Oxonium ions reaction with enol silanes

When the aldol reaction furnishes an intermediate oxonium ion, a Prins cydization may ensue in the case where a suitable internal nucleophile is present that intercepts this oxonium ion. Rychnovsky et al. have developed this strategy into a powerful tool for the straightforward synthesis of tetrahydropyrans [7]. Thus, enol ether 15 attached to an allyl silane reacted with various aldehydes under BFj activation to produce 2,6-cis-substituted-4-methylene tetrahydropyrans 16 in good to very good yields (Table 8.2). 

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