Synthesis of cytochalasans using intramolecular Diels-Alder reactions: an alternative approach to cytochalasin D

The macrocycle 25 which has the required functionality around the macrocycl ic ring for incorporation into a synthesis of cytochalasin D 1 has been syn thesized using an intramolecular Diels-Alder reaction to form the 11-member ed ring. The Diels-Alder precursor 24 was prepared in a convergent fashion from the dienyl phosphonate 17, the aldehyde 16 and the pyrrolidinone 21, w ith phenylselenenylation and oxidative elimination being used to convert th e pyrrolidinone 22 into the unstable pyrrolinone 24. The Diels-Alder reacti on of the pyrrolinone 24 under high dilution conditions gave the required e ndo-adduct 25 in a yield of 53% based on the phenylselenopyrrolidinone 23. N-Debenzoylation gave the NH-lactam 26 but preliminary attempts to effect r emoval of the SEM-groups led to the formation of the methylenedioxy compoun d 27.