作者:Sergey V. Pronin、Ryan A. Shenvi
DOI:10.1038/nchem.1458
日期:2012.11
Non-stop carbocationic polycyclizations of isoprenoids have been called the most complex chemical reactions occurring in nature. We describe a strategy for the initiation of tail-to-head polycyclization that relies on the sequestration of the counteranion away from the carbocation, which allows full propagation of the cationic charge. If the anion is mobile, Coulombic forces hold this species in close proximity to the carbocation and cause preemptive termination through elimination. Anion sequestration is crucial for effecting the biomimetic synthesis of complex and unstable terpenes, including the highly strained funebrenes. This study illustrates the deleterious role of the counterion in tail-to-head carbocationic polycyclization reactions, which to the best of our knowledge has not been rigorously explored. These observations are also expected to find use in the design and control of cationic polycyclization along biosynthetic pathways that have previously been inaccessible in bulk solvent. Sesquiterpenes are biosynthesized from linear isoprenols through the intermediacy of multiple, high-energy carbocations. Here a strategy is demonstrated for mimicking these reactions in bulk solvent to yield strained, acid-labile terpenes. Key to the success of these reactions is the sequestration of the counteranion away from the reactive carbocation, a strategy that should enable further study of challenging polycyclizations.
天然产物中,异戊二烯类化合物的连续碳正离子多环化反应被称作自然界中发生的最复杂的化学反应。近日,研究者提出了一种通过将反荷离子隔开离开碳正离子,使正电荷得以充分传递,从而实现头尾相接多环化反应的策略。如果反荷离子可以移动,则静电作用会将其紧紧束缚在碳正离子附近,导致通过消除反应而提前终止。在该项研究中,对反荷离子的隔开对于激动仿生合成具有复杂结构和高度不稳定性的萜类化合物来说至关重要,其中包括高度应变的大泪杉烷类萜物。据我们所知,这是首次通过严格的实验探究了反荷离子在头尾相连的碳正离子多环化反应中体现的有害作用。这些发现也被期望能进一步应用于在亲水有机溶剂中针对之前仍无法阐释的生物合成途径中的正离子多环化反应的设计和调控中。半萜类化合物是由线性异戊烯醇经由多种高能量中间体碳正离子代谢生成。在本研究中,采用上述反应策略,成功地在亲水有机溶剂中模拟实现了多环化的半萜类化合物。而整个反应过程成功的关键在于将反荷离子隔开离开活性碳正离子,此策略为进一步研究这些多环化反应奠定了基础。