Minimally competent lewis acid catalysts: Indium(III) and bismuth(III) salts produce rhamnosides (=6-deoxymannosides) in high yield and purity

Glycosylation of decan-1-ol (2), (±)-decan-2-ol (3), and (±)-methyl 3-hydroxydecanoate (4) with L-rhamnose peracetate 5 to produce rhamnosides (=6-deoxymannosides) 6, 7, and 8 in the presence of Lewis acids BF₃×Et₂O, Sc(OTf)₃, InBr₃, and Bi(OTf)₃ was studied (Table1). While the strong Lewis acids BF ₃×Et₂O and Sc(OTf)₃ were effective as glycosylation promoters, they had to be used in excess; however, glycosylation required careful control of reaction times and temperatures, and these Lewis acids produced impurities in addition to the desired glycosides. Enantiomerically pure rhamnosides (R)-1 and (S)-1 (Fig.) were obtained from L-rhamnose peracetate 5 and (±)-benzyl 3-hydroxydecanoate (9) via the diastereoisomeric rhamnosides 10 (Table2; Scheme3). The much weaker Lewis acids InBr₃ and Bi(OTfl)₃ produced purer products in high yield under a wider range of conditions (higher temperatures), and were effective glycosylation promoters even when used catalytically (<10% catalyst; Table2). We refer to these Lewis acids as 'minimally competent Lewis acids' (cf. Scheme4).