Dimethyl acetylenedicarboxylate sydnones

Laser flash photolysis (LFP) studies have been carried out on the generation of N,C-diaryl nitrile imines from sydnones and from tetrazoles in solution at 77 K. They were found to have hfetimes of milliseconds and were quenched by dimethyl acetylenedicarboxylate (DMAD) (A q=5-9 x 10 M s ) and by carboxyhc acids (feq= lO -lO s ) (13). The strong dependence of v ax on the nature of the aromatic substituents in N,C-diaryl nitrile imines was interpreted by a hnear free energy relationship as due to intramolecular charge transfer (14). 

Oxadiazoles. - 1,2,3-Oxadiazoles. Irradiation of the sydnone (466) affords the betaine iV-oxide (467) in low yield " the reaction with dimethyl acetylenedicarboxylate gives a mixture of the pyrrole (468) and the oxazinone (469) both originate from the intermediate cyclo-adduct... 

Miscellaneous. 1,3-Diazetidines (147) have been obtained by the thermal rearrangement of the bicyclic ketone (148). The ylide (149) is thought to be a reaction intermediate since it can be trapped by the addition of dimethyl acetylenedicarboxylate. Nitrile imines were shown to be intermediates in the photolysis of sydnones (150), which in one case (150 R = Bu ) gave diazetidinones. 

PROBLEM 23.39 Reaction of A -phenylalanine (1) with nitrous acid affords A -nitroso amino acid (2), which on treatment with acetic anhydride yields sydnone (3). Sydnones belong to a class of compounds known as mesoionic compounds. These compounds cannot be satisfactorily represented by Lewis structures not involving charge separation. The name sydnone derives from the University of Sydney where the first examples were prepared in 1935. Reaction of 3 and dimethyl acetylenedicarboxylate (DMAD) gives pyrazole (4). Propose mechanisms for the formations of 2, 3, and 4. Hint. For the formation of 4, it may be helpful to consider the other possible Lewis structures for sydnone 3. 

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See also in sourсe #XX -- [ , , ]

See also in sourсe #XX -- [ , , ]

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