Mechanism of 4,6‐<i>O</i>‐Benzylidene‐Directed β‐Mannosylation as Determined by α‐Deuterium Kinetic Isotope Effects

Abstract

Considerable oxacarbenium ion character may be in the transition state of a highly β-selective mannosylation reaction that proceeds via an α-mannosyl triflate. An α-deuterium kinetic isotope effect of 1.2 was measured at −78 °C (≡1.1 at 25 °C). This information may be interpreted in terms of a stereoselective trapping of a transient contact ion pair or, alternatively, as representative of an “exploded” transition state (see scheme).