Steric effects in the direct iodination and 1,3 dipolar cycloaddition of sydnones

The sydnones (4), (7) and (11) gave the corresponding pyrazoles (5), (8) an d (12) by 1,3-dipolar cycle addition with dimethyl acetylenedicarboxylate(D MAD). The highly sterically hindered 3-(6-bromo-2,4-dimethylphenyl)-4-iodos ydnone (9) failed to react with DMAD on heating for 3 days in boiling xylen e. The iodination of (7) required large excess of iodine monochloride and 8 hrs. reaction time, whereas iodination of (3) was complete in 20 minutes w ith an equivalent amount of iodination reagent. This result was explained b y steric hindering at the electrophilic center (C-4 in the sydnone ring).