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The sydnones 7a,b and 8a-c gave the corresponding pyrazoles 9a-e by 1,3-dipolar cycloaddition with dimethyl acetylenedicarboxylate (DMAD).The highly sterically hindered 3-(4,6-dibromo-2-methylphenyl)-4-iodosydnone (8d) failed to react with DMAD on heating in boiling xylene.The iodination of sterically hindered sydnone 7b required more drastic reaction conditions than the sydnone 7a.
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Dumitraşcu et al. (2002) studied this question.
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