杯芳烃构象的分子力学研究

杯芳烃是对位取代的苯酚与甲醛反应得到的环状缩合物,由于其独特的结构和易于衍生化的特点而受到广泛的关注［１］．杯芳烃中苯酚单元由亚甲基相连,由于酚羟基端可经杯中央翻转而产生多种构象异构体．杯［４］芳烃有４种典型构象：杯式、部分杯式、１,２－交替式和１,...     

杯芳烃衍生聚合物用作毛细管气相色谱固定相的研究

合成了两种不同构象的杯芳烃衍生聚合物（杯芳冠醚聚硅氧烷ｃ－ｃ［４］ｃ５－ｐｓｏ和二乙氧基杯［４］芳烃聚硅氧烷ｐ－ｃ［４］－ｐｓｏ），并将其用作毛细管气相色谱固定液，使用温度高达３２０℃，对不同的极性化合物，特别是对酚类异构体有很好的分离能力。同时也研究了分离机理。     

苄基和酯基杯[4]芳烃聚硅氧烷固定液热力学性质和分离机理

合成了两种新型杯芳烃衍生物苄基杯［4］芳烃聚硅氧烷（C［4］TECM-PSO）和酯基杯［4］芳烃聚硅氧烷,并用作毛细管气相色谱固定液。通过测定6组芳香族化合物在两种固定液上的热力学参数ΔH、ΔS、ΔG、ΔS/ΔH、Δ(ΔH)、Δ(ΔS),探讨了它们对于所测化合物的分子识别和色谱分离机理。结果显示,酯基杯［4］芳烃上测得的ΔH、ΔS远高于苄基杯芳烃上所测得的热力学参数,表明其与溶质的作用力强,构型效应更明显。同时对两种杯［4］芳烃的包结作用进行了探讨。     

杯芳烃固定液分子识别的热力学性质

用毛细管气相色谱法测得了５组芳香族位置异构体在杯芳冠醚聚硅氧烷（Ｃ［４］Ｃ５＿ＰＳＯ），二乙氧基杯芳烃聚硅氧烷（Ｃ［４］Ｅ＿ＰＳＯ）和二丁氧基杯芳烃聚硅氧烷（Ｍ＿Ｃ［４］Ｂ＿ＰＳＯ）３种杯芳烃固定液上的热力学参数ΔＨ、ΔＳ、ΔＧ，然后用热力学及超热力学的方法探讨了杯芳烃对异构体的分子识别特性和保留机理。     

C_(60)键合硅胶液相色谱固定相的合成及其性能评价

 以γ－（乙二胺基）丙基三乙氧基硅烷为偶联剂合成了Ｃ６０键合硅胶固定相；用元素分析法测定得到该固定相的Ｃ６０键合量为５７．５７μｍｏｌ／ｇ。考察了多环芳烃和杯芳烃在Ｃ６０键合硅胶固定相上的分离，多环芳烃在该固定相上的洗脱顺序与ＯＤＳ固定相相似；杯芳烃的洗脱顺序为杯［８］、杯［４］和杯［６］芳烃。     

逐步法合成杯[4]芳烃关环反应的改进

对位带有3种或4种不同取代基的杯「4］芳烃一般通过逐步合成法合成．50年代后期,wS和Hllfl比rI'·'a就用［1十1十1十工］的逐步法9步合成出了杯［4］芳烃,总产率相当低·1979年B6hmer等［'·'报道用对位取代酚的线型三聚体与2,6一双溪甲基对取代酚在乙酸中进行关环反应合成杯［4」芳烃,这种［3＋l」的逐步法只需3步连续反应就能合成带有3种不同取代基的杯卜」芳烃,但最后。步关环产率仍然很低（2％、15％）［"．后来＊dhmeTD'等将关环反应改进为在TICI。／二氧六环体系中进行,产率有所提高,这种逐步法曾用于不对称杯［4」芳烃的…     

C_(60)键合硅胶液相色谱固定相的合成及其性能评价

以γ－（乙二胺基）丙基三乙氧基硅烷为偶联剂合成了Ｃ６０键合硅胶固定相；用元素分析法测定得到该固定相的Ｃ６０键合量为５７．５７μｍｏｌ／ｇ。考察了多环芳烃和杯芳烃在Ｃ６０键合硅胶固定相上的分离，多环芳烃在该固定相上的洗脱顺序与ＯＤＳ固定相相似；杯芳烃的洗脱顺序为杯［８］、杯［４］和杯［６］芳烃。     

杯[4]芳烃衍生物的合成和构象研究

以对取代酚亚甲基桥三聚体和２，６－二溴甲基对取代酚为原料，在ＴｉＣｌ４催化下环化得到６种新的含不同取代基［Ｃｌ，Ｂｒ，ＣＨ３，Ｃ（ＣＨ３）３］的杯［４］芳烃衍生物，核磁共振谱研究表明：它们在溶液中的优势构象为锥形构象。     

杯芳烃化金属卟啉及其苯乙烯环氧化的催化性能

将单功能化四苯基卟啉与对叔丁基杯［４］芳烃通过单链相连，随后插入金属离子，合成了杯芳烃化金属卟啉１－３．在两相条件下对苯乙烯环氧化催化反应研究表明，化合物２的锰络合物中杯芳烃的存在对氧化反应有明显影响．可以根据连接于卟啉分子中憎水杯芳烃造成催化活性中心微环境差异来解释观察到的现象．各杯芳烃化锰卟啉的催化性能按以下次序降低：１ｂ＞２ｂ＞３ｂ－７．     

含杯[4]芳烃毛细管固定相的制备与色谱性能

杯芳烃是由对位取代的苯酚在邻位通过亚甲基连接起来的环状化合物,由于具有可调节大小的空腔,不仅能络合无机离子,而且还可络合有机分子,在分析分离方面有极好的应用前景［１,２］．杯芳烃已用于气相色谱［３,４］、离子选择性萃取［５,６］、液相色谱［７,８］和...     

