Chow Siew Yin, Williams Howard J, Huang Qiulong, Nanda Samik, Scott A Ian
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, USA.
J Org Chem. 2005 Nov 25;70(24):9997-10003. doi: 10.1021/jo0517489.
[reaction: see text] The cyclization of GGPP to taxadiene, catalyzed by taxadiene synthase, has been suggested to proceed through a series of monocyclic isocembrenyl- and bicyclic verticillyl-carbocationic intermediary stages. A set of GGPP analogues with abolished or perturbed pi-nucleophilicity at the delta10 double bond (GGPP numbering) was synthesized and incubated with taxadiene synthase to intercept the cyclization cascade at the monocyclic stage. Each analogue was transformed by taxadiene synthase in vitro to hydrocarbon products in varying yields, and the structures of the major product in each reaction were solved by GCEIMS and one- and two-dimensional (1H and 13C) NMR and found to be 14-membered monocyclic isocembrenyl diterpenes, indicating that the first C-C bond formation catalyzed by taxadiene synthase could be uncoupled from the other subsequent bond formation events by using suitably designed substrate analogues. The formation and isolation of these isocembrenyl diterpene products using taxadiene synthase supports proposals that the isocembrenyl cation is an intermediate in the cyclization of GGPP to taxadiene.
[反应：见正文] 紫杉二烯合酶催化牻牛儿基牻牛儿基焦磷酸（GGPP）环化生成紫杉二烯，这一过程被认为是通过一系列单环异贝壳杉烯基和双环垂叶基碳正离子中间体阶段进行的。合成了一组在δ10双键（GGPP编号）处消除或干扰了π亲核性的GGPP类似物，并将其与紫杉二烯合酶一起温育，以在单环阶段拦截环化级联反应。每种类似物在体外都被紫杉二烯合酶转化为产率各异的烃类产物，通过气相色谱-电子轰击离子质谱（GCEIMS）以及一维和二维（1H和13C）核磁共振确定了每个反应中主要产物的结构，发现它们是14元单环异贝壳杉烯基二萜，这表明通过使用适当设计的底物类似物，紫杉二烯合酶催化的第一个碳-碳键形成可以与随后的其他键形成事件解偶联。利用紫杉二烯合酶形成并分离这些异贝壳杉烯基二萜产物，支持了异贝壳杉烯基阳离子是GGPP环化生成紫杉二烯过程中的中间体这一观点。
