《有机化学》课程教学资源（文献资料）Determination of absolute configurations by X-ray crystallography and 1H NMR anisotropy

interScience CHRAITY 0 Review Article Determination of Absolute Configurations by X-ray Crystallography and H NMR Anisotropy ABSTRACT Todetermine the absolute configurations of chiral comounds,many ing chiral anisotrop s are relative and/or empirical meth- ired.As chiral orboth the by HPLC separation and the simultaneou ermi and 2-mempho 2-(1-naph f the e and NMR methods are explained using stly our 20:691-723,2008. ©200 7 Wiley-liss.ne KEY WORDS:a acid): separation on silica gel INTRODUCTION pounds. and their methodologies and applications are It is well re nized that most biologically active com explained in this review s the light-po METHODOLOGIES FOR DETERMINING ABSOLUTE ng in n nt inine absolut o Chiral Com of t It is well known that the absolute ration of chira e,in the field unds was t det desired enantiomers can be synthesized determined by the use of the Flack parameter,instead of noemolecrtodsseitiorbothcnaniorEsoittionc Culture ion (1).These chiral also for the abs olute 12 June 2007;Accepted 10 August 007 2007 Wiley-Liss.Inc

Review Article Determination of Absolute Configurations by X-ray Crystallography and 1 H NMR Anisotropy NOBUYUKI HARADA* Department of Chemistry, Columbia University, New York ABSTRACT To determine the absolute configurations of chiral compounds, many spectroscopic and diffraction methods have been developed. Among them, X-ray crystal￾lographic Bijvoet method, CD exciton chirality method, and the combination of vibra￾tional circular dichroism and quantum mechanical calculations are of nonempirical na￾ture. On the other hand, X-ray crystallography using a chiral internal reference, and 1 H NMR spectroscopy using chiral anisotropy reagents are relative and/or empirical meth￾ods. In addition to absolute configurational determinations, preparations of enantiopure compounds are strongly desired. As chiral reagents useful for both the preparation of enantiopure compounds by HPLC separation and the simultaneous determination of their absolute configurations, we have developed camphorsultam dichlorophthalic acid (CSDP acid) for X-ray crystallography and 2-methoxy-2-(1-naphthyl)propionic acid (MaNP acid) for 1 H NMR spectroscopy. In this review, the principles and applications of these X-ray and NMR methods are explained using mostly our own data. Chirality 20:691–723, 2008. VC 2007 Wiley-Liss, Inc. KEY WORDS: absolute configuration; X-ray crystallography; internal reference; chiral X￾ray reference reagents (CXR); camphorsultam dichlorophthalic acid (CSDP acid); 1 H NMR anisotropy; chiral 1 H NMR anisotropy reagents (CAR); 2- methoxy-2-(1-naphthyl)propionic acid; MaNP acid; diastereomers; HPLC separation on silica gel INTRODUCTION It is well recognized that most biologically active com￾pounds controlling physiological functions of living organ￾isms are chiral. Furthermore, studies on chiral molecular devices and molecular machines, such as the light-pow￾ered chiral molecular motors developed in our laboratory, have been rapidly progressing in recent years. The biologi￾cal and molecular functions of these compounds are closely related to the chirality of molecules, i.e., absolute configuration of molecules. Therefore, in the field of bio￾molecular and molecular material sciences, the unambigu￾ous determination of the absolute configuration of chiral compounds becomes the first major issue. The second issue is the chiral synthesis of target compounds and how efficiently the desired enantiomers can be synthesized with 100% enantiopurity or enantiomeric excess (%ee). We have developed some chiral carboxylic acids as novel molecular tools useful for both enantioresolution of various alcohols and simultaneous determination of their absolute configurations (see Fig. 1). These chiral molecu￾lar tools are very powerful for the facile preparation of chi￾ral compounds with 100% ee and also for the absolute con- figurational assignment. The methods using these chiral tools have been successfully applied to various com￾pounds, and their methodologies and applications are explained in this review. METHODOLOGIES FOR DETERMINING ABSOLUTE CONFIGURATIONS Nonempirical Methods for Determining Absolute Configurations of Chiral Compounds (Table 1) It is well known that the absolute configuration of chiral compounds was first determined by the X-ray crystallogra￾phy Bijvoet method using heavy atom effects.1 In X-ray crystallography, as the anomalous dispersion effect of heavy atoms can be measured accurately under proper conditions, absolute stereostructures have been clearly determined by the use of the Flack parameter,2 instead of Contract grant sponsors: Ministry of Education, Science, Sports, Culture, and Technology, Japan; Japan Society for the Promotion of Science. *Correspondence to: Nobuyuki Harada, Department of Chemistry, Columbia University, 3000 Broadway, MC3152, New York, NY 10027, USA. E-mail: nh2212@columbia.edu, n_harada@ma.mni.ne.jp Received for publication 12 June 2007; Accepted 10 August 2007 DOI: 10.1002/chir.20478 Published online 8 October 2007 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 20:691–723 (2008) VC 2007 Wiley-Liss, Inc

