Directing effect by remote electron-withdrawing protecting groups at O-3 or O-4 position of donors in glucosylations and galactosylations

Glucosylations and galactosylations of various acceptors with donors possessing an electronwithdrawing benzylsulfonyl, benzoyl, or acetyl group at the O-3 or O-4 position were performed. A beta-directing effect by the benzylsulfonyl group at O-3 of the glucosyl donors and by the benzylsulfonyl and acyl groups at O-4 of the glucosyl donors was observed. In contrast, acyl groups at O-3 of the glucosyl donors and acyl groups at O-3 and O-4 of the galactosyl donors exhibited an alpha-directing effect. The alpha-directing effect is partly considered to remote participation of the acyl groups, whereas the beta-directing effect is somewhat attributed to the S(N)2-like reaction of the acceptor with the glycosyl triflate or the contact ion pair, which is stabilized by remote electron-withdrawing groups. Further evidence for the stability of the alpha-glycosyl triflates was determined by a low-temperature NMR study. (C) 2015 Elsevier Ltd. All rights reserved.