Enantioselective total synthesis and absolute stereostructure of hippospongic acid A

A compound having the structure proposed for hippospongic acid A, a triterpene that specifically inhibits gastrulation of starfish embryos, was synthesized enantioselectively. The synthetic compound was not identical to the natural product. Comparison of the NMR spectra of the natural and synthetic compounds led us to propose an alternative structure, which was confirmed by enantioselective synthesis. The present synthesis established that the natural product has the (R)-configuration.