Taxadiene synthase-catalyzed cyclization of 6-fluorogeranylgeranyl diphosphate to 7-fluoroverticillenes
Taxadiene synthase-catalyzed cyclization of 6-fluorogeranylgeranyl diphosphate to 7-fluoroverticillenes
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DOI：
10.1021/ja050592r
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发表时间：
2005-06-01
影响因子：
15
通讯作者：
Coates, RM
中科院分区：
文献类型：
--
作者：
Jin, YH;Williams, DC;Coates, RM
紫杉二烯脱氢酶催化的（β）-香叶基香叶基二磷酸（GGPP，7）环化为紫杉二烯（5）的机理被提出为通过轮枝孢烯-12-基碳阳离子中间体（8）进行，该中间体经历11 -> 7质子转移，导致形成C环。合成底物类似物6-氟GGPP（17）以阐明推定的轮枝孢烯基中间体的立体化学。预计氟取代基的诱导吸电子效应将阻止关键的质子转移到A7双键，从而使双环阶段的环化脱轨。根据GC/MS分析，将氟类似物与重组紫杉二烯合酶孵育产生三种主要氟二萜和两种次要氟二萜的混合物。三种主要产物被鉴定为环外、环内和4（20）-亚甲基7-氟替西里烯，即，δ（3，7，12）（（18））、δ（37，12）和δ（4（20），7，11）异构体（22，23和24）。通过NOE实验和圆二色性光谱确定了22位的H_1 β，H_（11）α（1 S，11 R）构型。通过对（+）-轮枝菌醇对碘苯甲酸酯的反常色散X射线分析，修正了（+）-轮枝菌醇（4）的绝对构型。特别值得注意的是，文献中的轮枝烷二萜的所有绝对构型都应该颠倒过来。这项工作提供了令人信服的证据，支持在紫杉二烯糖苷酶催化的反应中轮枝孢烯-12-基（+）离子中间体的H11 α（11 R）立体化学，并说明了乙烯基氟类似物拦截复杂环化级联的能力。
The mechanism of the taxadiene synthase-catalyzed cyclization of (EEE)-geranylgeranyl diphosphate (GGPP, 7) to taxadiene (5) is proposed to proceed through a verticillen-12-yl carbocation intermediate (8) that undergoes an 11 -> 7 proton transfer leading to formation of the C ring. The substrate analogue 6-fluoroGGPP (17) was synthesized to elucidate the stereochemistry of the putative verticillenyl intermediate. It was expected that the inductive electron-withdrawing effect of the fluoro substituent would prevent the critical proton transfer to the A 7 double bond and thereby derail the cyclization at the bicyclic stage. Incubation of the fluoro analogue with recombinant taxadiene synthase yielded a mixture of three major and two minor fluoro diterpenes according to GC/MS analyses. The three major products were identified as the exocyclic, endocyclic, and 4(20)-methylene 7-fluoroverticillenes, i.e., Delta(3,7,12) ((18)), Delta(3 7,12), and Delta(4(20),7,11) isomers (22, 23, and 24) on the basis of H-1 NMR analyses and comparisons with the parent bicyclic diterpenes. The H1 beta, H11 alpha (1S,11R) configurations at the bridgehead positions of 22 were established by means of NOE experiments and CD spectra. The absolute configuration of (+)-verticillol (4) was revised after the anomalous dispersion X-ray analysis of (+)-verticillol p-iodobenzoate. Of particular note, all absolute configurations of verticillane diterpenes in the literature should be reversed. This work affords compelling evidence supporting the H11 alpha (11R) stereochemistry of the verticillen-12-yl(+) ion intermediate in the taxadiene synthase-catalyzed reaction and illustrates the capability of vinyl fluoro analogues to intercept complex cyclization cascades.