Enzymatic cyclization reactions of geraniol, farnesol and geranylgeraniol, and those of truncated squalene analogs having C<sub>20</sub>and C<sub>25</sub>by recombinant squalene cyclase
作者:Tsutomu Hoshino、Yuko Kumai、Isao Kudo、Shin-ichi Nakano、Shumi Ohashi
DOI:10.1039/b407001a
日期:——
The substrate specificity of squalene–hopene cyclase was investigated using the C10–C25 analogs including naturally occurring substances, e.g. geraniol (C10), farnesol (C15) and geranylgeraniol (C20). No cyclization occurred for geraniol, but a significantly high conversion ratio (64%) was observed for farnesol, yielding the cyclic sesquiterpenes consisting of 6/6-fused bicyclic ring systems. Among them, an attractive compound having C30 was produced, in the structure of which acyclic the farnesol unit is linked to the bicyclic skeleton through ether linkage. Conversion of geranylgeraniol was low (ca. 12%). The squalene analogs having C20 and C25 also were cyclized in yields of ca. 33–36%, but the analogs having the methyl group at C(7) and/or at C(11) underwent no cyclization; the large steric bulk size of C(7)–Me and/or C(11)–Me, which is arranged in α-disposition for all the pre-chair conformation, would have interacted repulsively with the cyclase recognition site near to the C(7) and/or C(11), resulting in no construction of the all-chair conformation inside the reaction cavity. A relatively low yield of geranylgeraniol indicated that a less bulky hydrogen atom must be located at C(14) for the efficient polycyclization reaction. The squalene cyclase shows remarkably broad substrate specificity to accept the truncated analogs having carbon-chain lengths of C15–C25 in addition to C30.
使用 C10-C25 类似物(包括天然物质,如香叶醇(C10)、法呢醇(C15)和香叶基香叶醇(C20))研究了角鲨烯-蒎烯环化酶的底物特异性。香叶醇没有发生环化反应,但法尼醇的转化率却很高(64%),产生了由 6/6 融合双环系统组成的环状倍半萜化合物。在这些化合物中,产生了一种具有吸引力的 C30 化合物,其结构中的非环芳樟醇单元通过醚键与双环骨架相连。香叶醇的转化率很低(约为 12%)。具有 C20 和 C25 的角鲨烯类似物也被环化,收率约为 33-36%。C(7)-Me和/或C(11)-Me的立体体积较大,在所有前椅构象中都呈α-排列,因此会与靠近C(7)和/或C(11)的环化酶识别位点发生排斥作用,导致无法在反应腔内构建全椅构象。香叶基苯二酚的产量相对较低,这表明必须在 C(14)处设置一个不太笨重的氢原子,才能有效地进行多环化反应。角鲨烯环化酶的底物特异性非常广泛,除了 C30 外,还能接受碳链长度为 C15-C25 的截短类似物。