Pyrroles Claisen rearrangements

The reactions of 2-hydroxy-2-(3-indolyl)ethanoamides and of ethyl 2-hydroxy-2-(2-pyrrolyl) ethanoates with trialkyl ortho-esters follow similar routes with the initial formation of the enol ethers (295) and (297), followed by an ortho- Claisen rearrangement to give the 2,3-disubstituted indoles (296) and pyrroles (298) (79JOC1885,80TL4335). 

A gold(I)-catalyzed domino reaction sequence involving pentenynyl tosylamides led to the formation of 2,3-disubstituted pyrroles containing a quaternary center in the 2-substituent <07OL3181>. The mechanism of the reaction involved a 5-endo-dig cyclization followed by an aza-Claisen rearrangement. 

SYNTHESIS OF FURANS AND PYRROLES VIA PROPARGYL CLAISEN REARRANGEMENT... 

Scheme 14 Synthesis of pyrroles via propargyl Claisen rearrangement.

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

Vedejs has reported an oxaza-Claisen rearrangement to construct the pyrrole derivative 38. In presence of ethyl propiolate, the vinylogous hydroximic acid 35 imdergoes a conjugate addition followed by sigmatropic rearrangement to produce the aldehyde 37. Cyclization and dehydration resulted 38. ... 

Gagosz has studied a gold(I)-catalyzed process to convert various allyl pentynyl tosylamides to functionalized pyrroles under mild conditions. Alkyne activation by Au(I) triggers a nuclephilic cyclization of tosylamide to generate the presumed alkenyl gold intermediate 385 which undergoes aza-Claisen rearrangement. Aromatization followed by protodemetalation leads to the pyrrole 387. ... 

A method for effecting ortho-Claisen rearrangements of benzyl vinyl ethers, reported last year (3, 189) has been extended to include furan, thiophen, and pyrrole derivatives, i.e. (100) (101). ... 

The first total synthesis of the intricate Stemona alkaloid (+ /—)-isostemofoline (224) was reported by Kende and coworkers 81) starting from 1,2-hexanediol (225) which was straightforwardly converted to 227 (Scheme 22) 82). Reductive cycUzation with sodium hydrosulfite in refluxing aqueous ethanol, and protection of the unstable pyrrole as tert-butyl carbamate, afforded 228 in five steps with 12% overall )deld. The key bicyclic ketone 231 was assembled by [4 + 3] cycloaddition of pyrrole 228 and diazoester 229 promoted by rhodium octanoate dimer, followed by enol silane deprotection, exo-specific hydrogenation, and nucleophilic decarboxylation (47% overall yield). Sodium methoxide-catalyzed aldol condensation of ketone 231 and furfural provided the Q-j/i-unsaturated ketone 232 whose olefin configuration was established by nOe studies. Allylation of 232 provided a 2.4 1 mixture of ketone 234 and the corresponding allylic enol ether 233, which could be converted to the former via a stereoselective Claisen rearrangement. 

The most probable way of the pyrrole ring assembly from ketoximes and acetylene in the system KOH/DMSO is a heteroatomic version of Claisen rearrangement of the intermediate O-vinyl oxime, preliminarily isomerized to 0,N-divinylhydroxylamine [4,7,16,18]. As shown in the previous section, O-vinyl oximes are isolable intermediates of the pyrrole synthesis capable of transformation to pyrroles under the action of KOH/DMSO and also in DMSO itself. However, other tentative intermediate stages of the rearrangement remained disputable for a long. 

Bermner and Organ (2008) reported [3,3]-aza-Claisen rearrangement under microwave irradiation to obtain good yield of pyrroles within 30 min as compared to one pot, domino aldehyde/amine condensation and imine-allene cyclization while Aza-Claisen rearrangement of the unexplored. A -thiophenojg acetyl-a-vinyl piperidine substrate and the oxone-induced transannulation via microwave accelerated, chiral transfer using the achiral phenylsulfide auxiliary has been described by Sim etal. (2012). 

A mixture of metals was utilized to promote a type Ilae synthesis of penta-substituted pyrroles <06OL2151>. Treatment of enyne 37 with a Ag(I) catalyst followed by BnNH2 and a Au(I) catalyst gave pyrrole 38 via a Claisen-type rearrangement and cyclocondensation. 

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