The reaction of ethyl 2-nitrosopropenoate 2a with 1 -methylpyrrole, 2.5-dimethylpyrrole and 2.5-
diphenylpyrrole has been investigated. In every case the products result from highly regioselective attack
at C-2 of the pyrrole by the electrophile. The azopropenoate esters 2b and 2c react similarly with pyrroles,
to the extent that even 1 -(triisopropylsilyl)pyrrole is attacked exclusively at the 2-position by the azo
ester 2b. In contrast, the nitrile oxide ethyl cyanoformate N-oxide 12 reacts at the 3-position of 2.5-
dimethyl- and of 2,5-diphenyl-pyrrole. The reactions of the nitroso and azo esters with pyrroles are
interpreted as Diels-Alder cycloadditions with inverse electron demand
