Lewis-acid catalyzed formation of dihydropyrans

A methodology is described for the synthesis of 2,6-disubstituted dihydro[2H]pyrans through a Lewis-acid catalyzed 6-endo-trig cyclization of β-hydroxy-γ,δ-unsaturated alcohols. Employing alkyl-substituted allylic diols and catalytic amounts of a Lewis acid, such as BF3·OEt2, the corresponding syn-pyrans are formed highly diastereoselectively and in good yields. The described process is simple to execute, proceeds readily at ambient temperature, and is scalable.