Pyrrolopyrazoline

The diazo compound (181a), prepared from the nitrosamine, cyclized to a pyrrolopyrazoline in 80% yield.98 The diazo compound (181b), prepared from diethyl diazomalonate and allylamine, cyclized similarly but at a much more rapid rate. This is consistent with the lowered LUMO of the dipole of (181b), substituted with an ester group here the dipole LUMO-dipolarophile HOMO is the likely dominant interaction. The N—N double bond of the pyrrolopyrazoline products was readily isomerized to afford A2-isomers. 

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