Determination of the absolute configuration of chiral compounds

The determination of the absolute configuration of chiral molecules is an extremely important and long-standing problem. At present, there are mainly four methods to determine the absolute configuration of chiral molecules :(1) organic chemical method; (2) NMR; (3) X-ray diffraction method; (4) Spectroscopic methods, such as optical rotation spectroscopy, circular dichroism, vibration circular dichroism, etc.

1. Organic chemistry

Organic synthesis is the earliest method to determine molecular chirality, mainly chemical correlation method. The reverse synthesis analysis of target molecules starts from the initial known chiral compounds, converts them into target compounds through chiral controlled organic chemical reactions, and then derives their absolute configurations from their optical rotation symbols or corresponding gas and liquid chromatography. The synthesis of many challenging chiral compounds has now been conquered by organic chemists, but organic synthesis has always been a tedious and laborious choice.

2. Nuclear Magnetic resonance (NMR)

NMR technique is the preferred method to obtain the structure of compounds, and its coupling constants and NOE spectra are important means to obtain the relative configuration of compounds, which is suitable for the confirmation of the configuration of diastereomers of rigid structures. However, for optical (enantiomer) isomers, the signal of their NMR spectrum is generally the same, that is, the application of NMR spectrum can not directly distinguish them, nor determine their absolute configuration. In recent years, some indirect methods have been developed to determine the absolute configuration of enantiomers by means of NMR derivatives of chiral samples.

Mosher method is the most common method to determine the absolute configuration of chiral compounds by NMR. That is, the 1H-NMR or 13C-NMR shift data of the product after the reaction between the sample molecule and the chiral reagent were determined by derivating the sample into diastereomers or similar to diastereomers, and the difference value of its chemical shift was obtained and compared with the model. Finally, the absolute configuration of the chiral center of the substrate molecule was estimated.

The NMR method of chiral derivatives has the advantages of low sample consumption, simple synthesis, rapid and accurate determination, and has been very mature in the absolute configuration determination of chiral alcohols, chiral amines and chiral carboxylic acids. Because the chiral recognition agent developed at present is mainly targeted at some groups in the chiral center (such as hydroxyl, amino, carboxylic acid), and it needs expensive chiral reagent for derivatization, its application scope is limited.

3. X-raydiffraction

The common X-ray method (molybdenum target) can only construct the relative configuration of the compound, but can not distinguish the corresponding isomers. If the molecule contains a heavy atom (usually with an atomic number greater than 16) or if a heavy atom is introduced into the molecule, the absolute chiral molecular configuration 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. With the development of technology, the X-ray single-crystal CCD diffractometer using CuKa as the incident light source has been able to determine the absolute configuration of organic molecules containing C, H, N and O atoms with relative molecular weight below 1000.

In the single crystal structure analysis, the internationally recognized parameters representing the absolute configuration are called Flack parameters. 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-ray diffraction method can be used to determine the final three-dimensional configuration because of its small amount of sample, rapid determination and reliable and intuitive results. However, due to the high cost of testing instruments, the strict requirements on single crystal limit the application of X-ray diffraction method.

4) spectroscopy

Among the spectral analysis methods, ORD and CD are the most famous and widely used methods for determining chiral molecular configuration. They have been widely used because of their low requirements on samples (such as purity, functional groups, crystallization, etc.) and no loss in the measurement process. In recent years, vibratory circular dichroism (VCD) has made great progress and has gradually become an important tool to identify the absolute configuration of chiral molecules.

4.1 Optical rotatory spectroscopy (ORD)

The early chiral optics methods were spin spectroscopy. When planar polarized light passes through a chiral material, it can rotate its plane of polarization. This phenomenon is called optical rotation. The instrument records the deflection Angle of the vibration surface of the plane polarized light passing through the chiral compound solution, which is the optical rotation. The optical rotation we usually measure is the specific optical rotation under the yellow light of the Na lamp with the wavelength of 589.6nm. The optical rotation spectrum (ORD) can be obtained by varying 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 the direction of its optical rotation. 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 should be determined under the same experimental conditions with compounds with known absolute configuration and the same or similar structure as the drugs to be tested, so as to ensure the reliability of the comparison results.

Compared with circular dichroism (CD), ORD is sharp, simple and easy to analyze. ORD has been replaced by CD, a modern chiral optical technology.

4.2 Circular dichroism (CD)

The wavelength range of planar polarized light used by traditional circular dichroism is generally in the ultraviolet region (200-400 nm). The difference of absorption coefficient () of chiral compounds (solutions) in left-handed and right-handed circularly polarized light varies with the wavelength of incident polarized light. The resulting pattern is circular dichroism (CD), also known as electron circular dichroism (ECD).

The method mainly through determination of the optical active substances () object under test under a circularly polarized light Cotton effect of the Cotton effect according to the symbol for drug structure found chromophore surrounding three-dimensional chemical information, and with an absolute configuration of known and drug structure under test in comparison with the Cotton effect of similar compounds, or with the aid of the method of computational chemistry experiments, measurements and the calculated value is to deduce the absolute configuration of the object under test.

For a long time, electronic circular dichroism has been widely used because of its low interference and easy determination. However, this method is based on the premise that the chiral center of the compound to be tested contains the appropriate chromophore (uv absorption) or is able to introduce the appropriate chromophore. This method is not suitable for compounds with no chromophore in the chiral center or where no chromophore can be introduced.

4.3 Vibrating circular dichroism (VCD)

Traditional circular dichroism requires chiral molecules to have ultraviolet absorption, which has become a major problem restricting its application. In the 1970s, Holzwart, Nafie and Stephens had successfully determined the circular dichroism (VCD) at infrared region frequencies. When the wavelength range of plane polarized light is in the infrared region (4,000-750 cm-1), since its absorption spectrum is caused by the vibrational rotational energy level transition of molecules, the VCD spectrum is the spectrum given by the difference of absorption coefficient of left-handed and right-handed circular polarization in infrared light varying with the wavelength.

Due to the complexity of vibration spectrum, IT is difficult for VCD to develop appropriate theories to interpret structural-spectrum correspondence like traditional Electronic Circular Dichroism (ECD), and the absolute configuration of chiral molecules is mainly determined by the comparison between theoretical calculated values and measured values.

Compared with ECD, the biggest advantage of VCD is that there is no need to contain chromophore (ULTRAVIOLET absorption) in the molecules. Almost all chiral molecules are absorbed in the infrared region, which will produce VCD spectrogram. In addition, THE VCD test is in the solution state, no single crystal is required, and the non-chiral impurities in the sample do not affect the determination results. With more and more attention and research, vibrational circular dichroism will be a powerful tool to identify the absolute configuration of chiral molecules.

In addition to the four classical configuration determination methods mentioned above, infrared spectroscopy and ULTRAVIOLET spectroscopy are also used to assist the determination of compound configuration. More methods also hope colleagues to discuss the summary together.