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 1H/15N correlations were applied to elucidate
the structures of the cycloadducts.