A facile access to natural and unnatural dialkylsubstituted maleic anhydrides

A facile new route to the potential building blocks 2-bromomethyl-3-alkylmaleic anhydrides 15a/b for the synthesis of natural and unnatural dialkylsubstituted maleic anhydrides has been demonstrated, starting from dimethyl citraconate (9) via NBS-bromination, S_N2′ Grignard coupling reactions, hydrolysis, molecular bromine addition and dehydrative ring closure reactions pathway with 49-51% overall yield in 5-steps. Chemoselective allylic substitution of bromoatom in 15a/b with Grignard reagents has been described to obtain the unsymmetrical maleic anhydride 16 and symmetrically dialkylsubstituted maleic anhydrides 25a/b in 55% yield. The naturally occurring 2-carboxymethyl-3-hexylmaleic anhydride (1) has been synthesized from 16 via esterification, ozonolysis and an oxidation route. The synthesis of two naturally occurring 2-(β-carboxyethyl)-3-alkylmaleic anhydrides 2a/b have been completed via a chemoselective diethylmalonate coupling reaction followed by acid induced hydrolysis. In our hands the S_N2 or S_N2′ coupling of Grignard reagent with 21 to obtain 1 and Reformatsky reaction with 15a/b to obtain 2c/d met with failure.