Generation of diazo-2-methylenecyclopropane and its 1,3-dipolar cycloaddition to acrylates

N-(2-Methylenecyclopropyl)-N-nitrosourea (2) was synthesized for the first time (yield 70%) and its decomposition induced by bases was studied. In par ticular, treatment with MeONa/MeOH at -30 degrees C in the presence of meth yl methacrylate gives the corresponding 1-pyrazoline stereoisomers, the pro ducts of [3+2]-cycloaddition of diazo-2-methylene-cyclopropane (1) generate d in situ. Decomposition of 2 on treatment with K2CO3 at 0-5 degrees C in t he presence of acrylonitrile also proceeds as [3+2]-cycloaddition; however, the expected 2-pyrazoline easily isomerizes into 5(3)-isopropenyl-3(5)-cya nopyrazole. Buta-1,2,3-triene is the main product of base-induced decomposi tion of 2 in the absence of unsaturated substrates.