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 Professor Albert Eschenmoser on the occasion of his 75th birthday The cycloadducts of isoquinolinium N-phenyl imide 2 with CC bonds are derivatives of 2-amino-1,2-dihydroisoquinoline. Their N b -vinylphenylhydrazine system is amenable to an acid-catalyzed [3,3]-sigmatropic shift; the formation of pentacyclic aminals is exemplified by 6 3 8. The dimethyl maleate adduct 11, C 21 H 20 N 2 O 4 , is exceptional by being converted on treatment with acid to bright-yellow crystals, C 24 H 22 N 2 O 6 (additional C 3 H 2 O 2 ). X-Ray crystal-structure analysis and NMR spectra reveal structure 13, and mechanistic studies indicated an initial b-elimination at the NÀN bond of 11 to yield 18; this step is followed by a retro-Mannichtype cleavage that gives methyl isoquinoline-1-acetate (14) and methyl 2-(phenylimino)acetate (15), according to the sequence C 21 H 20 N 2 O 4 (11) 3 18 3 C 12 H 11 NO 2 (14) C 9 H 9 NO 2 (15). In the second act of the drama, electrophilic attack by 15-H on the ene-hydrazine group of a second molecule of 11 furnishes 13 by a polystep intramolecular redox reaction. All rate constants must be fine-tuned in this reaction cascade to give 13 in yields of up to 78% with an overall stoichiometry: 2 C 21 H 20 N 2 O 4 (11) 3 C 24 H 22 N 2 O 6 (13) C 12 H 11 NO 2 (14) aniline. Interception and model experiments confirmed the above pathway. A by-product, C 33 H 31 N 3 O 6 (62), arises from an acid-catalyzed dimerization of 11 and subsequent elimination of 15.

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Das Kohlenstoffgerüst der Titelverbindung (VII) wird über eine Diels‐Alder‐Reaktion des durch Photoenolisierung aus (II) erzeugten Dienols mit Dimethylfumarat aufgebaut.