3+2] Cycloadditions of N-Protected ‘(S)-Diazoproline’ with Selected Acetylenes

Abstract

Acetylene carboxylates and an acetylene phosphonate reacted with N-protected (S)-2-(diazoacetyl)pyrrolidines ((S)-diazoprolines) to give optically active bis-heterocyclic pyrazole derivatives in a regioselective [3+2] cycloaddition. The reactions occurred at 60 °C in THF solution in the absence of a catalyst. However, diethyl ethynylphosphonate reacted significantly slower than the carboxylates. The obtained products were shown to exist in CDCl3 solution at room temperature as mixtures of rotamers. The reactions of diethyl ethynylphosphonate with a selected cyclic α-oxodiazo compound, i.e. 2-diazoacenaphthen-1-one, yielded a fused tricyclic pyrazole derivative.