Unique reactivity of the Mukaiyama glycosidation catalyst (SnCl3ClO4) toward beta-mannopyranosides

Glycosidation of a mannosyl donor in the presence of the Mukaiyama catalyst was found to give exceptionally high alpha/beta selectivity. A systematic study was conducted to reveal that selective beta-to-alpha anomerization accounts for the observed high alpha/beta stereoselectivity. Furthermore, the Mukaiyama catalyst was shown to exhibit an unusual level of substrate and anomer selectivity for the anomerization. On the basis of the combined anomeric and Delta 2 effects, a mechanistic rationale was proposed, thereby suggesting the minimum structural moiety essential for the anomerization in question. With this analysis, beta-talo-, beta-altro-, and beta-idopyranosides are predicted to exhibit a reactivity profile similar to beta-mannopyranosides, but all other pyranosides should not. This prediction was verified by using beta- and alpha-talopyranosides as an example.