Boren Brant, Hirschi Jennifer S, Reibenspies Joseph H, Tallant Matthew D, Singleton Daniel A, Sulikowski Gary A
Department of Chemistry, Texas A&M University, College Station, TX 77842, USA.
J Org Chem. 2003 Nov 14;68(23):8991-5. doi: 10.1021/jo034462h.
Diels-Alder reactions of vinylazepines with N-phenylmaleimide afforded exclusively the exo cycloadduct, while high endo stereoselectivity was observed, as previously reported, in analogous reactions of vinylpiperideines. This curious contrast was confirmed by X-ray analysis of cycloadducts not susceptible to epimerization. The stereoselectivity of Diels-Alder reactions of vinylazepines, vinylpiperideines, and vinylcycloalkenes exhibits surprising divergence depending on the detailed diene structure, and DFT calculations (Becke3LYP) were undertaken to shed light on these observations. The model calculations correctly predict the major stereoisomers in these reactions, though they tend to significantly underestimate the stereoselectivity. The results suggest some general considerations in predicting or controlling the stereochemistry of this class of Diels-Alder reactions.
乙烯基氮杂环庚三烯与N-苯基马来酰亚胺的狄尔斯-阿尔德反应只生成外型环加成产物，而正如之前报道的，在乙烯基哌啶的类似反应中则观察到了较高的内型立体选择性。通过对不易发生差向异构化的环加成产物进行X射线分析，证实了这种奇特的对比。乙烯基氮杂环庚三烯、乙烯基哌啶和乙烯基环烯烃的狄尔斯-阿尔德反应的立体选择性，根据二烯的具体结构表现出惊人的差异，并且进行了密度泛函理论计算（Becke3LYP）以阐明这些观察结果。模型计算正确地预测了这些反应中的主要立体异构体，尽管它们往往会显著低估立体选择性。结果表明，在预测或控制这类狄尔斯-阿尔德反应的立体化学时需要一些一般性的考虑。
