Cladospolide D

A new antibiotic termed cladospolide D was isolated along with the known cladospolides A and B from the fermentation broth of Cladosporium sp. FT-0012 by solvent extraction, ODS column chromatography and preparative HPLC. The structure of cladospolide D was deduced to be (E)-2-dodecen-5-hydroxy-11-olide-4-one. Cladospolide D showed antifungal activity against Pyricularia oryzae and Mucor racemosus.

An eleven-step synthesis of the title compound (1) from biocatalytically-derived and enantiomerically pure 'building blocks' alcohol (R)-(-)-9 and ester 13 is described. Attempts to construct the twelve-membered lactone ring of cladospolide A in a direct manner by using a ring-closing metathesis (RCM) reaction failed. However, a ten-membered lactone 19, could be constructed by such means and this was then subject to a two-carbon homologation sequence involving, inter alia, Wadsworth-Horner-Emmons and Yamaguchi lactonisation reactions in the closing stages of the synthesis. The impact of substituent stereochemistries and protecting groups on the RCM reaction leading to various ten-membered lactones is also described.

The enantioselective synthesis of cladospolide B, C, and (ent)-cladospolide D has been achieved in 11-15 steps from 1-nonyne. The route relies upon an alkyne zipper reaction to relay an ynone and dienoate functional groups across a nine carbon fragment, which enables a highly enantioselective Noyori ynone reduction and a diastereo- and regioselective Sharpless dihydroxylation of a dienoate. In addition to being a flexible approach to three members of the cladospolide natural products, this route for the first time correctly established the structure for cladospolide D.