Total Synthesis of Halichondrin A, the Missing Member in the Halichondrin Class of Natural Products

A total synthesis of halichondrin A, the phantom member in the halichondrin class of natural products, is reported. The highlights of synthesis include: (1) synthesis of C1-C19 building block 6b via a catalytic asymmetric Cr-mediated coupling of 12 and 13b; (2) synthesis of the right-half of 19 via an asymmetric Ni/Cr-mediated coupling, followed by base-induced furan formation, and Shiina macrolactonization; (3) synthesis of enone 20 via Ni/Cr-mediated coupling of 5 with 19, followed by oxidation; (4) synthesis of halichondrin A from 20, with use of a newly discovered, highly selective TMSOTf-mediated equilibration of C38-epi-halichondrin A to halichondrin A. Two pieces of evidence are presented unambiguously to establish the structure of halichondrin A thus synthesized: one is the synthesis of norhalichondrin A (2,4) from 19 and 23, and the other is the study of the proton chemical shift difference between synthetic halichondrin A and known members of this class of natural products.