Recent Developments in Total Syntheses of Cephalosporolides, Penisporolides, and Ascospiroketals.

The possible biogenetic connection of the 10-membered lactone (decanolide) and the tricyclic [5,5]-spiroacetal-cis-fused-γ-lactone (SAFL) natural products was proposed by Hanson in 1985. Mimicking such biosynthetic hypothesis led us to develop a general synthetic strategy for the total syntheses of both decanolide-type cephalosporolides and SAFL-type natural products. This biomimetic strategy features two key transformations: i) oxidative ring expansion of bicyclic β-hydroxy tetrahydropyrans to the decanolides and ii) ring contraction rearrangement of the decanolides to the tricyclic SAFLs. In particular, the phenol derivatives are exploited as the starting materials for the decanolides and then the SAFLs. The successful biomimetic total synthesis allows us to revise the structures and biosynthetic hypothesis of several natural products, along with development of an NMR analysis method for determination of the relative stereochemistry of SAFL-type compounds. In addition, this account will summarize recent synthetic work by other research groups.