环加成反应势能面上后过渡态分叉现象的研究

The importance of post-transition state bifurcation, which is a unique mechanism for chemoselectivity, was somewhat underestimated previously. It is manifested in various organic reactions. Post-transition state bifurcation usually occurs when two products arise after the same energy barrier on potential energy surfaces. Due to the special shapes of post-transition state bifurcations’ potential energy surfaces, quasi-classical trajectory calculation is the most effective method of predicting chemoselectivity. Our group has discovered a novel post-transition state bifurcation behaving as [3+2]/[4+2] competition in the transition-metal catalyzed isochromenylium cycloadditions. We will investigate how different transition metals and different substituent groups influence the ratio of products. In the collaboration with other groups, we found that an enzyme-catalyzed reaction in the biosynthesis of streptoseomycin has the post-transition state bifurcation behaving as the competition between [6+4] and [4+2] products. We will investigate how solvent and enzyme influence the ratio of products. We are also interested in whether the biosyntheses of other natural products with ten-membered rings involve post-transition state bifurcations behaving as [6+4]/[4+2] competition.

后过渡态分叉现象作为一种独特的化学反应选择性机制，过去研究中其重要性被低估了。它其实在许多不同种类的有机反应中都有体现。后过渡态分叉现象表现为势能面上经过同一个能垒而得到不同产物，由于其特殊的势能面形状，直接动力学轨迹计算是目前预测其选择性的最有效手段。本课题组在对过渡金属催化的苯并异吡喃氧鎓中间体的环加成反应的机理研究中，发现了一类新颖的[3+2]/[4+2]竞争的后过渡态分叉。我们已经采用直接动力学轨迹计算的手段对其进行了一些初步研究，拟探究不同过渡金属、取代基对不同产物间比例的影响。我们在与其他课题组的合作中，发现汉城链霉素的生物合成过程中涉及的一步酶催化反应表现出[6+4]/[4+2]竞争的后过渡态分叉。我们将采用直接动力学轨迹计算的手段探究溶剂、酶对不同产物间比例的影响。我们还将探究其他的含有十元环结构的天然产物的生物合成中是否涉及到[6+4]/[4+2]竞争的后过渡态分叉。

后过渡态分叉是指单一过渡态生成多个产物的现象。近十年来，涉及后过渡态分叉的生物合成和酶催化反应逐渐被科研工作者所关注。本项目主要研究了汉城链霉素生物合成中酶催化的[6+4]/[4+2]后过渡态分叉和天然产物Heronamide A生物合成中非酶催化的[6+4]/[4+2]后过渡态分叉。此外，还研究了介质环境对酶催化的后过渡态分叉反应的影响。

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