The astounding chemistry of a 2-amino-1,2-dihydroisoquinoline derivative

The cycloadducts of isoquinolinium N-phenyl imide 2 with C=C bonds are deri vatives of 2-amino-1,2-dihydroisoquinoline. Their N-beta-vinylphenylhydrazi ne system is amenable to an acid-catalyzed [3,3]-sigmatropic shift; the for mation of pentacyclic aminals is exemplified by 6 --> 8. The dimethyl malea te adduct 11 C21H20N2O4, is exceptional by being converted on treatment wit h acid to bright-yellow crystals, C24H22N2O6 (additional C3H2O2). X-Ray cry stal-structure analysis and NMR spectra reveal structure 13, and mechanisti c studies indicated an initial beta-elimination at the N-N bond of 11 to yi eld 18; this step is followed by a retro-Mannich-type cleavage that gives m ethyl isoquinoline-l-acetate (14) and methyl 2-(phenylimino)acetate (15), a ccording to the sequence C21H20N2O4 (11)-->18-->C12H11NO2 (14) + C9H9NO2 (1 5) 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 intr amolecular redox reaction. All rate constants must be fine-tuned in this re action cascade to give 13 in yields of up to 78% with an overall stoichiome try: 2 C21H20N2O4 (11) --> C24H22N2O6 (13) + C12H11NO2 (14) + aniline. Inte rception and model experiments confirmed the above pathway A by-product, C3 3H31N3O6 (62), arises from an acid-catalyzed dimerization of 11 and subsequ ent elimination of 15.