Synthesis and Absolute Configuration of (−)-Phytocassane D, a Diterpene Phytoalexin Isolated from the Rice Plant,Oryza sativa

To determine the absolute configuration of the phytocassane group of phytoalexins, the naturally occurring (−)-phytocassane D was synthesized from the (R)-Wieland−Miescher ketone, in an approach based on the preliminary model synthesis of the unnatural (+)-2-deoxyphytocassane A. By comparison of their CD spectra with those of synthetic phytocassanes of known absolute configuration, phytocassanes A−E were shown to possess the ent-cassane skeleton.