Isoquinolinium N-arylimides and electron-deficient ethylene derivatives

Article abstract of DOI:10.1002/(SICI)1099-0690(199802)1998:2<387::AID-EJOC387>3.0.CO;2-J

The 1,3-cycloadditions of isoquinolinium N-phenylimide (2a) and N-(2-pyridyl)imide (2b) to twelve α,β-unsaturated carboxylic esters and nitriles proceeded at room temp, with high yields; the reactions furnished tetrahydropyrazolo[5,1-α]isoquinoline derivatives and could be visually followed by the loss of the red color, in this class of azomethine imines, the imide nitrogen of 2 is the nucleophilic center which determines the regiochemistry of the additions to methyl acrylate, acrylonitrile, and their α-methyl and α-chloro derivatives. The diastereoselectivity is low; pairs of adducts were also formed with dimethyl fumarate and maleate. The configurations were elucidated by ¹H NMR analysis, which likewise provided the clue to the favored conformation of the tricyclic system. The N-arylimides 5 do not react with ethylene, but the formal ethylene adducts were accessible from the cycloadducts of 5a,b to triphenylvmylphosphoniurn bromide by alkaline cleavage. The statistical analysis of the δ_H values of 39 cycloadducts provided a consistent set of substituent increments for the pyrazolidine protons.