Department of Chemistry, University of Science and Technology of China, Hefei 230026, China.
Org Biomol Chem. 2012 Apr 7;10(13):2673-82. doi: 10.1039/c2ob07079k. Epub 2012 Feb 27.
Bioorthogonal reactions are useful tools to gain insights into the structure, dynamics, and function of biomolecules in the field of chemical biology. Recently, the Diels-Alder reaction has become a promising and attractive procedure for ligation in bioorthogonal chemistry because of its higher rate and selectivity in water. However, a drawback of the previous Diels-Alder ligation is that the widely used maleimide moiety as a typical Michael acceptor can readily undergo Michael addition with nucleophiles in living systems. Thus, it is important to develop a nucleophile-tolerant Diels-Alder system in order to extend the scope of the application of Diels-Alder ligation. To solve this problem, we found that the theoretical protocol M06-2X/6-31+G(d)//B3LYP/6-31G(d) can accurately predict the activation free energies of Diels-Alder reactions with a precision of 1.4 kcal mol(-1) by benchmarking the calculations against the 72 available experimental data. Subsequently, the electronic effect and ring-strain effect on the Diels-Alder reaction were studied to guide the design of the new dienophiles. The criteria of the design is that the designed Diels-Alder reaction should have a lower barrier than the Michael addition, while at the same time it should show a similar (or even higher) reactivity as compared to the maleimide-involving Diels-Alder ligation. Among the designed dienophiles, three substituted cyclopropenes (i.e. 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes) meet our requirements. These substituted cyclopropene analogs could be synthesized and they are thermodynamically stable. As a result, we propose that 1,2-bis(trifluoromethyl)-, 1,2-bis(hydroxylmethyl)- and 1,2-bis(hydroxylmethyl)-3-carboxylcyclopropenes may be potential candidates for efficient and selective Diels-Alder ligation in living systems.
生物正交反应是深入了解化学生物学中生物分子结构、动态和功能的有用工具。最近，Diels-Alder 反应因其在水中更高的速率和选择性而成为生物正交化学中一种有前途和吸引人的连接方法。然而，以前的 Diels-Alder 连接的一个缺点是，广泛使用的马来酰亚胺部分作为典型的迈克尔受体，很容易在活体内与亲核试剂发生迈克尔加成。因此，开发一种亲核试剂耐受的 Diels-Alder 系统对于扩展 Diels-Alder 连接的应用范围非常重要。为了解决这个问题，我们发现，通过将计算结果与 72 个可用的实验数据进行基准测试，M06-2X/6-31+G(d)//B3LYP/6-31G(d) 理论方案可以准确地预测 Diels-Alder 反应的活化自由能，精度为 1.4 kcal mol(-1)。随后，研究了电子效应和环应变效应对 Diels-Alder 反应的影响，以指导新的双烯体的设计。设计的标准是，设计的 Diels-Alder 反应的势垒应低于迈克尔加成，同时，与涉及马来酰亚胺的 Diels-Alder 连接相比，它应表现出相似（甚至更高）的反应性。在所设计的双烯体中，三个取代的环丙烯（即 1,2-双（三氟甲基）-、1,2-双（羟甲基）-和 1,2-双（羟甲基）-3-羧基环丙烯）符合我们的要求。这些取代的环丙烯类似物可以合成，并且热力学稳定。因此，我们提出 1,2-双（三氟甲基）-、1,2-双（羟甲基）-和 1,2-双（羟甲基）-3-羧基环丙烯可能是活体内高效和选择性 Diels-Alder 连接的潜在候选物。