HARADA ☐N Phenomena. COOH Method and/or reagents Refs. ChiralAcid Non L Heavy atom effect 12 CD spectroscopy 5 Twisted electron system 6 CDORD Ab initio calculation (5 901213 ofahsoltio MaNP acid (-(+3 MPA acid MA,2NMA. the r 特8 How the method has been exten 品泾51-0 ab Chemical correlation ecause the abs eoagdd third n unstable this no chromophore.and so it should be thi more widely used in the near future. ted by the The obt mine their absolute urations theoretically determined have been established configuration stituent with La ch Raertaheahneakhtoanehodofbrnaional known abso configurati rotatory dispersion,and CD.has been developed as the Chirality DOI 10.1002/chir

the measurement of Bijvoet pairs. The obtained results are unambiguous and reliable. In addition, the molecule can be projected as a three-dimensional structure, and there￾fore, the method has been extensively employed. How￾ever, the X-ray method needs single crystals of a large size suitable for X-ray diffraction experiments, and so the criti￾cal problem is how to obtain such large single crystals. The CD (circular dichroism) exciton chirality method3–6 is also useful because the absolute configuration can be determined in a nonempirical manner, and it does not require crystallization. Furthermore, chiral chemical and biological reactions are traceable by CD, and even the absolute configurations and conformations of unstable compounds can be obtained by this method. However, since some compounds are not ideal targets for this method, the results must be carefully interpreted. In addi￾tion, the CD spectra of chiral compounds with a strongly twisted p-electron chromophore can be calculated by the p-electron SCF–CI–DV MO (self-consistent field/configu￾ration interaction/dipole velocity molecular orbital) method.7 The method has been successfully applied to various natural and synthetic chiral compounds to deter￾mine their absolute configurations.8,9 The absolute config￾urations theoretically determined have been established by total syntheses of those chiral compounds. Recently the ab initio calculation method of vibrational circular dichroism (VCD), Raman optical activity, optical rotatory dispersion, and CD, has been developed as the third nonempirical method.10–13 By comparison of the observed spectra with calculated ones, one can determine the absolute configurations. The method is applicable to compounds having no chromophore, and so it should be more widely used in the near future. Relative and/or Empirical Methods for Determining Absolute Configuration Using an Internal Reference of Known Absolute Configuration (Table 1) X-ray crystallography. Absolute configuration can be obtained by determining the relative configuration at the position of interest against a reference compound or sub￾stituent with known absolute configuration. A typical example is the X-ray crystallography performed after the introduction of a chiral X-ray internal reference (CXR) with known absolute configuration (see Fig. 2).14–28 In this case, the absolute configuration of the point in question can be automatically determined using the chirality of the TABLE 1. Methods for determining the absolute configurations of chiral compounds Method Phenomena, key points, and/or reagents Refs. Nonempirical methods X-ray crystallography Heavy atom effect 1 2 CD spectroscopy Coupled oscillator 3 Exciton coupling 4,5 6 Twisted p-electron system 7 8,9 CD, VCD, ROA, and/or ORD Ab initio calculation 10 11 12 13 Relative and/or empirical methods X-ray crystallography Internal reference of absolute configuration: recent examples CSDP acid and HPLC 14–20 MaNP acid and HPLC 21,22 Inclusion complex 23–25 Part of systems 26–28 1 H NMR spectroscopy Diamagnetic anisotropy effect MTPA acid 39 MPA acid 40 MTPA, 1NMA, 2NMA, 2ATMA acids 41–43 MTPA, 1NMA, 2NMA, 9AMA acids 44–48 CFTA acid 49,50 HPLC and diamagnetic anisotropy effect MaNP acid 19–22, 51–60 M9PP acid 61,62 Comparison of CD spectra Chemical correlation Fig. 1. Chiral carboxylic acids useful for enantioresolution and deter￾mination of absolute configuration of alcohols. 692 HARADA Chirality DOI 10.1002/chir

