Synthetic studies in the 5-thio-D-xylopyranose series -: part 3:: Preparations of O-phenyl and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides

Treatment of the tri-O-acetyl 5-thio-alpha-xylopyranosyl 1-O-trichloroacetimidate 1 and phenol in the presence of BF3. OEt2 at low temperature led to the corresponding O-phenyl (4: similar to 30 % yield) and C-(4-hydroxyphenyl) 5-thio-D-xylopyranosides (5: similar to 40 % yield) as a result of competitive O-glycosidation and aromatic electrophilic substitution. At higher temperatures, O-phenyl 5-thio-beta-D-xylopyranoside was shown to rearrange readily to give C-(4-hydroxyphenyl) 5-thio-beta-D-xylopylanoside as the thermodynamically favoured product. In the presence of zinc oxide, the sugar alpha-bromide 2 and phenol led also to a mixture of 4 (similar to 20 % yield) and 5 (similar to 40 % yield). Compound 5 was produced from 2 on treatment with dibutyltin diphenoxide and from the 1-O-trimethylsilyl sugar derivative 3 and trimethylsilyloxybenzene, when reacted in the presence of trimethylsilyl triflate in dichloromethane (35 % yield). Under these conditions, a tetrahydrothiophene derivative was observed (6: similar to 5 % yield). However, the sulfur transannular participation pathway which accounted for its formation became predominant when the alpha-bromide 2 and phenol were treated for several hours with zinc chloride, thus affording 6 in a 42 % yield. (C) 1998 Elsevier Science Ltd. All rights reserved.