Differential O-3/O-4 regioselectivity in the glycosylation of alpha- and beta-anomers of 6-O-substituted, N-dimethylmaleoyl-protected D-glucosamine acceptors

MARÍA L. BOHN; MARÍA I. COLOMBO; PABLO L. PISANO; CARLOS A. STORTZ; EDMUNDO A. RÚVEDA

CARBOHYDRATE RESEARCH

Elsevier

Lugar: Amsterdam; Año: 2007 vol. 342 p. 2522 - 2536

0008-6215

An assessment of the relative O-3/O-4 reactivities of both  a- and  b-methyl glycosides of N-dimethylmaleoyl (DMM) d-glucosamine acceptors protected at O-6 with benzoyl (Bz), benzyl (Bn) and tert-butyldiphenylsilyl (TBDPS) groups is presented using per-O-benzoylated b-d-galactofuranosyl and per-O-acetylated a-d-galactopyranosyl trichloroacetimidates as glycosyl donors. Using the former donor, the a-anomer of the 6-O-benzoylated compound gave exclusively substitution at O-3, whereas the other two compounds with a-configuration kept this site as preferential. The b-anomer of the 6-O-benzoylated compound gave the same amounts of reaction products on O-3 and O-4, whereas the other b-analogs carried a more reactive O-4. The same reactions were carried out using as donor the less reactive per-O-acetylated a-d-galactopyranosyl trichloroacetimidate. Although the same trend was found to occur, the O-4 was always relatively more reactive with the pyranosyl donor than with the furanosyl donor, when keeping the remaining factors constant. Furthermore, the b-anomers of the acceptor gave almost exclusively substitution at O-4. These observations confirmed and extended the outlook of these “matching” donor and acceptor reactivities.