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 (Table 1). While the strong Lewis acids BF3.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 (Table 2; Scheme 3). 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; Table 2). We refer to these Lewis acids as minimally competent Lewis acids (cf. Scheme 4).