Diastereoselective access to hexahydro- and octahydrofuro[ f]indolizines analogues of phenanthro[ f]indolizidines alkaloids

Graphic Enantiospecific syntheses of furo[ f]indolizines with different degrees of unstauration ( 4a, b, 8a, 9a, and 10a) as analogues of phenanthro[ f]indolizidine alkaloids have been completed from ( S)-glutamic acid in a few-step sequence. This was achieved from the known optically active alcohol-lactams 2a and 2b by utilizing chemical and catalytic reduction processes. During these transformations, we have shown that partial furan ring reduction can be achieved conveniently. The resulting products 5a and 8a readily constituted platforms to access stereoselectively the partially 6a and 7a or totally 9a and 9b reduced furo[ f]indolizines. The key step of the stereocontrolled reduction seems to be the catalytic hydrogenation of the furan nucleus in the (4a S,9a S)-(+)- 5a product. Assignments of the absolute stereochemistry are made and some mechanistic aspects of these transformations also discussed.