Antineoplastic Agents. 561. Total Synthesis of Respirantin1a

Total synthesis of the 18-membered ring cyclodepsipeptide believed to be respirantin (1b) has been achieved. The key step in the synthesis is an intramolecular transesterification of the beta-ketoester alcohol 6 to afford the protected macrocycle 5. The synthetic product was shown to be identical to a natural product presumed to be respirantin (1b), and the absolute stereochemistry of six of the seven asymmetric centers of cyclodepsipeptide 1b was unequivocally established. Respirantin (1b) was found to be a remarkable inhibitor of cancer cell growth and related to the antimycin family of antibiotics.