A new synthesis of pyrroles

Fates of imine intermediates in radical cyclizations of N-sulfonylindoles and ene-sulfonamides

Two new fates of imine intermediates formed on radical cyclizations of ene-sulfonamides have been identified, reduction and hydration/fragmentation. Tin hydride-mediated cyclizations of 2-halo-N-(3-methyl-N-sulfonylindole)anilines provide spiro[indoline-3,3'-indolones] or spiro-3,3'-biindolines (derived from imine reduction), depending on the indole C2 substituent. Cyclizations of 2-haloanilide derivatives of 3-carboxy-N-sulfonyl-2,3-dihydropyrroles also presumably form spiro-imines as primary products. However, the lactam carbonyl group facilitates the ring-opening of these cyclic imines by a new pathway of hydration and retro-Claisen-type reaction, providing rearranged 2-(2'-formamidoethyl)oxindoles.

Tandem catalytic oxidative deacetylation of acetoacetic esters and heteroaromatic cyclizations

Coupling of ethyl acetoacetate with MVK and with 2-nitrobenzyl bromides, Mn(iii)/Co(ii)-catalyzed deacetylation, and heterocyclizations provide pyrroles and indoles, respectively.

Radical cyclizations of cyclic ene sulfonamides occur with β-elimination of sulfonyl radicals to form polycyclic imines

Radical cyclizations of cyclic ene sulfonamides provide stable bicyclic and tricyclic aldimines and ketimines in good yields. Depending on the structure of the precursor, the cyclizations occur to provide fused and spirocyclic imines with five-, six-, and seven-membered rings. The initial radical cyclization produces an α-sulfonamidoyl radical that undergoes elimination to form the imine and a phenylsulfonyl radical. In a related method, 3,4-dihydroquinolines can also be produced by radical translocation reactions of N-(2-iodophenylsulfonyl)tetrahydroiso-quinolines. In either case, very stable sulfonamides are cleaved to form imines (rather than amines) under mild reductive conditions.

Enamides: valuable organic substrates

Enamides display a fine balance of stability and reactivity, which is now leading to their increasing use in organic synthesis. Enamides offer multiple opportunities for the inclusion of nitrogen based functionality into organic systems. Recent examples of these compounds as substrates are discussed in this article.

Efficient Pyrrole Synthesis Using Double Nucleophilic Addition to α,β-Unsaturated Imines with Plural Nucleophiles

[reaction: see text] 2,3,5-Trisubstituted pyrroles were prepared in a regioselective manner using the double nucleophilic addition of alpha,alpha-dialkoxy ketene silyl acetals and ketene sily thioacetals or trimethylsilyl cyanide to alpha,beta-unsaturated imines followed by acid-promoted cyclization and oxidation with DDQ. Using this methodology an imidazole glycerol phosphate dehydratase inhibitor (IGPDI) possessing a monopyrrole aldehyde moiety was synthesized.

Synthesis of 2-Alkylidenepyrrolidines and Pyrroles by Condensation of 1,3-Dicarbonyl Dianions with α-Azidoketones and Subsequent Intramolecular Staudinger−Aza-Wittig Reaction

The condensation of 1,3-dicarbonyl dianions with alpha-azidoketones afforded open-chained condensation products that were transformed into pyrroles by Staudinger-aza-Wittig reactions and the subsequent treatment with trifluoroacetic acid.

A novel pyrrole synthesis

The silver(I)-promoted oxidative cyclization of homopropargylamines at room temperature provides a novel access to pyrroles. Homopropargylamines are readily available by the addition of a propargyl Grignard reagent to Schiff bases.