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 BF 3 ⋅Et 2 O, Sc(OTf) 3 , InBr 3 , and Bi(OTf) 3 was studied ( Table 1 ). While the strong Lewis acids BF 3 ⋅Et 2 O 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 InBr 3 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 ).