Scalable Sn-Catalyzed Regioselective Allylation of 1-Methyl-l-α-rhamnopyranoside

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A robust selective allylation of 1-methyl-l-α-rhamnose was developed using di-n-butyltin oxide (n-Bu2SnO) as the catalyst. Proton sponge was found to be the optimal base for high regioselective control. The optimized condition afforded the 3-O-allylated rhamnose in excellent regioselectivity (>20:1) and 82% isolated yield on a 50 g scale. A scalable isolation/purification process was developed which afforded the desired product in 72% yield with 1530 ppm of Sn. The residual Sn level can be further reduced to <20 ppm by treatment with scavengers. This catalytic system also showed outstanding potential in the selective benzylation of the same substrate.