Synthesis of C-glycosylic compounds using three-membered cyclic intermediates

Numerous methods for preparation of C-glycosylic compounds (C-glycosides) h ave been developed. One general approach to the synthesis of these importan t O-glycoside analogs is based on the use of three-membered cyclic intermed iates. The review is focused on the application of glycal and exo-glycal de rived epoxides and episulfonium and iodonium ions for preparation of C-glyc osides. Reactions of glycal epoxides with organocuprates, Grignard and orga nolithium reagents, allylsilane, sodio malonate, and lithium alkynyl deriva tives have been shown to be convenient for stereoselective synthesis of bot h alpha- and beta-C-glycosides. The unprotected C(2)-hydroxyl group in the products can be removed in two steps providing an easy excess to 2-deoxy-C- glycosides. Electrophilic addition of arylsulfenyl chloride (ArSCl) to glyc als has afforded 2(arylthio)pyranosyl chlorides. Upon the treatment with a Lewis acid, the chlorides have been converted to episulfonium-like intermed iates. Reactions of the latter species with silyl enol ethers, TMSCN, allyl silanes, vinyl ethers, and heteroaromatic compounds have opened a new synth etic route to 2(arylthio)-beta-C-glycosides having a variety of functional groups in the lateral chain. The 2-arylthio group in the products can be se lectively removed using Raney Ni or n-Bu3SnH/AIBN. Episulfonium ions genera ted from ArSCl adducts of 1-methylene sugars have reacted with O- and C-nuc leophiles to afford O-ketopyranosides and 1,1-dialkyl C-glycosides, respect ively. Acid catalyzed and nucleophilic ring openings of spiro epoxides obta ined from exo-glycals have occurred with opposite stereoselectivity. Iodoni um-promoted reactions of exo-glycals have led to O-ketopyranoside