Total synthesis of citreomontanin, a putative polyene precursor to citreoviridin and citreoviridinol produced by Penicillium sp.

A total synthesis of all-E-citreomontanin, produced by Pencillium pedemontanum and believed to be the putative precursor of citreoviridin and citreoviridinol, involving successive epoxidations of its terminal triene portion, is described. Elaboration of the pyrone 9a to the phosphonium salt 13b was smoothly accomplished via the key pyrone aldehyde intermediates 10 and 12. The all-E-trienal 7 required for Wittig coupling with compound 13b was readily produced from the aldehyde 14 by use of two successive stereoselective Wittig olefination reactions involving the stabilised phosphorane 15; i.e., 14+15→16, and 17+15→18. A Wittig coupling reaction between the ylide 8 derived from 13b and the trienal 7 then produced the all-E-polyene 1, as an orange solid, which was identical with citreomontanin isolated from P. pedemontanum.