On the mechanism of conversion of N-acyl-4-acyloxy-β-lactams into 2-substituted 1,3-oxazin-6-ones. Can a low-barrier transition state be antiaromatic?
On the mechanism of conversion of N-acyl-4-acyloxy-β-lactams into 2-substituted 1,3-oxazin-6-ones. Can a low-barrier transition state be antiaromatic?
复制标题
DOI：
10.1021/jo015922e
复制
发表时间：
2001-12-14
影响因子：
3.6
通讯作者：
Lecea, B
中科院分区：
文献类型：
--
作者：
Alajarín, M;Sánchez-Andrada, P;Lecea, B
使用从头算和密度泛函理论研究了 N-酰基-4-酰氧基-β-内酰胺转化为 1,3-恶嗪-6-酮的机理。研究发现整个过程涉及两个拟周环反应。第一个关键反应是逆[4-外-位]环化而不是热旋转电环开环。尽管与该过程相关的活化能较低，但相应过渡结构的磁性特征显示出反芳香特征。第二步是非常放热的并且没有活化势垒。它对应于另一种假周环反应，而不是六电子解旋电环化。这些结果证实芳香性和伪周环反应之间不存在相关性。相反，热对称允许的周环反应总是芳香族的。因此，相应过渡结构的磁分析构成了区分这两种过程的有用工具。
The mechanism of the conversion of N-acyl-4-acyloxy-beta -lactams into 1,3-oxazin-6-ones has been investigated using ab initio and density functional theories. It has been found that two pseudo-pericyclic reactions are involved in the whole process. The first key reaction is a retro-[4-exo-dig] cyclization instead of a thermal conrotatory electrocyclic ring opening. Magnetic characterization of the corresponding transition structure shows antiaromatic character, despite the low activation energy associated with this process. The second step is very exothermic and has no activation barrier. It corresponds to another pseudopericyclic reaction instead of a six-electron disrotatory electrocyclization. These results confirm that there is no correlation between aromaticity and pseudopericyclic reactions. In contrast, thermal-symmetry-allowed pericyclic reactions are always aromatic. Therefore, magnetic analysis of the corresponding transition structures constitutes a useful tool to distinguish between both kinds of processes.