AbstractReductive cyclopropanation of triethyl methanetricarboxylate (4a) with ethylmagnesium bromide in the presence of titanium tetraisopropoxide furnished diethyl 2‐(1′‐hydroxycyclopropyl)malonate (5a) in 50% yield. Dehydromesylation of the mesylate of cyclopropanol 5a, generated in situ, by treatment with an excess of triethylamine gave diethyl cyclopropylidenemalonate 3‐Et as a highly reactive, unstable compound, which could be isolated as an oil in 39% yield, but more favorably could also be generated in situ from the mesylate or the acetate 11 (prepared from 5a in 90% yield) and trapped without isolation with various N‐, O‐, S‐, and C‐nucleophiles. Thus, ammonia, dimethylamine, isopropylamine, diethylamine, dibenzylamine, morpholine, N‐methylpiperazine, piperazine, aniline, p‐cresol, 1‐thionaphthol, and cyanide anion easily add to the double bond of the diester 3‐Et generated in situ at ambient temperature, forming 1′‐substituted cyclopropylmalonates 10a−i in 18−95% yields. The α,β‐unsaturated diester 3‐Et also easily enters into 1,3‐dipolar and Diels−Alder cycloadditions with nitrones, ethyl diazoacetate, azide anion, and dienes to give the corresponding cycloadducts in 12−83% yields. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)