Studies in Stereochemistry. XII. Mechanism of Racemization in Schiff Bases of D-Methionine Ethyl Ester.

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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.
公益社団法人日本薬学会の論文
1957-04-20