Total syntheses of the 3 S, 8 S, 10 S, 19 R, 43 S isomer 4a and the 3 S, 8 S, 10 S, 19 R, 43 R isomer 4b of the unique crambescidin alkaloid crambidine are reported. These studies confirm the tetracyclic structure proposed by Braekman and co-workers for crambidine, and establish the rel-3 R, 8 R, 10 R, 19 S relative configuration for this moiety. Natural crambidine is most likely the 3 S, 8 S, 10 S, 19 R, 43 S isomer 4a. These syntheses were ...