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 delta2 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.