Sydnones, tautomerism

For the type A meso-ionic heterocycles the valence tautomerism 19 45 is now considered. Examples include the possibility that the sydnones (1) could be associated with a valence tautomerism involving the nitrosoketene isomer (47). Similarly, the relation between the isosydnones (48) and their acylaminoisocyanate isomers (49) have been... 

In addition, ab initio calculations for the unknown imsubstituted sydnone molecule (1, R = R = H) and the tautomeric species 2H-l,2,3-oxadiazol-5-one (442) and 5-hydroxy-1,2,3-oxadiazoIe (443) have been reported. The meso-ionic tautomer (1) is calculated to be less stable... 

This is exactly what has been observed for the cycloaddition reactions of sydnones 6.336 with all three kinds of dipolarophile. An intermediate is produced in the first instance with the general structure 6.337 this loses carbon dioxide in a retro 1,3-dipolar cycloaddition, followed by tautomerism giving the 3-substituted pyrazolines 6.338 as the major products.860... 

---