INTERMOLECULAR AND INTRAMOLECULAR HETERO-DIELS-ALDER REACTIONS .36. SYNTHESIS OF DIHYDROPYRANS BY HETERO-DIELS-ALDER REACTION OF ENAMINONES - AN EFFICIENT ROUTE TO 3-AMINO SUGAR-DERIVATIVES

The hetero Diels-Alder reaction of the N-monoacyl- and N,N-diacyl-enamino ketones 4a-g containing an electron-with-drawing group at C-2 of the oxadiene moiety with the electron-rich dienophiles 5a-d affords the diastereomeric adducts 6a-i and 7a-i in 78-98% yield with the cis adducts 7a-i as the main products. The highest selectivity is obtained with the N,N-phthalimido-protected enamino ketones 4d-g.