Diastereoselective ring-expansion reaction of methanochromanone with aldehydes: Formation of trans-fused tetrahydrofuro-[2,3-b][1]benzo-pyranones andtheir isomerization

In the presence of SnCl4, 2,3-dimethoxycarbonylmethanochromanone (4) was tr ansformed into a zwitter-ion which easily reacted with aldehydes to give th e trans-fused tetrahydrofuro[2,3-b] [1]benzopyranones in good yields with h igh diastereoselectivity. cis-l;used furobenzopyranone derivatives were als o obtained in good yields by isomerization of the trans isomers.