Δ3-DIHYDROPYRANS AND TETRAHYDROPYRANS BY REDUCTION OF PYRYLIUM SALTS WITH SODIUM BOROHYDRIDE IN ACETIC ACID

Article abstract of DOI:10.1016/S0040-4039(00)95365-6

The major reduction products with triacetoxyborohydride (NaBH4 in AcOH) of 2,4,6-trisubstituted pyrylium salts bearing alkyl substituents in the 2- and/or 6-position are the Δ³-dihydropyrans with cis 2- and 6-substituents and all-cis-2,4,6-trisubstituted tetrahydropyrans. Δ³-dihydropyrans are shown to be formed via 2H-pyrans by a 1,4 reduction while tetrahydropyrans result from 4H-pyrans by reduction of both enol-ether double bonds.