The basic mechanistic hypothesis (Scheme 8) suggests that the ^-mannosides are formed in an SN2-like manner from the a-triflate. The NMR experiments provide overwhelming support for the formation of these triflates and, in the 4,6-benzylidene series, for their stereochemistry. However, there remains the possibility, which we cannot rule out, that the covalently bound triflates simply serve as a reservoir for the storage of extremely reactive contact ion pairs. In this hypothesis, which echoes that of Schuerch (see above), the a-triflate dissociates to a contact ion pair in which the anion is intimately associated with the a face of the oxacarbenium ion and so ster-ically prevents approach from that face. In this hypothesis, reduced selectivity, as

Scheme 12

seen with the per-ether protected donors, arises from intervention of solvent and of solvent-separated ion pairs. This hypothesis therefore requires a change in the equilibrium between contact ion pairs and solvent-separated ion pairs in going from the 4,6-benzylidene series to the per-ether protected series. Again we fall back on the extra strain imposed on the oxacarbenium ion by the 4,6-benzylidene group, which will have the effect of shifting the CIP/covalent triflate equilibrium toward the covalently bound species, hence of reducing the likelihood of intervention of a solvent-separated ion pair. This variation of the original mechanism is presented in Scheme 12.

Although we have provided compelling evidence for the formation of mannosyl triflates in our system, it is not implied that related triflates are formed in all sulfoxide glycosylation reactions. Indeed, Kahne and his coworkers subsequently investigated the mechanism of their reaction and found that in many cases the original sulfoxide is rearranged to isolable sulfenate esters, which also serve as glycosyl donors as shown, for example, in Scheme 13 [34]. The precise reasons for the different reactivity patterns are not clear and are the subject of ongoing research in our laboratory.

In the course of our investigations into the mechanism of the reaction, we became aware of the extreme electrophilicity of the by-product benzenesulfenyl tri-

MeO /OMe iXo


Scheme 14

PhSOTf (53)

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