ABSOLUTE CONFIGURATIONS BY X-RAY AND'H NMR 693 HoR2 chiral alcoho 2R.3R7 o 1)condensation 2)X-raycrystallography RRX 2二放 cied with (1- can be treated asa isted the we had sy iastereomeric bisadducts chira otesneG finally d that the product mea品 rotHoeitetgored ntionats of 1-2 m.Those crysta therefore the synchroting was us the f 11 the Althoug COOH of bisa 2R.3R ration of the tether moiety as an inteml refere arget mo ule have been developed. For example and the recent developed complexes ster (IS:R)-6,X-ray raction of crystalline Chirality DOI 10.1002/chir

auxiliary introduced as an internal reference. Conse￾quently, samples do not need to contain heavy atoms for anomalous dispersion effect. An example of this method is shown in Figure 3, where alcohol (2)-5 has a heavy bromine atom, but no single crystals suitable for X-ray analysis were obtained.29 There￾fore, alcohol (–)-5 was esterified with (1S)-(–)-camphanic acid 4 (CXR), yielding ester 6 as single crystals. From the X-ray stereostructure of ester 6, the absolute configuration of (–)-5 was unambiguously determined to be R. The results obtained by this internal reference method are very clear and reliable, even when the final R-value is not small enough due to poor quality of the single crystal, as exemplified by the recent determination of the absolute configurations of chiral C60 fullerene cis-3 bisadducts by X￾ray crystallography (see Fig. 4).26 For the absolute config￾uration of chiral C60 fullerene cis-3 bisadducts, there had been many controversies among research groups.30–33 They had synthesized cis-3 bisadducts with different chiral tethers, and had calculated the molecular energies of pos￾sible diastereomers by the molecular mechanics force field method, because they had assumed that the product obtained should be the most stable diastereomer. From the calculation results, they had determined the absolute configurations of cis-3 bisadducts. However, their reported assignments were inconsistent with one another.30–35 We had calculated the CD spectra of cis-3 bisadducts by the p-electron SCF–CI–DV MO method, because the p￾electron system of cis-3 bisadducts can be treated as a twisted chromophore. From the CD calculation results, the absolute configuration of cis-3 bisadduct was deter￾mined.36 The Diederich group had applied the CD exciton chirality method to derivatives to determine their absolute configurations.37 Later we had synthesized two possible diastereomeric cis-3 bisadducts with a pertinent chiral tether, and carefully analyzed the 1 H NMR (nuclear mag￾netic resonance) chemical shift and coupling constant data. From the 1 H NMR studies, we had come to the same absolute configurations, which had been previously deter￾mined by the CD calculation.38 The absolute configura￾tions of cis-3 bisadducts determined by us have been finally established by X-ray crystallography as follows. Starting from (2R,3R)-(2)-2,3-butanediol 7, chiral C60 fullerene cis-3 bisadduct [CD(1)280]-8 was synthesized (see Fig. 4), and the product was recrystallized from chlo￾roform/hexane (1:1) giving extremely thin plate single crystals, which had a thickness of 1–2 lm. Those crystals are too thin for conventional X-ray diffractometers, and therefore the synchrotron X-ray radiation at the SPring-8 in Hyogo, Japan, was used for the X-ray analysis. Although the final R-value remained large (R 5 0.180), the absolute configuration of bisadduct [CD(1)280]-8 was clearly determined as f,sA (5f C) by using the (2R,3R) absolute configuration of the tether moiety as an internal reference. On the basis of this X-ray analysis and comparison of CD spectra, the absolute configurations of C60 fullerene cis-3 bisadducts have been conclusively determined.26 A variety of methods to link an internal reference to the target molecule have been developed. For example, there are ionic bonds such as conventional acid–base salts, covalent bonds such as esters or amides, and the recently developed inclusion complexes.23–25 Very recently, structure analyses have became possible also by X-ray dif￾fraction of crystalline powder. Therefore, the X-ray crystal￾lography using an internal reference is expected to find widespread applications. Fig. 3. An example of the determination of absolute configuration by the use of CXR. Fig. 4. Absolute configuration of chiral C60-fullerene cis-3 bisadduct 8 by X-ray internal reference method.26 Fig. 2. A general scheme for determining the absolute configuration by X-ray crystallography using a CXR, where the case of ester formation is shown. ABSOLUTE CONFIGURATIONS BY X-RAY AND 693 1 H NMR Chirality DOI 10.1002/chir

