Reaction with pyrroles

Furan undergoes 1,4-addition with ethoxycarbonyinitrene to give, after rearrangement, the pyrrolinone (121). The corresponding reaction with pyrrole gives a mixture of (122) and (123) (64TL2185). 

Alkynes substituted with one or two trifluoromethyl groups are also highly reactive dienophiles [9] Indeed, hexafluoro-2-butyne is used increasingly as a definitive acetylenic dienophile in "difficult Diels-Alder reactions. It was used, for example, to prepare novel inside-outside bicycloalkanes via its reaction with cir,trnns -l,3-undecadiene [74] (equation 67) and to do a tandem Diels-Alder reaction with a l,l-bis(pyrrole)methane [75] (equation 68) Indeed, its reactions with pyrrole derivatives and furan have been used in the syntheses of 3,4-bis(tri-fluoromethyl)pyrrole [76, 77] (equation 69) and ],4-bis(trifluoromethyl)benzene-2,3-oxide [78] (equation 70), respectively. 

The first example of a nitrone reaction with pyrroles and furan in the presence of HC1 as an activating agent was recently reported (589). Depending on reaction conditions, these acid-catalyzed reactions make it possible to obtain both, 2-heteroaromatic N -benzylhydroxylamines and symmetric as well as asymmetric 2,2 -bis -(heteroaryl) alkanes (Scheme 2.155). 

Wittig reactions with pyrrole-2-aldehyde led to the esters (79) which were cyclisized to 3a-azaazulen-4-ones (80).104,105 4-Methylene-3a-aza-azulenes (81) have been obtained from 80 with stabilized phos-phoranes.36 Reaction of dimethyl acetylenedicarboxylate with 81 could not be achieved. A similar cycloaddition was successful in the synthesis of cycl[3,3,3]azines (2) (Section V). 

The reaction of an azepinium ion (3), generated in situ, with a number of aromatic substrates (benzene, anisole, phenol, furan and thiophene) usually gave aryl-2//-azepines [e.g. (4) from benzene] as the major products.7 In the case of reaction with pyrrole, however, a ring-opened compound (5) was the major product. Some condensed thiophenes have been shown8 to give products of substitution at C(l). For example, (6a) gave (6b) on diazo coupling. 

Pyrroles and thiophenes have been found to react with alkynes in the presence of dinuclear palladium complexes with high cis stereoselectivity in almost all cases. Whereas regioselectivity in the reaction with pyrroles depends on the substituents on the nitrogen atom and alkynes, all reactions of thiophenes afforded 2-alkenylthiophenes.85... 

With amines, the variety of results is larger. There is no reaction with pyrrole, diphenylamine or N-methylaniline, but with aniline, the equilibrium phosphane + amine aminophosphorane is shifted to the right side (>65%). With propylamine and methyl-amine, only the phosphorane is present. 

Indoles, pyrroles and carbazoles themselves are suitable substrates for palladium-catalyzed amination. An initial study of this reaction using DPPF-ligated palladium as catalyst showed that these reactions occurred readily with electron-poor aryl halides. With unactivated aryl bromides, the reaction with pyrrole or indole resulted in good yield, but reaction times were long and the temperature was 120 °C. Thus, an improved catalyst system was necessary for reactions to occur in a more general fashion and with temperature- or base-sensitive substrates. 

The perfluorinated cyclopropenes (292, X = C(CF3)NH) may be prepared from (293) by 1,3-dipolar addition of diazomethane followed by desulphurisation with, triphenylphosphine to produce, (294), and then thermolysis. Addition of (292, X = C(CF3)0) to 2,3-dimethylbutadiene occurs predominantly in an endo-manner, the intermediate undergoing an intramolecular ene-reaction to produce (295). In the same way reaction with pyrrole leads to (296), in this case presumably by an intramolecular nucleophilic attack in the initially formed endo-adduct 237). 

Nucleophilic substitution is a relatively rare reaction with pyrrole, thiophene, or furan and requires an activating group such as nitro, carbonyl, or sulfonyl, just as it does with benzene (Chapter 23). Here is an intramolecular example used to make the painkiller ketorolac. 

Diazetine, see Diazete, dihydro-Diazirines, calculations, 56, 373 Diazirine, 3-chloro-3-trifluoromethyl-, reaction with pyrrole, 59, 10 3//-Diazirine, 3-fluoro-3-methoxy-, 59, 3 Diazo coupling... 

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