SYNTHESIS OF FUROFURANS BY REARRANGEMENT OF 4-ACETYLPYRANS

The hetero-Diels-Alder reaction of 1-oxabutadienes 1a-c bearing electron-withdrawing groups with ethyl vinyl ether and 2,3-dihydrofuran gave the functionalized 5,6-dihydro-4H-pyrans 2a-c, 4c and 4H-4a,5,6,6a-tetrahydrofuro[2,3-b]pyrans 5a-c and 6c. Cycloadducts 2a and 4c easily rearranged to furo[2,3-b]furans 3a, 3c and 9c and cycloadducts 5a and 6c to difuro[2,3-b:3',4'-d] furans 7a, 7c and 8c. The stereochemistry of compounds 5c and 7a was determined by single crystal X-ray analysis. Relative configurations of the other compounds were established by nOe data. Some aspects of the reaction mechanisms are discussed.