Cu(I)-catalyzed [3+2] Cycloadditions of tert-Butyl (S)-(3-Oxopent-4- yn-2-yl)carbamate to 1-Benzylidenepyrazole-3-one-derived Azomethine Imines

Abstract

Parallel screening of suitable reaction conditions for Cu(I)-catalyzed [3+2] cycloadditions of (1Z,4R*,5R*)-4-benzoylamino-1-benzylidene-5-phenyl-3-oxopyrazolidin-1-ium-2-ide (1a) to methyl propiolate (2) has established that this reaction proceeds smoothly at room temperature in acetonitrile in the presence of CuI and Hünig’s base. The optimized reaction conditions were then applied in regio- and stereo-selective 1,3-dipolar cycloadditions of racemic azomethine imines 1a-e to tert-butyl (S)-(3-oxopent-4-yn-2-yl)carbamate (6) leading to mixtures of diastereomeric non-racemic chromatographically separable cycloadducts 7a-d, 7'a-d, 8e, and 8'e. The structures of the products were confirmed by NMR spectroscopy.