Reductive cleavage of 2-alkoxy- and 2-aryloxy-tetrahydropyrans with LiAlH₄–AlCl₃

Abstract

A series of 2-alkoxy- and 2-aryloxy-tetrahydropyrans has been hydrogenolyzed in ether by LiAlH 4 –AlCl 3 . As the alkyl group attached to the exo oxygen atom is changed from primary to tertiary, the proportion of ring cleavage to side-chain cleavage increases. Only side-chain cleavage occurs with the 2-aryloxytetrahydropyrans. The results are explained in terms of the polar properties of the alkyl or aryl group attached to the ring oxygen or the side-chain oxygen. Any control by steric factors is considered to be submerged by the polar effects, but does have a clearly apparent directive effect when the polar factors are equal for the two routes of cleavage. The results of the hydrogenolysis of 6-substituted 2-alkoxytetrahydropyrans could be explained on the same basis.The configurations of the cis and trans isomers of the disubstituted tetrahydropyrans were determined by nuclear magnetic resonance spectroscopy. Their conformational preference is discussed.