Reaction of amino substituted heterocycles with one heteroatom in a five-membered ring as enamines. A revision

2-Aminoheterocycles (pyrrole, furan, and thiophene) exist in solution in the amino form of type 1 and 5 . They undergo protonation at the exocyclic nitrogen in DMSO/TFA and protonation at position 5 in TFA. However when the electrophile is different from proton the 2-aminoheterocyles do not react as enamines (C-acylation is never observed), but their behavior is typical of aromatic amines, giving rise to N-acylated product of type 9 .