Stereoselective synthesis of tetrahydropyran new derivatives underlain by α-allyl ketones

Abstract

Decarboxylation of α-allyl-substituted acetoacetic esters afforded α-allyl ketones that were reduced with L-selectride [LiBH(s-Bu)₃] in alcohols RCH(OH)CH₂CH₂CH=C(Me)CH₂R'. The latter reacted with methyl 4-hydroxy-3-formylbenzoate and methyl orthoformate in the presence of p-toluenesulfonic acid to provide trans-tetrahydropyrano[3,2-с][1]benzopyran. In reaction of the E-isomer of alcohol Me₂CHCH(OH) CH₂CH₂CH=C(Me)CH₂CH₂Ph with CF₃SO₃H a stereoselective cyclization occurred with the formation of 2,6-disubstituted tetrahydropyran; Prins reaction with 4-bromobenzaldehyde and salicylaldehyde in the presence of boron trifluoride etherate also proceeded stereoselectively giving a substituted tetrahydropyrano-[3,2-c][1]-benzopyran.