Translation of pseudo-cross-conjugation into chemistry:: Cycloadditions of mesomeric betaines to the new ring system spiro[indazole-3,3′-pyrrole]

Indazolium-3-amidates (X-ray analysis), readily available on trapping the N-heterocyclic carbene indazol-3-ylidene with isocyanates, underwent [3 + 2]-cycloadditions with activated triple bonds to spiro[indazole-3,3'-pyrroles]. A combination of NMR techniques such as heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond correlation (HMBC), nuclear Overhauser enhancement spectroscopy (NOESY), and H-1/N-15 correlations were applied to elucidate the structures of the cycloadducts.