for his valuable help in obtaining the 500-MHz NMR and high-resolution mass spectra, molecular mechanics calculations, and elemental analysis determinations. We also thank CONICET (Consejo Nacional

A stereoselective synthesis of different oxacycles is reported starting from aldehydes and unsaturated alcohols in the presence of A1C13 or AlBr3.Sir: Acetal-initiated cyclization reactions have been recently reported for the stereoselective preparation of oxygenated heterocycles.1 Vinylsilanes gave different unsaturated oxacycles with a good control of the double bond stereochemistry11" (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 substituents13 (route 2 in Scheme I).We and others recently reported2 that the coupling between allylsilanes and aldehydes could be conveniently used to prepare 2,6-disubstituted 4-halotetrahydropyrans (Scheme II).

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