Arylidene-enamino acid compounds

Grigg and J. Kemp 148) prepared benzylidene compounds of dehydrovaline ester, dehydroleucine ester, dehydroisoleucine ester and dehydrophenylalanine ester by dehydrogenation of the corresponding saturated benzylidene compounds with diethyl azodicarboxylate. This dehydrogenation is assumed to be an ene reaction . 

Schmidt and coworkers 287, 352) condensed dehydrovaline ester with aromatic aldehydes and pyridoxal to yield Schifif bases directly, which were employed in biomimetic experiments (VI. A. 2.1). 

The simplest and most satisfactory method of preparing arylidene-enamino acid esters is the P-elimination from 2-aryl-4-thiazolidinecarbo-xylic acid esters (40) on treatment with silver carbonate 287A). Arylidene-enamino acid N-hydroxysuccinimide esters were prepared and combined with amino acid esters to yield Schiff bases of dehydro-dipeptides. This elimination reaction of thiazolidine carboxylic acid esters has been applied to prepare the extremely unstable benzyUdene dehydroalanine ester, although it was impossible to isolate it. Nevertheless its existence could be proved without a doubt when the reaction was carried out at 0° in trideuterioacetonitrile solution and was followed by NMR spectrometry. Within one half hour at room temperature the unstable substance started to decompose. 

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