Conformational Properties, Chiroptical Spectra, and Molecular Self-Assembly of 2,3-Piperazinodiones and Their Dithiono Analogues

A family of chiral cyclic oxamides was prepared by the condensation of optically active 1,2-diamines
with diethyl oxalate. Thionation of the products with Lawesson's reagent afforded a series of chiral
2,3-piperazinedithiones. Molecular geometries of the title compounds were studied with the use of quantum
mechanical DFT calculations and were compared to the X-ray crystallographic results. The heterocyclic
six-membered ring adopted a half-chair conformation with the C-5 substituent preferably at the equatorial
position, whereas a substitution at the nitrogen atoms resulted in domination of the axial form in the
conformational equilibrium. The opposite helicity of the twisted oxamide chromophore in the axial and
equatorial conformers led to the opposite signs of the Cotton effects corresponding to two π−π* electronic
transitions. The CD signs can be predicted by a simple helicity rule. The same rule is valid for 2,3-piperazinodithiones, where a substitution of sulfur for oxygen in the carbonyl groups results in
bathochromic shifts of the absorption and CD bands. The crystal packing analysis of several
2,3-piperazinodiones revealed that strong NH···OC intermolecular hydrogen-bonding interactions
generating the chain motif resulted in the formation of 3-D networks as well as with the use of the cyclic
hydrogen-bond motif tape structures.

本研究通过光学活性1,2-二胺与草酸二乙酯的缩合反应，制备了一类手性环状草酰胺化合物。采用劳森试剂（Lawesson's reagent）对所得产物进行硫化反应，得到一系列手性2,3-哌嗪二硫酮。借助量子力学密度泛函理论（Density Functional Theory, DFT）计算，对标题化合物的分子几何结构展开研究，并将计算结果与X射线晶体衍射（X-ray crystallography）结果进行对比。该六元杂环采取半椅式构象，其中C-5位取代基优先处于平伏键位置；而氮原子上的取代基则使构象平衡中直立键形式占据主导地位。轴向与平伏键构象体中，扭曲的草酰胺生色团具有相反的螺旋手性，这使得对应于两个π−π*电子跃迁的科顿效应（Cotton effect）符号相反。可通过一条简单的螺旋规则预测圆二色谱（Circular Dichroism, CD）信号符号，该规则同样适用于2,3-哌嗪二硫酮类化合物：羰基中氧被硫取代后，会导致吸收带与圆二色谱带发生红移（bathochromic shift）。对多款2,3-哌嗪二酮的晶体堆积分析表明，生成链状基元的强NH···O=C分子间氢键相互作用，不仅形成了三维网状结构，同时还通过环状氢键基元形成了带状（tape）结构。