Regioselective dehydrative intramolecular heteroannulation of β-allyl-β-hydroxy dithioesters: facile and straightforward entry to 2H-thiopyrans

β-Allyl-β-hydroxy dithioesters have been employed in the synthesis of hitherto unreported and synthetically demanding 2H-thiopyrans via regioselective intramolecular annulation strategy. Lewis acid BF3·Et2O efficiently mediates the regioselective dehydration followed by intramolecular thioannulation at room temperature. The attractive features of this protocol include mild conditions, high atom-economy and excellent yields with the elimination of water as the only by-product.