Intramolecular Diels-Alder Reactions of Ester Linked 1,3,9-Decatrienes: Cis/Trans Selectivity in Thermal and Lewis Acid Promoted Reactions of Ethylene-Tethered and Benzo-Tethered Systems

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• 作者：Emma L. Pearson ; Laurence C. H. Kwan ; Craig I. Turner ; Garth A. Jones ; Anthony C. Willis ; Michael N. Paddon-Row ; Michael S. Sherburn
• 刊名：Journal of Organic Chemistry
• 出版年：2006
• 出版时间：August 4, 2006
• 年：2006
• 卷：71
• 期：16
• 页码：6099 - 6109
• 全文大小：539K
• 年卷期：v.71,no.16(August 4, 2006)
• ISSN：1520-6904

文摘

High cis (i.e., endo) diastereoselectivities are witnessed in heat-promoted intramolecular Diels-Alder(IMDA) reactions of ethylene-tethered hexadienyl acrylates. The cis stereoselectivity is improved bypromotion with Et₂AlCl. The first examples of Et₂AlCl catalyzed intramolecular Diels-Alder reactionsof ester-activated dienophiles are reported. In contrast, the corresponding benzo-tethered hexadienylacrylates undergo moderately trans (i.e., exo) selective IMDA reactions. Very high trans stereoselectionis obtained upon promotion with ATPH. The outcomes of these reactions are essentially insensitive todienophile (C10) geometry and substitution. DFT (B3LYP/6-31+G(d)) computed cis/trans productdistributions-based on Boltzmann transition structure populations-are in good agreement with theexperimental results. These computational investigations provide useful insights into the origins ofstereoselection in these systems. The stereoselectivity exhibited by the ethylene-tethered hexadienylacrylates is ascribed to stabilizing secondary orbital interactions at play in the cis-transition structures(TSs). In the benzo-tethered series, this effect is overridden by stabilizing -conjugative interactions,between the benzo moiety and the 1,3-diene component, which are stronger in trans TSs, compared tothe cis TSs. The computed TS geometries generally exhibit advanced peripheral bond formingasynchronicity, with the tether carbonyl group in conjugation with the dienophile. Such TS featuressignificantly weaken the stereodirecting influence of terminal dienophile substituents.