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The red isoquinolinium N‐arylimides 19−23 are azomethine imines of which the C=N bond is part of an aromatic ring. The N‐(4‐nitrophenyl)imide 22 and the N‐(2‐pyridyl)imide 23 were obtained crystalline; in solution the latter equilibrates with the hexahydrotetrazine 24 as its dimer. The N‐phenylimide 19 is not stable; an isolated solid appears to be a tetramer. Generated by deprotonation of 11−13, the N‐arylimides 19−21 undergo in situ cycloadditions to carbon disulfide, phenyl isocyanate, phenyl isothiocyanate, and diphenylketene. The storable CS2 adduct 29 offers a neutral source of theN‐phenylimide 19, since a cycloreversion equilibrium is established in solution.

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Dedicated to Paul von R. Schleyer on the occasion of his 70th birthday The thermal conversion of 4-isoxazolines to 4-oxazolines involves the transposition of two ring members. The ring-contraction and ring-expansion sequence in the reaction 2 3 5 has been previously clarified. The low NÀN bond energy should favor an analogous conversion of 3-pyrazolines 6 to 4-imidazolines 7; the first example of such a transformation is reported here. In the yellow 16, the 3-pyrazoline is part of a pyrazolo-[5,1-a]isoquinoline system. Daylight induces a ring contraction, which affords the 2-isoquinolylaziridine derivative 21. The latter is converted at 658 to the tricyclic 4-imidazoline 26 by a sequence of electrocyclic aziridine ring-opening and 1,5-electrocyclization of a CN-conjugated azomethine ylide 25.