Tautomerism of Pyrroles Not Involving the Functional Group

The tautomeric forms 11 and 12, called pyrrolenines, have often been postulated for pyrrole (10), but there is no conclusive evidence for their existence. The report that two isomers exist as the pyrro-lenine forms 13 and 14 (Ar = 3,4-dimethoxyphenyl) must be regarded with considerable doubt. 

Chemical evidence led to the conclusion that the conjugate acids of pyrrole probably exist predominantly as 16 or 17 rather than as X52,4,5 Although infrared spectra were initially interpreted on the 

Dipyrromethenes rapidly exchange the hydrogen atom attached to nitrogen, and the two isomers of unsyramctrical compounds (e.g, 19 and 20) cannot be separately isolated/ Dipyrromethenes form meso- 

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