Steric and electronic requirements of amide and ester groups in benzylidenemalonates

Stereoisomeric benzylidinemalonates have been synthesised. In the case of N-(2-ethoxycarbonyl-3-phenylpropenoyl)-2-oxazolidinone, both E- and Z-isomers could be obtained, but the Z-isomer isomerised >95%) to the E-isomer in the presence of catalytic amounts of Rh(I) complex catalysts. For ethyl tertbutylamidobenzalmalonate, the Z-isomer initially predominates under Knoevenagel conditions, but is very readily converted into the more stable E-isomer, which is here isolated stereoisomerically pure and is the only product of other synthetic routes. The configuration of benzylidinemalonates was established in two cases by X-ray crystallography, and the relative stability of the isomeric forms correctly simulated by MM2 calculations. In both the amide and oxazolidinone series, asymmetric hydrogenation catalysed by cationic Rh complexes gave a configurationally stable product cleanly but in low e.e..