Bis-esters from maleic anhydrides under neutral conditions: Protection of the anhydride of the natural product cornexistin

Trimethylsilyldiazomethane converts maleic anhydride derivatives in alcoholic THF to bis-esters. The highly acid and base sensitive natural product comexistin was converted to its bis-methyl ester in 70-75% yield and to the mixed benzyl/methyl ester in 61% yield. The mixed ester can be converted back to comexistin via transfer hydrogenation.