Šmit Biljana M, Pavlović Radoslav Z, Milenković Dejan A, Marković Zoran S
Faculty of Science, University of Kragujevac, Radoja Domanovića 12 P.O. Box 60, 34000 Kragujevac, Serbia.
Bioengineering Research and Development Center, 34000 Kragujevac, Serbia.
Beilstein J Org Chem. 2015 Oct 7;11:1865-75. doi: 10.3762/bjoc.11.200. eCollection 2015.
The mechanism and selectivity of a bicyclic hydantoin formation by selenium-induced cyclization are investigated. The proposed mechanism involves the intermediates formed by an electrophilic addition of the selenium reagent on a double bond of the starting 5-alkenylhydantoin prior the cyclization. These intermediates are readily converted into the more stable cyclic seleniranium cations. A key step of the mechanism is an intramolecular cyclization which is realized through an anti-attack of the internal nucleophile, the amidic nitrogen, to the seleniranium cation yielding the intermediate imidazolinium cations. Their deprotonation is followed by the formation of the fused bicyclic reaction products. Important intermediates and key transition states are studied by using density functional theory (DFT) methods. The pathways of the reaction are investigated in detail. There are two regioselective pathways related to 5-exo and 6-endo products. Theoretical calculations and the monitoring of the cyclization reaction using (1)H NMR spectroscopy are in a good agreement with the proposed mechanism and are consistent with our experimental results. The preferred pathway for formation of 5-exo products is confirmed.
研究了硒诱导环化形成双环乙内酰脲的机理和选择性。提出的机理涉及在环化之前，硒试剂对起始5-烯基乙内酰脲的双键进行亲电加成所形成的中间体。这些中间体很容易转化为更稳定的环状硒鎓阳离子。该机理的关键步骤是分子内环化，它是通过内部亲核试剂（酰胺氮）对硒鎓阳离子的反式进攻实现的，生成中间体咪唑啉鎓阳离子。它们去质子化后形成稠合双环反应产物。使用密度泛函理论（DFT）方法研究了重要中间体和关键过渡态。详细研究了反应途径。有两条与5-外向和6-内向产物相关的区域选择性途径。理论计算以及使用核磁共振氢谱（1H NMR）对环化反应的监测与提出的机理高度吻合，并且与我们的实验结果一致。确定了形成5-外向产物的优选途径。
