Concise syntheses of (+)-macrosphelides A and B.

[reaction: see text]. Unified and highly convergent total syntheses of (+)-macrosphelides A and B are described. Key features of the syntheses include (1) concise synthesis of the optically active delta-hydroxy-gamma-keto alpha,beta-unsaturated acid fragment via the direct addition of a trans-vinylogous ester anion equivalent to the readily available Weinreb amide and (2) facile construction of the 16-membered macrolide core of the macrosphelide series via an intramolecular nitrile-oxide cycloaddition (INOC).