ChemInform Abstract: Model Studies for a Ring‐Closing Metathesis Approach to the Bafilomycin Macrolactone Core from a 2,2‐Dimethoxy Tetraenic Ester Precursor.

Abstract

A ring-closing metathesis strategy is reported for the construction of the 16-membered macrolactone core of the bafilomycins. One decisive key feature is the presence of a 2,2-dimethoxyketal functionality at C-2 that provides the required flexibility to the tetraenic ester precursor, allowing the ring-closing metathesis reaction to take place. Three different model esters of increasing complexity were successfully subjected to the 1,3-diene-ene ring-closing metathesis reaction. The best promoter for the simplest esters was the Grubbs first-generation precatalyst. A Hoveyda–Grubbs-type trifluoromethylamido-containing precatalyst developed by Mauduit's group gave satisfactory results for the most complex ester. In all experiments, the 12-Z-configured isomer was obtained as the major product. Subsequent microwave-promoted methanol elimination was achieved on the simplest model compound using camphorsulfonic acid (CSA) as a catalyst. Under these conditions, a E to Z isomerization of the double bond at C-4, as well as ca. 50 % isomerization of the 12-Z double bond into the corresponding 12-E isomer, were observed.