RITTER REACTIONS .12. REAPPRAISAL OF THE REACTIVITY OF METHYL SCHIFF-BASES WITH DIMETHYL ACETYLENEDICARBOXYLATE

The reaction of the methyl Schiff base compound 2 with dimethyl acetyl enedicarboxylate (DMAD) has been reinvestigated in order to determine whether it is initiated through the imine form 2 or the enamine tautom er 3. The structure of the reported 1:1 adduct is reassigned as 6 foll owing elucidation by a combination of NMR techniques including natural abundance gradient enhanced H-1-N-15 HMBC and 1D HSQC, as well as H-1 , C-13, DEPT, DQF-COSY, NOESY, H-1-C-13 HMQC and HMBC. Similar experim ents, plus C-13-C-13 INADEQUATE data, for the 1:1 adduct of imine 10 a nd DMAD require its reassignment as structure 15. (NH)-N-3J values wer e used as an indicator of the likely geometries of these products. In both cases, the products are the eventual outcome of initial attack on DMAD by the nitrogen of the imine form. (C) 1997 Elsevier Science Ltd .