694 HARADA Meo.COOH .COOH H NMR of alcoho Denzene plane Meo.LCOOH Meo.LCOOH aCC shif)are move S-18At出 igh e pro hen c substituentRis aboveo NC COO f eand hence the ◇ the par rou x for Mr py effects i CH. bining g the anisotropy effects discus d above and the Fig5.Chiral acids usefulfor the advanced Mosher method H NMR advanced Mosher m ethod.Recently.the resonance (H NMR anisotropy 5.g 5XH-ISA solute co have been succes fully determined by n-17 luoro red con 品文n院 forme MTPA mis the 18 时he e been develope ed mos ter】 8≤0 d Fig.5) R 40■S,18A-R,018E O-MTPA MTPA pian 40=50-R S0-18A ()-17 is si (R Chirality DOI 10.1002/chir

1 H NMR advanced Mosher method. Recently, the proton nuclear magnetic resonance (1 H NMR) anisotropy method has often been used as a relative and empirical method, and it is useful for the study of the absolute con- figurations of natural products and chiral synthetic com￾pounds.39–63 In particular, the absolute configurations of secondary alcohols have been successfully determined by the Trost’s method using a-methoxyphenylacetic acid (MPA, (S)-(1)-10) 40 or by the advanced Mosher method developed by Kusumi and coworkers.41 In the advanced Mosher method, the absolute configuration of a chiral aux￾iliary, the Mosher’s reagent [a-methoxy-a-(trifluoro methyl)phenylacetic acid (MTPA, (S)-(2)-9)]39 is known, and the preferred conformations of the esters formed with a chiral secondary alcohol and MTPA have been rational￾ized. The 1 H NMR chemical shift of alcoholic part protons are affected by the diamagnetic anisotropy effect gener￾ated by the phenyl group of MTPA. From Dd values reflecting anisotropy effects, the absolute configuration of alcohol part can be determined. Since then, various chiral acids suitable for this method have been developed mostly by the Kusumi group41–43 and the Riguera group,44–48 and have been applied to various chiral compounds (Table 1 and Fig. 5). The outline of the advanced Mosher method is as fol￾lows (see Fig. 6). To determine the absolute configuration of a chiral alcohol, a chiral alcohol (X)-17 with unknown absolute configuration X is esterified with MTPA acid (S)- (2)-9 giving ester (S,X)-18A. Alcohol (X)-17 is similarly esterified with MTPA acid (R)-(1)-9 giving ester (R,X)- 18B. These MTPA esters take preferred conformations shown in Figure 6b, where CF3 group is synperiplanar to carbonyl oxygen atom, and the methine proton of second￾ary alcohol moiety is also synperiplanar to carbonyl oxy￾gen atom.39,41 Therefore, these moieties lie on a single plane, the MTPA plane, in the preferred conformations. In the preferred conformation of ester (S,X)-18A, the alco￾holic substituent R1 is above of the benzene plane of the MTPA moiety. In MTPA esters, the aromatic substituent (phenyl group) generates a diamagnetic anisotropy effect due to the ring current induced under the external magnetic field, and so the proton NMR signals of the alcohol moi￾ety, which faces the phenyl group in the preferred confor￾mation, are moved to a higher magnetic field (high field shift). So, in ester (S,X)-18A, the protons of group R1 feel a diamagnetic anisotropy effect, and hence their 1 H NMR signals show high-field shifts. On the other hand, in the ester (R,X)-18B, the alcoholic substituent R2 is above of the benzene plane of the MTPA moiety, and hence the protons of group R2 show high-field shifts. The parameter Dd reflecting the anisotropy effects is defined as Dd5d(S,X) 2 d(R,X) for MTPA esters.41 By combining the anisotropy effects discussed above and the definition of Dd parameter, the empirical rule for determin￾ing the absolute configurations of chiral secondary alco￾Fig. 5. Chiral acids useful for the advanced Mosher method. Fig. 6. The advanced Mosher method for determining the absolute configuration of chiral secondary alcohols using both (S)- and (R)-MTPA acids: (a) preparation of MTPA esters, (b) preferred conformations of MTPA esters, and (c) definition of Dd value and sector rule. 694 HARADA Chirality DOI 10.1002/chir

ABSOLUTE CONFIGURATIONS BY X-RAY AND'H NMR 695 OH We also consider that a highly efficient method for pre 回 C DM Chiral Acid 21 HPLC method 店 OH be separated reagent us .the ent-alooho nsotoeymethod hols from the diaste 不hi aination of absolute configuration alcohol moiety in the the p and Simu te Configurations by X-ray Crystallography As a chiral a useful for preparation of ena ve deve oped a chiral gen a sy eed se of its good affinity with a gel us ted bet ddition th ng aparation on th or pre ing pris me 1△8 ntly large a a re able dis ibution pa and phtha the H NMR ani the method could be extended to othe uSR ABSOLUTE CONFIGURATIONS The rthat the most reliableand practi hy by th of points in from the X-ray ais already known.Therefore.it is easy to determine 0s52 n2竖&DaneoabroalcSpmdcpahomtn Chirality DOI 10.1002/chin

