The cytotoxic metabolites of pulmonate of the family of Onchidacea, a shelless marine mollusc phylum, were extensively investigated by some research groups. Their biosynthetic origin and stereochemical relationships have attracted some interest. The γ-pyrones are one of major class of those metabolites. Ilikonapyrone (1), peroniatriols (2, 3), and onchitriols (4, 5), are new members of the class; polyhydroxylated linear polypropionates with two fully substituted γ-pyrone rings. The structures of this family were in general, proposed by comparison with that of ilikonapyrone (1), which had been determined by an X-ray crystallographic analysis, but ambiguities on relative stereochemistries of asymmetric centers existing across γ-pyrone or trisubstituted olefin, could not be avoided. In order to confirm these points, we describe herein the effective cyclization of triketides under DMSO-(COCl)_2 or Ph_3P - CCl_4 conditions to the corresponding γ-pyrones without any serious epimerization and/or elimination of adjacent stereogenic centers, which must establish the absolute stereochemistries of peroniatriols and ilikonapyrone by synthesis of their degradation products. We also report the first total synthesis of onchitriol II (5) and some of its diastereoisomers, using the new synthetic methodology for γ-pyrone developed by us. This synthesis allowed the complete assignment of the stereochemistry for onchitriol II (5), and the structural revision of onchitriol I (1).
