Stereoselective glycosylations using chiral auxiliary

SCHEME 3.2 Stereoselective glycosylation by using chiral auxiliaries. 

H. Kunz Stereoselective syntheses using carbohydrates as chiral auxiliaries G. W. Hart Ubiquitous and temporal glycosylation of nuclear and cytoplasmic proteins... 

New strategies for preparing either 1,2-cis- or 1,2-trans-glycosides stereoselectively have been reported.61 A chiral auxiliary containing a nucleophile is used to control the anomeric selectivity of the glycosylation. A sample reaction is shown in Scheme 8. 

A novel strategy for stereoselective glycosylations has been reported in which a chiral auxiliary at the 2-position of a glycosyl donor is used (Scheme 126).189 Upon formation of an oxonium ion, participation of the nucleophilic moiety of the auxiliary is (g) selective and depedent on the chirality. 

An alternative stereoselective synthesis of chiral heterocycles based on carbohydrate-induced stereodifferentiation includes nucleophilic addition reactions on heterocyclic systems already bound to the carbohydrate auxiliary. An example of this strategy has been shown by stereoselective addition of Grignard reagents to carbohydrate-linked 4-pyridones [61]. For this purpose, trimethylsiloxypyridine was glycosylated regioselectively using pivaloyl-protected glycosyl fluorides. 

Carbohydrates are inexpensive and renewable natural products which contain numerous functional groups and chiral centers. Utilizing their pronounced complexing abilities and their content of chiral information, carbohydrates are applied as chiral auxiliaries in Diels-Alder and aza-Diels-Alder reactions. The method involves the use of Lewis acid catalysts with different complexing properties. Thus, dienes of low reactivity can be transformed to their cycloadducts with high asymmetric induction. The use of N-glycosyl imines as the dienophiles offers stereoselective access to enantiomerically pure piperidine derivatives, e.g. the alkaloids coniin and anabasin. 

A review examines the use of carbohydrates as versatile starting materials for chiral auxiliaries in glycosylation, Mannich-type, stereoselective Strecker condensation and Ugi reactions. ... 

Carbohydrate-derived auxiliaries exhibit an efficient stereoselective potential in a number of nucleophilic addition reactions on prochiral imines. a-Amino acids, P amino acids and their derivatives can be synthesized in few synthetic steps, and with high enantiomeric purity. A variety of chiral heterocycles can readily be obtained from glycosyl imines by stereoselective transformations, providing evidence that carbohydrates have now been established as useful auxiliaries in stereoselective syntheses of various interesting classes of chiral compounds. 

N-Glycosylnitrones have received considerable attention as chiral equivalents in enantioselective synthesis because the glycosyl moiety acts as an auxiliary, which can be easily removed by acid treatment at an advanced stage of a synthetic sequence. Concerning cycloadditions, it has mainly been AT-glycosylnitrones derived from formaldehyde (C-unsubstituted) [43-45], from glyoxylates (C-alkoxycarbonyl) [46-51], or from aromatic aldehydes (C-aryl) [52,53] that have been used, and these compounds usually showed very interesting stereoselectivities. 

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