IsoquinoliniumN-Arylimides and Electron-Deficient Ethylene Derivatives

Abstract

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-a]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 triphenylvinylphosphonium bromide by alkaline cleavage. The statistical analysis of the δH values of 39 cycloadducts provided a consistent set of substituent increments for the pyrazolidine protons.