Synthetic studies in the 5-thio-D-xylopyranose series part 2:: Coupling of 5-thio-D-xylopyranosyl donors with electron-rich aryl moieties:: Access to C-aryl 5-thio-D-xylopyranosides

Electron-rich benzene derivatives (anisole, 1,4-dimethoxybenzene) undergo electrophilic substitution by tri-O-acetyl-5-thio-D-xylopyranosyl oxonium ions produced at -78 degrees C from the corresponding alpha-trichloroacetimidate 1 in the presence of BF3 . OEt2 complex to yield C-aryl 5-thio-D-xylopyranosides, and in particular, the para-subdtuted regioisomer in the case of anisole. With the corresponding acetylated 5-thio-alpha-D-xylopyranosyl bromide 2 and the same aromatic compounds, no reaction occurred in the presence of zinc oxide while zinc chloride at room temperature led to tetrahydrothiophene derivatives in moderate to good yields, by a sulfur transannular participation mechanism. With 1,3-diphenol and 1,3,5-triphenol, those sugar derivatives also reacted under quite different conditions (1: -78 degrees C, BF3 . OEt2, 2: 50 degrees C, ZnO) to afford the corresponding C-aryl 5-thio-D-xylopyranosides exclusively. Although O-aryl 5-thio-D-xylopyranosides could not be isolated, their formation as intermediates seemed reasonable under basic conditions only. With acidic catalysts, our observations, in accordance with other results, suggested that C-aryl 5-thio-D-xylopyranosides were formed essentially by electrophilic substitution of electron-rich aromatic derivatives. (C) 1998 Elsevier Science Ltd. All rights reserved.