Stereochemical vs• polar control in the preparation and LiAlH4 reduction of cis-and trans-2-alkoxy-3,4-epoxytetrahydropyrans

Abstract

2-Methoxy- and 2-t-butoxy-5,6-dihydro-2H-pyran react with m-chloroperoxybenzoic acid to give mixtures of trans- and cis-2-alkoxy-3,4-epoxytetrahydropyrans in which the trans to cis ratio is 3:1 and 9:1 for the 2-methoxy and 2-t-butoxy homologues respectively. This selectivity is attributed to the steric effect of the 2-alkoxy group.Lithium aluminium hydride attacks the ring in these cis and trans compounds exclusively at the epoxide carbon remote from the alkoxy substituent to form only 2-alkoxy-3-hydroxytetrahydropyrans. This selectivity of hydride attack is attributed to the polar influence of the two geminal oxygen atoms. Steric factors in this hydride reduction are apparently completely submerged by this polar effect.