Synthesis of (E)-Enaminoesters by a 1,6-Addition/Fragmentation Cascade Involving Coumalic Acid Esters and Secondary Amines

Coumalic acid methyl ester (methyl coumalate), a biomass-derived building block, was converted into (E)-enaminoesters by an original uncatalyzed stereoselective 1,6-addition/fragmentation cascade involving secondary amines. The transformation occurred under useful experimental conditions as a simple heating of a stoichiometric mixture of the starting compounds led to the desired products. The reaction could be extended to a range of alkyl coumalates and a variety of secondary amines. Given the original character of the transformation, the reaction mechanism was discussed. Therefore, the cascade is supposed to involve a 1,6-conjugate addition followed by a fragmentation sequence leading ultimately to the decarboxylative elimination of (E)-enaminoesters. Using this procedure, renewable coumalic acid proved to be a valuable alternative to fossil fuel-based propiolates for the preparation of enaminoesters.