Pyrroles trimer

Cation (13) could react with pyrrole in two ways leading either to (15) or (16). Here again, neither can be seen as a likely intermediate for pyrrole trimer (15) would lead to the wrong structure, and (16) would be expected to be unreactive, migration of the double... 

In general it has been found that polypyrrolc is extremely poorly crystalline, limiting the information that can be obtained from direct structural techniques such as X-ray crystallography, and hence much of our knowledge has been obtained from indirect measurements and/or experiments on model compounds (e.g. X-ray studies on pyrrole trimers and dimers). It is now generally accepted that the ideal structure of the polymer is a planar (a a>bonded chain in which the orientation of the pyrrole molecules alternates. 

The ornithine related phosphonate (n = 3) can be guanidylated at the 4-amino group by treatment with formamidinesulfonic acid to provide Argp(OPh)2. This method can be used for the synthesis of substrate-related thrombin inhibitors such as Ac-Phe-Pro-Argp(OPh)2. 41 Proline analogues required a special approach and a few synthetic methods are reported since dipeptides of Prop(OPh)2 are excellent inhibitors of dipeptidyl peptidase IV (DPP IV or CD 26), the serine protease involved in immune response. 4 Diphenyl phosphite and dialkyl phosphites react smoothly with 1-pyrrolidine (3,4-dihydro-2//-pyrrole) trimer 20 to give the corresponding pyrrolidine-2-phosphonate 21 (Scheme 14). 42 ... 

D. Discussion of a Possible Free-Radical Mechanism for the Formation of Pyrrole Trimer... 

The mineral-acid-catalysed polymerisation of pyrrole involves a series of Mannich reactions, but under controlled conditions, pyrrole can be converted into an isolable trimer, which is probably an intermediate in the polymerisation. The key to understanding the formation of the observed trimer is that the less stable, therefore more reactive, P-protonated pyrrolium cation is the electrophile that initiates the sequence, attacking a second mole equivalent of the heterocycle. The dimer , an enamine, is too reactive to be isolable, however pyrrole trimer , relatively protected as its salt, reacts further only slowly. ... 

Reactions involving primary amines have been successfully carried out via Schiff s bases derived from a number of aldehydes. Thus, the reaction of pyrrole with the protonated enamine (1), which results in the formation of the pyrrole trimer (2 equation 1), constitutes a good example of a Mannich reaction involving an aldehyde other than formaldehyde. ... 

Pyrrole forms with sodium bisulphite an addition compound, C4H7N0eS2Na2.2H20, which has been formulated as the dihydrate of the disulphonate (49). The reaction has been represented as a nucleophilic attack upon the polarized pyrrole molecule. Mechanistically it could be very satisfactorily represented as a nucleophilic attack by bisulphite anion upon the highly reactive -protonated cation of pyrrole in a manner analogous to the formation of pyrrole trimer (p. 86). However, under the conditions used 4, the concentration of cation must be extremely small. 

Physically this account of the acid catalysis is obscure. The clue to the true nature of the acid catalysis is contained in the mechanism of pyrrole trimer formation, discussed above the catalyst functions by activating the entity which is to attack the pyrole nucleus. Thus, the acid catalysis of the reactions of pyrroles with carbonyl compounds is caused by protonation of the carbonyl compound. 

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