Preparation of chiral compounds

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

Obviously, in a relatively small work such as this it is not possible to be comprehensive. Preparations of bulk, achiral materials (e.g. simple oxiranes such as ethylene oxide) involving key catalytic processes will not be featured. Only a handful of representative examples of preparations of optically inactive compounds will be given, since the emphasis in the main body of this book, i.e. the experimental section, is on the preparation of chiral compounds. The focus on the preparation of compounds in single enantiomer form reflects the much increased importance of these compounds in the fine chemical industry (e.g. for pharmaceuticals, agrichemicals, fragrances, flavours and the suppliers of intermediates for these products). 

Simon H, Bader J, Gunther H, Neumann S, Thanos J (1984) Biohydrogenation and elec-tromicrobial and electroenzymatic reduction methods for the preparation of chiral compounds, Ann NY Acad Sci 434 171, and refs therein... 

In addition to providing a novel approach to the preparation of chiral compounds, this type of chemistry may allow one to inquire into the subtle stereochemical details of some crystal-state reactions. For example, what are the approach geometry and the preferred side of attack in the addition of bromine to a chiral olefin (259) What can be learned of the geometry of the labile electronically excited species involved in (2 + 2) photocycloaddition reactions (260) ... 

Although asymmetric reactions using chiral Lewis acids are of great current interest as one of the most efficient methods for the preparation of chiral compounds, examples using imines as electrophiles are rare compared to those us-... 

In recent times, the term deracemization has been used for describing techniques becoming increasingly important for the preparation of chiral compounds as single enantiomers. The term refers to a process in which one single enantiomer is obtained when starting from a racemate (1). 

FIGURE 16.1. Preparation of chiral compounds from amino acids... 

The preparation of chiral compounds is an important and challenging area of contemporary synthetic organic chemistry, mainly in connection with the fact that most natural products are chiral and their physiological or pharmacological properties depend on their recognition by chiral receptors, which will interact only with molecules of the proper absolute configuration. 

A monograph has been published on the use of naturally-occurring chiral compounds as synthons for the preparation of chiral compounds, with special reference to carbohydrates. Reviews... 

Although kinetic resolution is an established method for the preparation of chiral compounds, it requires a large difference in the rate constants of the enantiomers of the substrate in order to obtain the product and recovered starting material with... 

The preparation of chiral compounds by catalytic asymmetric hydrogenation is now well established. This year has again seen several reports on both the synthesis of and mechanistic studies on such systems, optical yields of 90% being achieved with a variety of catalysts. One such system which is worthy of note is the rhodium complex of the extremely simple chiral ligand (i )-l,2-bis(diphenyl-phosphino)propane [(i )-prophos] (7). This system is an efficient hydro-... 

Chiral Auxiliary Chemistry The introduction of a sulfur atom to alkanes through a C—H activation is currently limited to enolizable C—H bonds such as a-sulfeny-lation of carbonyl compounds. Although there are other synthetic routes to a-sulfenylated carbonyl compounds, the preparation of chiral compounds has relied on the chiral auxiliary-assisted diastereoselective thioalkylation. In 1985, Paterson et al. reported that the chiral (9-silylated imide enolates 385 of Evans oxazolidinone could be sulfenylated with high diastereoselectivities by phenylsulfenyl chloride (Scheme 46.44). ... 

We have developed some chiral carboxylic acids as novel molecular tools useful for both enantioresolution of various alcohols and simultaneous determination of their ACs (Figure 55.1). These chiral molecular tools are powerful for facile preparation of chiral compounds with 100% ee and for the absolute configurational assignment. The so-called asymmetric syntheses are useful for preparation of chiral compounds, but reaction products are not always enantiopure, and in some cases, it is necessary to determine their ACs by independent chemical and/or physical methods. The methodologies explained in this chapter are useful for preparation of enantiopure authentic sample and for determination of their ACs in an unambiguous manner. The protocols using these chiral reagents have been successfully applied to various compounds, and their principle and applications are explained in this chapter. 

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