Sir: Acetal-initiated cyclization reactions have been re- cently reported for the stereoselective preparation of ox- ygenated heterocycles.' Vinylsilanes gave different unsaturated oxacycles with a good control of the double bond stereochemistrylb (route 1 in Scheme I), and (2-methoxyethoxy)methyl (MEM) ethers were also employed to give 4-halotetraydropyrans with a complete control of the stereochemistry of the substituents'* (route 2 in Scheme I). We ...
