Unified Synthesis of Right Halves of Halichondrins A-C

The right halves of halichondrins A-C were synthesized by coupling the common C20-C37 building block 9 with the C1-C19 building blocks 10a-c, respectively. Catalytic, asymmetric Ni/Cr-mediated coupling was used for three C-C bond formations. For all cases, the stereochemistry was controlled with the Cr catalyst prepared from the chiral sulfonamide identified via the toolbox approach. For (3 + 4)-, (6 + 7)-, and (9 + 10)-couplings, the stereoselectivity of 28:1, >40:1, and ∼20:1 was achieved by the Cr catalysts prepared from (S)-H, (S)-I, and (R)-L, respectively. Unlike the first and second couplings, the third coupling used the structurally complex nucleophile. It was demonstrated that the coupling efficiency was excellent even with the electrophile/nucleophile molar ratio = 1.0/1.1. In addition, the third coupling was achieved with the substrate bearing a free hydroxyl group. The products obtained in the Ni/Cr-mediated couplings were converted to the right halves of halichondrins A-C in excellent overall yields. The right halves of halichondrins A-C (1a-c) were synthesized in 28, 24, and 24 steps from commercial d-galactal in 13.4%, 21.1%, and 16.7% overall yield, respectively.