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.