“…As a possible source of the highly strained spiropentene ring system,2 the reaction of diazoalkanes with the exp double bond of a suitable methylenecyclopropene appeared attractive.3 We therefore undertook an examination of the reaction of diazomethane with a methylenecyclopropene (1) which was available to us from previous work in these laboratories. 4 The purpose of this paper is to report our finding that this reaction does not proceed by attack of the diazomethane on the exocyclic double bond but, instead, by attack on the endo double bond to give the unstable ring-opened diazoalkene 3, which isomerizes under very mild conditions to pyrazole 5 via what we believe to be the pyrazolenine 4.…”