Silatropic Carbonyl Ene Cyclizations in the Synthesis of Pseudosugars and Hydroxylated Piperidines

We describe two applications of silicon-tethered thermal allyl transfer reactions of α-silyloxyaldehydes; formally, these processes can be regarded as silatropic carbonyl ene reactions in which the silicon tether is transferred to the aldehyde oxygen concurrent with carbonyl allylation. In the first application, isoserinal substrates, which bear side-chain nitrogen functionality, are elaborated to dihydroxypiperidines. In the second application, a product of cyclohexadienyl transfer is taken on to carbocylic analogues of, for example, mannose. In both series, the silatropic ene reactions are effected thermally, with no added Lewis acid, and are both stereospecific and highly stereoselective.