Theoretical Insights Regarding the Cycloaddition Behavior of Push−Pull Stabilized Carbonyl Ylides
| Autor: | James P. Snyder, Michael Prein, Alan T. Price, Albert Padwa, M. David Weingarten |
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| Rok vydání: | 1997 |
| Předmět: | |
| Zdroj: | The Journal of Organic Chemistry. 62:2001-2010 |
| ISSN: | 1520-6904 0022-3263 |
| DOI: | 10.1021/jo962184z |
| Popis: | A series of diazoamido keto esters were prepared by the reaction of N-substituted 3-carbethoxy-2-piperidone with n-butylmagnesium chloride followed by the addition of ethyl 2-diazomalonyl chloride. Treatment of these diazo amides with rhodium(II) acetate afforded transient push-pull carbonyl ylide dipoles which could be readily trapped with electron deficient dipolarophiles. All attempts to induce the dipolar cycloaddition to occur across tethered alkenyl pi-bonds failed to give internal cycloadducts. However, placing a sp(2) center on the tethered side chain was found to result in the formation of a tricyclic adduct in 95% yield. The stereochemistry of the cycloadduct was firmly established by an X-ray crystallographic study and occurred endo with respect to the amido carbonyl ylide dipole. A detailed computational study was undertaken to provide better insight into the factors that influence the intramolecular cycloaddition process. The calculations indicate that a severe cross-ring 1,3-diaxial interaction caused by the bridgehead methyl group promotes a boat or twist-boat conformation in the piperidine ring fused to the newly forming one. The presence of a carbonyl group in the dipolarophile tether helps to relieve the steric congestion by virtue of favoring a second boat in the latter ring. Without the C=O group, both nascent and piperidine rings are in the chair conformation at lowest energy, and the reaction barrier is disadvantaged by 5.6 kcal/mol, allowing other competing processes to intervene. |
| Databáze: | OpenAIRE |
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