samples with the help of NMR.
. That is, by derivatizing the sample into diastereomers or similar diastereomers, the 1H-NMR or 13C-NMR shift data of the product after the reaction between the sample molecule and the chiral reagent were determined, and the difference of its chemical shift was obtained and compared with the model. Finally, the absolute configuration of the chiral center of the substrate molecule was deduced. For example, the Mosher method is to react the chiral secondary alcohol group (or secondary amine group) of the sample to be tested with (R) or (s) - α - methoxy - α - trifluoromethyl - α - phenylacetic acid (also known as Mosher acid, abbreviated as MTPA, see Figure 1) to generate the corresponding ester or amide, and then determine the NMR spectrum of the ester or amide. According to the shielding effect of the aromatic ring of MTPA, the chemical shift differences of 1H-NMR or 13C-NMR signals before and after the ester (or amide) formation of the tested substance and MTPA were compared, and the absolute configuration of the secondary alcohol (or secondary amine) was inferred from the difference of chemical shifts in the spectrum and the model diagram.
Figure 1: stereoconformation of Mosher acid
. At present, the developed chiral recognition agents mainly target some groups in the chiral center (such as hydroxyl, amino, carboxylic acid), and need expensive chiral reagents for derivatization, so their application scope is limited.
3. X-ray diffraction
Ordinary X-ray method (molybdenum target) can only construct the relative configuration of compounds, and cannot distinguish the corresponding isomers. If a molecule contains heavy atoms (generally atomic number greater than 16) or introduces a heavy atom into the molecule, the absolute configuration of the chiral molecule of the heavy atom can be determined by X-ray. In addition, the absolute configuration of the structure can also be obtained by introducing another chiral molecule with known absolute configuration. .
In the analysis of single crystal structure, the internationally recognized parameter characterizing the absolute configuration is called Flack parameter. When the structure analysis enters the final refinement stage, if the parameter is equal to or close to 0, or its parameter is within ± 0.3, then the absolute configuration is generally considered to be determined.
The single crystal X-diffraction method has the advantages of less sample consumption, rapid determination, reliable and intuitive results, and can be used as a method to determine the final stereo configuration. However, the application of X-ray diffraction method is also limited due to the high price of the testing instrument and strict requirements for single crystals.
4. Spectroscopic method
Among the spectral analysis methods, the most famous and widely used chiral molecular configuration determination methods are optical rotation spectroscopy (ORD) and circular dichroism chromatography (CD). This method has been widely used because of its low requirements for samples (such as purity, functional groups, crystallization, etc.) and no loss in the measurement process. .
4.1 Optical rotation spectrometry (ORD)
The early chiral optical method is the rotation spectroscopy method. When a plane polarized light passes through a chiral substance, it can make its polarization plane rotate. This phenomenon is called optical rotation. Use the instrument to record the deflection angle of the vibration surface of the plane polarized light passing through the chiral compound solution, which is the optical rotation α. The optical rotation we usually measure is the specific rotation under the yellow light of the Na lamp with the wavelength of 589.6 nm. The optical rotation spectrum (ORD) can be obtained by changing the optical rotation with the wavelength.
In homologues, the same chemical reaction causes the optical rotation value to change in the same direction without changing its optical rotation direction. Therefore, the configuration information of chiral compounds can be obtained by comparing the optical rotation of related compounds. When using this method to determine the absolute configuration of drugs, the optical rotation spectrum of compounds with known absolute configuration and the same or similar structure as the drug to be tested should be determined under the same experimental conditions to ensure the reliability of the comparison results.
Compared with circular dichroism (CD), CD spectrum is sharp, simple and easy to analyze. Ord has been replaced by modern chiral optical technology CD.
4.2 Circular dichroism (CD)
The wavelength range of plane polarized light used in traditional circular dichroism is generally in the ultraviolet region (200 ～ 400 nm). The difference between the absorption coefficients (ε) of chiral compounds (solutions) in left-handed and right-handed circularly polarized light changes with the wavelength of incident polarized light. The spectrum obtained is circular dichroism spectrum (CD), also known as electronic circular dichroism (ECD).
This method mainly measures the cotton effect of optically active substances (substances to be tested) under circularly polarized light, obtains the stereochemical information of the environment around the chromophore in the drug structure according to the symbol of cotton effect, and compares it with the cotton effect of a compound with known absolute configuration and similar drug structure to be tested, or with the help of computational chemistry, compares the experimental and theoretical values, that is, it is possible to deduce the absolute configuration of substances to be tested. Figure 2 shows the structures and CD spectra of osmium heterobenzene chiral compounds 1a and 1b.
Fig. 2 osmium heterobenzene chiral compounds 1a and 1b and their CD spectra,
(a) ; (b) Measured values of CD spectra of 1a and 1b;
For a long time, electronic circular dichroism has been widely used because of its less interference and easy determination. . This method is not suitable for compounds without chromophores in chiral centers or compounds that cannot introduce chromophores.
4.3 Vibrating circular dichroism (VCD)
Traditional circular dichroism requires chiral molecules to have UV absorption, which has become a major problem limiting its application. In the 1970s, holzwart, nafie and Stephens successively successfully measured circular dichroism (VCD) at the frequency of infrared light region. When the wavelength range of plane polarized light is in the infrared region (4000 ~ 750 cm − 1), because its absorption spectrum is caused by the transition of the vibrational rotational energy level of molecules, the VCD spectrum is the spectrum given by the difference of the absorption coefficient ∆ε between the left-handed circularly polarized light and the right-handed circularly polarized light in infrared light as a function of wavelength.
Due to the complexity of the vibrational spectra, it is difficult for VCD to develop a suitable theory to interpret the structure spectra like the traditional electronic circular dichroism (ECD), which mainly relies on the comparison of theoretical and measured values to determine the absolute configuration of chiral molecules.
Compared with ECD, the biggest advantage of VCD is that it does not need chromophores (UV absorption) in the molecule. Almost all chiral molecules have absorption in the infrared region and will produce VCD spectra. In addition, the VCD test is carried out in solution without single crystals, and the achiral impurities in the sample do not affect the determination results. With more and more attention and research, vibrational circular dichroism will become a powerful tool to identify the absolute configuration of chiral molecules.
. More methods are expected to be discussed and summarized by peers.
Transferred from: roaming pharmaceutical official account