C-H-deprotonation mediated by a remote syn-axial acetoxy group--an unprecedented double bond formation upon cyanation of the dimer from L-fucal

Replacement of the anomeric acetate by a cyanide group in the dimer of di-O-acetyl-L-fucal by the action of mild Lewis acid Hg(CN)(2)-HgBr(2)-Me(3)SiCN], resulted not only in the desired transformation but also in the introduction of an additional double bond between C-2A and C-3A. Due to its configuration, the remote C-4A acetoxy group may facilitate the deprotonation by functioning as an internal base. 1H NMR spectroscopy and X-ray crystallography indicate that the conformations of both rings A and B and their relative orientation in the resulting C-linked disaccharidic glycosyl cyanide, 4-O-acetyl-2-C-(4-O-acetyl-2,3-dideoxy-alpha-L-threo-hex-2-enopyranosyl)-2,3-dideoxy-2-eno-alpha-L-fucopyranosyl cyanide, in solution are virtually identical to the crystal structure.