[2+3] Cycloaddition Approach to Multisubstituted N-Arylpyrroles

Significance

Reported is the synthesis of tetra- and trisubstituted pyrroles by the [2+3] cycloaddition of quinazolinium N1-salts and electron-poor alkynes followed by an interesting fragmentation reaction. The quinazolinium N1-salts were prepared by reaction of 3-methyl-4(3H)-quinazolin­one with 2-bromoacetophenones. Butyl oxirane is used as solvent and hydrogen bromide scavenger. The proposed mechanism is shown above and is supported by the isolation of intermediate A (R^¹, R^² = CO₂Me) when dimethyl acetylenedicarboxylate (DMAD) is used as dipolarophile. The original targets of this reaction were the pyrrolo-[1,2-a]quinazolines B, whose synthesis by this methodology was precedented in different systems.