- THE SYNTHESIS AND CHEMISTRY OF AZOLENINES. PART 18. PREPARATION OF 3-ETHOXYCARBONYL-3H-PYRROLES VIA THE PAAL-KNORR REACTION, AND SIGMATROPIC REARRANGEMENTS INVOLVING COMPETITIVE ESTER MIGRATIONS TO C-2, C-4 AND N.
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3H-Pyrrole-3-carboxylic esters (4) have been prepared, in some cases together with isomers (5) and (6) having exocyclic double bonds, by cyclization of suitably substituted 2-ethoxycarbonyl-1,4-diketones (1) with liquid ammonia, followed by dehydration of the isolable 2-hydroxy-3,4-dihydro-2H-pyrrole intermediates (2) and (3) with aluminia in boiling solvents.Prolonged heating in toluene or p-xylene converts the 3H-pyrroles (4) quantitatively into isomeric 4-esters (11) and N-esters (13) of 1H-pyrroles via competitive sigmatropic rearrangements.Isolable intermediate 2H-pyrrole-2-carboxylic esters (12) are converted similarly into the same products, under the same conditions.Detection of 3H-pyrroles (4) as intermediates in the latter reaction demonstrates for the first time the reversibility of the thermal 2H-pyrrole to 3H-pyrrole interconversion.
- Chiu, Pak-Han,Sammes, Michael P.
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p. 3439 - 3456
(2007/10/02)
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- THERMAL REARRANGEMENT OF 3H-PYRROLES BY COMPETITIVE -SIGMATROPIC SHIFTS, AND THE REVERSIBILITY OF THE 3H- TO 2H-PYRROLE INTERCONVERSION
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3-Ethoxycarbonyl-3H-pyrroles are converted via thermal -ester shifts to the isomeric 1H-pyrrole-4- and N-esters.Isolable intermediate 2H-pyrroles are converted into the same products, and also into the 3H-pyroles, demonstrating conclusively the reversibility of the 3H- to 2H-pyrrole interconversion.
- Chiu, Pak-Kan,Sammes, Michael P.
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p. 2775 - 2778
(2007/10/02)
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