摘要：Ethyl esters of N-isopropylidene-, N-benzylidene-, and N-p-nitrobenzylidene-D-methionine (II, I, and III) were prepared. The relative velocities of racemization of the Schiff bases were measured resulting in the order of (II)«(I)«(III). It has been postulated by Holland that prototropy in Schiff bases derived from α-amino acid esters occurs through tautomerism in the >C*H-N=C< structure. The relative velocities found experimentally were in reverse of the theoretical assumption that they would be (II) > (I) > (III) by the electronic effect of the N-substituent, if the said postulation were correct. The findings support the view that the prototropic change occurs originally in the >C*H-C=O structure and is additionally helped by the azomethine structure in the molecule.