hols was proposed as illustrated in Figure 6c, where the MTPA moiety is placed in the down and rear side, while the methine proton of the secondary alcohol moiety in the up and rear side. The substituent R2 showing positive Dd values is placed at the right side, while the substituent R1 showing negative Dd values is at the left side. So, the abso￾lute configuration (X) of alcohol 17 can be determined.41 This method is thus convenient, since it does not require crystallization of compounds. In the case of the Trost’s method using MPA,40 the methoxyl group is synperiplanar to the ester carbonyl oxy￾gen atom in their preferred conformations, and the param￾eter reflecting the 1 H NMR anisotropy effect, Dd is defined as Dd5d(R,X) 2 d(S,X). One problem of this method is that it is based on the assumption of preferred conformation of molecules in solu￾tion. However, it is reliable in most cases since the method itself has a self-diagnostic function.41–47 Namely, in some exceptional cases of MTPA esters,48 to which the NMR ani￾sotropy method is not applicable, the observed Dd values reflecting the anisotropy effect are small and/or distribute randomly. On the other hand, if the Dd values are suffi- ciently large and show a reasonable distribution pattern, it leads to a reliable assignment. Therefore, the applicability of the 1 H NMR anisotropy method can be judged from the magnitude and distribution pattern of the observed Dd val￾ues. Although the method has been widely applied to sec￾ondary alcohols, the method could be extended to other kinds of compounds. PREPARATION OF ENANTIOPURE COMPOUNDS AND SIMULTANEOUS DETERMINATION OF THEIR ABSOLUTE CONFIGURATIONS The author considers that the most reliable and practi￾cal method for determining the absolute configuration is the X-ray crystallography by the use of internal reference, as described above. Namely, the absolute configurations of points in question can be unambiguously determined from the X-ray stereoview showing a relative stereochem￾istry, because the absolute configuration of the chiral aux￾iliary is already known. Therefore, it is easy to determine the absolute configuration, and there is no possibility of making a mistake in the assignment. We also consider that a highly efficient method for pre￾paring an appropriate amount of various chiral compounds with 100% enantiopurity in a laboratory scale is the enantio￾resolution method, as illustrated in Figure 7, although it is called a classical method. In the method, a chiral auxiliary is covalently bonded to racemates, and therefore the obtained diastereomeric mixture can be separated by conventional HPLC on silica gel. If the chromatogram shows a baseline separation and the chiral reagent used is enantiopure, the diastereomers separated are also enantiopure. It is easy to recover enantiopure alcohols from the diastereomeric esters obtained. Therefore, if the absolute configuration of one of diastereomers can be determined by X-ray crystallography and/or by 1 H NMR anisotropy, enantiopure alcohols with established absolute configurations are obtained. (2)-CSDP Acid, a CXR Useful for the Enantioresolution of Alcohols by HPLC and Simultaneous Determination of their Absolute Configurations by X-ray Crystallography As a chiral auxiliary useful for preparation of enantio￾pure alcohols and simultaneous determination of their absolute configurations, we have developed a chiral molec￾ular tool, camphorsultam dichlorophthalic acid (CSDP acid) (2)-164 connecting (1S,2R,4R)-2,10-camphorsultam and 4,5-dichlorophthalic acid (see Fig. 8), and have applied this chiral tool to various compounds.14–20 The 2,10-cam￾phorsultam was selected because of its good affinity with silica gel used in HPLC, allowing good separation of two diastereomers. In addition, the sultam amide moiety is effective for providing prismatic single crystals suitable for X-ray diffraction experiment. Furthermore, the (1S,2R,4R) absolute stereochemistry of 2,10-camphorsultam is useful as the internal reference of absolute configuration. To con￾nect alcohols, an ester bond was chosen, because it could be readily formed and cleaved off. Accordingly, 4,5-dichlor￾ophthalic acid was selected as a linker for CSDP acid (2)- 1, and phthalic acid for CSP (camphorsultam phthalic) acid (2)-2 (see Fig. 8).64,65 Fig. 7. Enantioresolution and determination of absolute configuration of alcohols using chiral carboxylic acid. Fig. 8. Design of a chiral molecular tool, CSDP and CSP acids contain￾ing 2,10-camphorsultam moiety. ABSOLUTE CONFIGURATIONS BY X-RAY AND 695 1 H NMR Chirality DOI 10.1002/chir