Total synthesis of antheliolide A.

The first synthesis of the structurally unique marine natural product antheliolide A (1) has been accomplished by the pathway outlined in Scheme 1. The sequence is stereocontrolled and has led to the synthesis of (+/-)-1, and ent-1 as well as 1. The route contains a number of noteworthy or novel steps including (1) formation of the mixed acetal 7, (2) the diastereoselective bicyclization to form 8 in which stereocenters are correctly established at each carbon of the four-membered ring, (3) the chain extension 8 --> 11, (4) the efficient closure of the nine-membered ring of 12, (5) the mild oxidative cleavage sequence 14 --> 17, and (6) the successful and quick formation of the last three rings of 1 from aldehyde 17 via 18. The synthesis of 1 has also resulted in the clarification of its absolute configuration, which had not been determined previously.