Abstract
The methyl ester of the pentacyclic nucleus of ptilomycalin A (9) has been prepared by an efficient, convergent, biogenetic, 14-step route. The key steps involve the conversion of acyclic bis enone 39 to 9 in four steps. Michael addition of 0-methylisourea to 39 afforded 52% of a mixture of isoureas 40 and 41, which were both converted to 72% of tricyclic aminals 42 and 43 by ammonolysis. Deprotection of the silyl ethers with HF and cyclization with EtjN in MeOH afforded 9 («34% from 42) and the diastereomer 45 with an equatorial methyl ester group («26% from 42).