Felkin-Anh stereoselectivity in cycloadditions of acetylketene: evidence for a concerted, pseudopericyclic pathway
Felkin-Anh stereoselectivity in cycloadditions of acetylketene: evidence for a concerted, pseudopericyclic pathway
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乙酰乙烯酮环加成中的 Felkin-Anh 立体选择性：协同伪周环途径的证据
作者：
Shumway;Ham;Moer;Whittlesey;Birney
乙酰烯酮与α手性醛和酮的环加成具有非对映选择性，在缩醛或缩醛碳上形成三级或四级手性中心，具有良好的立体控制性。次级产物（5b）的x射线晶体学表明，主要产物（如4b）是Felkin-Anh模型预测的产物。在MP2/6-31G水平上，乙醛加入到甲酰基烯中，在B3LYP/6-31G水平上，在2-苯丙醛的加入上，报告了过渡态。反应物和生成物的基态构象被用来使过渡态的相对能量和几何形状合理化，而不需要援引Cieplak假说。然而，α -氧烯酮上的手性取代基没有非对映选择性。这些实验和计算结果仅与先前提出的近平面、伪周期过渡态相一致。
The cycloadditions of acetylketene with alpha-chiral aldehydes and ketones are shown to be diastereoselective, forming a tertiary or quaternary chiral center at an acetal or ketal carbon with good stereocontrol. X-ray crystallography of a minor product (5b) shows that the major products (e.g., 4b) are those predicted by the Felkin-Anh model. Transition states are reported at the MP2/6-31G level for the addition of ethanal to formylketene and at the B3LYP/6-31G level for the addition of 2-phenylpropanal. The ground-state conformations of the reactants and products are used to rationalize the relative energies and geometries of the transition states without the need to invoke the Cieplak hypothesis. However, chiral substituents on the alpha-oxoketene show no diastereoselectivity. These experimental and computational results are only consistent with the nearly planar, pseudopericyclic transition state previously proposed.