Shumway W, Ham S, Moer J, Whittlesey BR, Birney DM
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, USA.
J Org Chem. 2000 Nov 17;65(23):7731-9. doi: 10.1021/jo0002644.
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.
乙酰基乙烯酮与α-手性醛和酮的环加成反应显示出非对映选择性，在缩醛或缩酮碳上形成叔或季手性中心，并具有良好的立体控制。一种次要产物（5b）的X射线晶体学表明，主要产物（例如4b）是费尔金-安模型预测的那些产物。报道了在MP2/6-31G水平下乙醛与甲酰基乙烯酮加成以及在B3LYP/6-31G水平下2-苯基丙醛加成的过渡态。反应物和产物的基态构象用于合理化过渡态的相对能量和几何结构，而无需援引西普拉赫假设。然而，α-氧代乙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乎平面的假周环过渡态一致。
