The Elusive Paal Knorr Intermediates in the Trofimov Synthesis of Pyrroles: Experimental and Theoretical Studies

摘要

We have used isoxazolo [3,4-b]pyridin-3(H-1)one and isoxazolo[3,4-b]quinolin-3(H-1)-one as "masked" heterocyclic hydroxylamines to generate Paal Knorr intermediates of the Trofimov pyrrole synthesis. The previously inaccessible intermediates, trapped by ethyl propiolate, were obtained by reacting corresponding isoxazolones with 4-fold excess of ethyl propiolate under basic conditions at ambient temperature, and characterized by means of IR and NMR spectroscopic data as well as by single crystal X-ray analysis. Quantum chemical calculations of a [3,3]sigmatropic rearrangement of the N,O-divinyl hydroxylamines to corresponding imino-aldehydes (Paal-Knorr intermediates) revealed that this reaction exothermic. proceeds via chairlike transition state and is exothermic.