Sydnones with acetylenic esters

The reactions of sydnones with acetylenic esters are generally thermally induced concerted processes which are allowed on orbital symmetry considerations.8 On the basis of Huisgen s mechanistic study,805 the conversion of 71 into 73 proceeds through the formation of 72 in a slow rate-determining step, followed by rapid loss of carbon dioxide, giving the pyrazole 73. 

Sydnones behave as 1,3-dipolar systems and undergo addition reactions with various dipolarophiles. Huisgen, Grashey, Gotthardt, and Schmidt503,504 were the first to react acetylenic esters with sydnones and obtained pyrazoles e.g., 71 with DMAD (1 hour, 120°, in xylene) gave 99% of the pyrazole 73. These reactions have also been carried out with propiolic and phenylpropiolic esters, and their kinetics have been studied.505... 

Almost simultaneously, papers by Schmid et al.509 and by Gotthardt and Reiter510 appeared on the photoaddition of DMAD to sydnones the last authors also used propiolic and phenylpropiolic esters. Ohta et al.511 showed that similar reactions could be carried out with 3-alkyl-4-phenylsydnones, and other papers on the addition of acetylenic esters to sydnones have appeared.512-514... 

In the reaction of the same sydnone with an acetylenic dipolarophile, slightly more dipole-HO control can be expected, and this has been observed. Propiolic ester still gives mainly the 3-substituted pyrazole 6.339, but also a substantial amount of the 4-substituted pyrazole 6.340.861... 

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