Pyrrole photochemical addition

The photochemical addition of alkenes to the pyrrole ring of indole to give cyclobutane products was first reported by Julian and Foster in 1973 [43], They found that both electron-rich and electron-poor alkenes add to indole, but only if an acyl group is present on the indole nitrogen atom. With monosubstituted alkenes the reaction was regioselective and, with the exception of vinyl acetate, the exo stereoisomer was produced as either the major or the only product (Scheme 15). A mechanism involving the indole triplet excited state was suggested, based on the observation that the reaction could be sensitized by acetophenone and quenched by naphthalene. 

Photochemical addition of dimethyl acetylenedicarboxylate to the enamine H2Ca C -NH-CE=>CH2 (E => COOMe) yields the bridged cyclohexene (90), irtiich decomposes at 90 °C in a retro-Diels-Alder reaction to ethylene and the ester (91). The adducts (92 R = PhCO or COOBu ") of perf luorobut-2-yne, CP CHCCF, to N-substituted pyrroles have been degraded to 3,4-bls(trlfluoromethyl)pyrroles (93) by two methods in the former, the adducts were first hydrogenated at the unsubstituted double bond and then heated to eliminate ethylene,idille in the latter, the adducts were treated with mesitonitrile oxide, 2,4,S-Me C H CEN -0, which removed the ethylene unit in the form of the isoxazole (94). ... 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

Of the 5-membered heteroaromatic systems, pyrrole reacts most like benzene in alkyne photocycloaddition, giving a 3,4-disubstituted azepine by 2,3-cycloaddition followed by electrocyclic ring-opening fequation 73) . Azepines with a different substitution pattern have been made by therpial 2,5-addition of an alkyne to a pyrrole, followed by photochemical ring-closure and thermal ring-opening of the tetracyclic photoproduct (equation 74) . 

Heteroarenes have been photochemically functionalized by PET reactions forming new C—C bonds both in an inter- and intramolecular fashion via a similar mechanism [46]. The heteroarenes could serve both as electron donor (e.g. pyrroles or indoles) or electron acceptor (e.g. cyanopyridines or cyanopyrazines). Again, fragmentation of the radical cation, coupled with the radical anion and loss of the anion, led to overall ipso-substitution. In addition to the cyano group, halides could also function as leaving groups, such that in some cases an attack at an unsubstituted position took place [46],... 

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