Studies on intramolecular alkylation. III. The preparation of γ-lactams from podocarpic acid: Models for diterpene alkaloid synthesis

Several methods for the introduction of a 6β,16 nitrogen bridge in podocarpane derivatives are described. Attempted intramolecular N-alkylation in the 6α-bromo amide (9) gave ketol (13) via imino ether (10). Attempted lactam formation from the 6-oxo nitrile (22) or 6-oxo ester (24) gave no useful result, but the diosphenol(15) gave lactam (16), which was reduced to the target lactam (28). A further route to (28) via the 6-ene derivative of amide (7) is also described. The lack of identity between (28) and the γ-lactam formed from photolysis of O-methylpodocarpoyl azide indicated that this lactam has the 2β,19 structure (31).