But-I -enyl(methyl)maleic anhydride (4) has been synthesised and shown to act as an efficient (55% incorporation) precursor of the cyclononadienes, glauconic and glaucanic acids, in Penicillium purpurogenum. Base-catalysed dimerisation of compound (4) yields 5% of an epimer of glaucanic acid.IN Part V evidence, based on feeding of labelled acetate, succinate, glucose, and n-butyl(methy1)maleic anhydride, was presented supporting the hypothesis that a key step in the biosynthesis of glaucanic acid (1) was the dimerisation of a C, unit and that glaucanic acid was further oxidised to glauconic acid (2). The anhydride (4) appeared to be the most likely C, unit which dimerises to (1) since it is of the correct oxidation level and functionality.The synthesis of the anhydride (4) was based on our findings that a-keto-esters condense with cc-phosphonato-ester anions in a Wadsworth-Emmons reaction4 to give high yields of maleic esters with little or none of the corresponding fumarates. The required keto-ester (5) was synthesised from ethyl 2-oxohexanoate, which, was first converted into the enol acetate
This is a general reaction of y-bromo-a /3-unsaturated esters.The general reaction of a-keto-esters with the phosphonate anions derived from a-bromo-esters gives maleic, but not fumaric, esters.
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