The mechanisms of heterocyclic ring closures

Katritzky, Alan R.; Ostercamp, Daryl L.; Yousaf, Taher I.

doi:10.1016/s0040-4020(01)87693-6

Cited by 117 publications

(29 citation statements)

References 21 publications

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“…24 The present computational results are also nicely corroborated by experimental findings on the related reaction of hydroxylamine with bketo esters. 21,22 Based on the calculations, therefore, preferential formation of 9 and 10, respectively, depending on whether 4-amino-(7) or 4-hydroxypyridinones (8A or 8B) are used as starting materials, can be fully rationalized.…”

Section: Resultsmentioning

confidence: 99%

“…21 Specifically, reaction of hydroxylamine 11 with b-keto esters preferentially occurs via attack of the nitrogen atom of 11 at the b-keto group. Under stopped-flow conditions, 22 the primary carbinol amine (corresponding to e.g.…”

Section: Path Dmentioning

confidence: 99%

“…Depending on the pH, formation of the isomeric isoxazolin-3-one is also observed. 21 It has been shown 21 that formation of this isomer occurs via b-keto hydroxamic acids rather than addition of the oxygen atom of 11 at the b-keto group. Since in the present molecules the ester functionality is lacking, such an alternative mechanism, if at all operative, must involve formation of an O,O-acetal by attack of the oxygen atom of 11.…”

Section: Path Dmentioning

confidence: 99%

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Effect of substituents on the formation of isomeric isoxazolo heterocycles: rationalization by semi-empirical PM3 molecular orbital calculations

Fabian

Schweiger

Weis

1999

J. Phys. Org. Chem.

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The different behavior of 4-amino-vs 4-hydroxypyridinones 7 and 8, respectively, towards hydroxylamine is rationalized with the aid of semi-empirical PM3 molecular orbital calculations including solvent effects. Four different mechanisms leading to either isoxazolo [4,3-c]pyridinones 9 or isoxazolo[4,5-c]pyridinones 10 are considered. Based on computed activation energies reaction of hydroxylamine via its oxygen atom as nucleophile is highly disfavored. For compound 7 a b-carbon addition of NH 2 OH at C-4 of the pyridinone accompanied by amine exchange and ring closure to 9 is by far the most feasible pathway. In contrast to 7, according to both NMR spectroscopy and molecular orbital [semi-empirical PM3 and hybrid density functional/Hartree-Fock (B3LYP/6-31G*)] calculations, 8 exists as a mixture of tautomers 8A (ca 20%) and (Z)-8B (ca 80%). Both tautomers of 8 are predicted to react with hydroxylamine at the hydroxyethylidene carbon atom [(Z)-8B] or acyl functionality (8A) to give hydroxyaminoethylidene compound 38 (oxime 23). Subsequent cyclization of either of these intermediates leads to compound 10.

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Section: Resultsmentioning

confidence: 99%

Section: Path Dmentioning

confidence: 99%

Section: Path Dmentioning

confidence: 99%

See 1 more Smart Citation

Effect of substituents on the formation of isomeric isoxazolo heterocycles: rationalization by semi-empirical PM3 molecular orbital calculations

Fabian

Schweiger

Weis

1999

J. Phys. Org. Chem.

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“…They are also used as starting materials foron cycloaddition and electrophilic reactions [1,2,3,4]. Recently we reported the preparation of 4-substituted-2-cyclo-hexenones by reductive cyclization of Hantszch esters using sodium and withand ethanol as the solvent [5].…”

Section: Introductionmentioning

confidence: 99%

Rearrangement of o-Nitrobenzaldehyde in the Hantzsch Reaction

et al. 2001

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The reaction of 5-hydroxy-2-nitrobenzaldehyde with ethyl acetoacetate in ammonia gave the two expected isomeric 1,4- and 1,2-dihydropyridines resulting from the normal Hantzsch reaction. However, the combination of 2-nitrobenzaldehyde with ethyl acetoacetate under the same conditions yielded four products: the two normal isomeric dihydropyridines and two tricyclic compounds. When we attempted to independently synthesize the two tricyclic compounds by reductive cyclization of 4-(2-nitrophenyl)-2,6-dimethyl-3,5-dicarbetoxy-1,4-dihydropyridine and 2-(2-nitrophenyl)-4,6-dimethyl-3,5-dicarbetoxy-1,2-dihydropyridine with tin (II) chloride in hydrochloric acid media, we obtained instead an indole and a quinoline derivative, respectively.

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“…The synthetic routes described in the literature involve in many cases cyclization by dehydration using strong acids and high temperatures, microwave irradiation [11,12], mesoporous material catalysis [13] or by metal-based catalysis [14][15][16][17][18][19][20].…”

Section: Introductionmentioning

confidence: 99%