Stereoselective synthesis of 2,6-syn-dimethyl-tetrahydropyran derivatives, important segments of marine polycyclic ethers, by unique insertion of the methyl group.

Treatment of 6-methyl-tetrahydropyran derivatives, which have a 1′-mesyloxy group at the C2-side chain, with Me3Al effected stereoselective insertion of a methyl group at the C2-position to give 2,6-syn-dimethyl-tetrahydropyran derivatives. This reaction proceeds via removal of the mesyloxy group, 1,2-hydride shift, and stereoselective insertion of a methyl group into the resulting oxonium ion.