Cu(II)-Catalyzed Aminocyclization of -Propargyl Hydrazones to Substituted Pyrazolines.

An efficient route for the copper(II)-catalyzed synthesis of substituted pyrazolines from readily accessible -propargyl hydrazones has been reported under open flask conditions via intramolecular C-N bond formation. -acyl and -tosyl-substituted pyrazolines have been prepared in moderate to excellent yields. Mechanistic investigations using NMR, high-resolution mass spectrometry (HRMS), and Hammett analyses suggest that the Cu(II) catalyst generally acts as a Lewis acid to form an iminium-ion intermediate via cyclization, which afforded the desired pyrazolines upon hydrolysis. One progesterone receptor antagonist has also been synthesized utilizing this reaction methodology.