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) unde rgo electrophilic substitution by tri-O-acetyl-5-thio-D-xylopyranosyl oxonium ions produced at -78 degrees C from the corresponding alpha-tr ichloroacetimidate 1 in the presence of BF3 . OEt2 complex to yield C- aryl 5-thio-D-xylopyranosides, and in particular, the para-subdtuted r egioisomer 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 chlorid e at room temperature led to tetrahydrothiophene derivatives in modera te to good yields, by a sulfur transannular participation mechanism. W ith 1,3-diphenol and 1,3,5-triphenol, those sugar derivatives also rea cted under quite different conditions (1: -78 degrees C, BF3 . OEt2, 2 : 50 degrees C, ZnO) to afford the corresponding C-aryl 5-thio-D-xylop yranosides exclusively. Although O-aryl 5-thio-D-xylopyranosides could not be isolated, their formation as intermediates seemed reasonable u nder basic conditions only. With acidic catalysts, our observations, i n accordance with other results, suggested that C-aryl 5-thio-D-xylopy ranosides were formed essentially by electrophilic substitution of ele ctron-rich aromatic derivatives. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.