Multistep conformational interconversion mechanism of cyclododecane. A simple and fast analysis using potential energy curves

An ab initio and Density Functional Theory (DFT) study of the conformational properties of cyclododecane was carried out. The energetically preferred equilibrium structures, their relative stability, and some of the transition state (TS) structures involved in the conformational interconversion pathways were analyzed from RHF/6‐31G(d), B3LYP/6‐31G(d,p) and B3LYP/6311++G(d,p) calculations. Aug‐cc‐pVDZ//B3LYP/6311++G(d,p) single point calculations predict that the multistep conformational interconversion mechanism requires 11.07 kcal/mol, which is in agreement with the available experimental data. These results allow us to form a concise idea about the internal intricacies of the preferred forms of cyclododecane, describing the conformations as well as the conformational interconversion processes in the conformational potential energy hypersurface. Our results indicated that performing an exhaustive analysis of the potential energy curves connecting the most representative conformations is a valid alternate tool to determine the principal conformational interconversion paths for cyclododecane. This methodology represents a satisfactory first approximation for the conformational analysis of medium‐ and large‐size flexible cyclic compounds. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Restricted motion of guests confined in carceplexes and capsules

Guest orientation within carceplexes and capsules was determined qualitatively from NMR data, and the molecular mobility of guests was determined via coalescence of (1)H NMR signals. Both are highly dependent on guest size and shape, as is interconversion of twistomers. Incarceration of 1,4-thioxane results in a large (1.8 kcal/mol) constraint on thioxane's conformational mobility (chair-to-chair interconversion). Similar conformational constraints (1.6 kcal/mol) were determined for 1,4-dioxane both when incarcerated in carceplex 1b and when encapsulated reversibly in capsule 3b. Encapsulation-induced conformational constraints of this magnitude are unprecedented, and are particularly striking for the noncovalently linked capsules.     

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out     

On the mechanism of conformational interconversion in cyclohexene derivatives

The free energy barriers to ring inversion for 3,3-dimethylcyclohexene, 4,4-dimethylcyclohexene and 3,3,6,6-tetramethylcyclohexene were determined by low temperature PMR studies. The results were interpreted in terms of a mechanism whereby the boat conformation is of highest energy on the energy profile for conformational interconversion. The very high ΔG^≠ value for 3,3,6,6-tetramethylcyclohexane (8·4 kcal/mole) is considered a reflection of the serious non bonded interaction that exists between methyl groups at the “prow” positions of the boat form.     

Total synthesis of abyssomicin C, atrop-abyssomicin C, and abyssomicin D: implications for natural origins of atrop-abyssomicin C

In this article, the total syntheses of the antibiotic abyssomicin C (1) and its biologically inactive sibling abyssomicin D (3) are described. A number of unforseen roadblocks in our synthetic plan encouraged innovation, which culminated in the discovery of a new Lewis acid-templated Diels-Alder reaction. En route to abyssomicin C, we prepared and characterized its stable conformational isomer atrop-abyssomicin C (57), which in the presence of a strong acid underwent an unusual interconversion with the targeted natural product. Close inspection of the X-ray crystallographic structures of these compounds led to hypotheses on the mechanism of their interconversion. Attempted reduction of both atropisomers revealed that atrop-abyssomicin C afforded abyssomicin D much more readily, suggesting that this previously unknown atropisomer may be synthesized by the host organism and serves as a direct precursor of abyssomicin D. Finally, to gain insight into the mechanism of antiobiotic activity, several synthetic intermediates and designed analogues were evaluated for biological activity.     

Structure, conformation, stereodynamics, and absolute configuration of the atropisomers of fluorenylidene derivatives

The barrier for the interconversion of the conformational atropisomers of an aryl fluorenylidene derivative was determined by variable-temperature NMR technique. In the case of a more hindered compound the two atropisomers were isolated and the structure determined by X-ray diffraction. The absolute configuration was assigned by theoretical interpretation of the Electronic Circular Dichroism spectrum (ECD).     

An exhaustive conformational analysis of N-acetyl-L-cysteine-N-methylamide. Identification of the complete set of interconversion pathways on the ab initio and DFT potential energy hypersurface

The full conformational space of N-acetyl-l-cysteine-N-methylamide was explored by ab initio (RHF/ 6-31G(d)) and DFT (B3LYP/6-31G(d)) computations. Multidimensional conformational analysis predicts 81 structures in N-acetyl-l-cysteine-N-methylamide, but only 47 relaxed structures were previously determined at the RHF/3-21G level of theory. These structures were now optimized using RHF/6-31G(d) and B3LYP/6-31G(d) approaches. Seven conformational migrations were observed when recalculated at higher level of theory. Besides these major changes, only smaller conformational shifts were operative for the remaining stationary points. The exploration of the whole conformational space of N-acetyl-l-cysteine-N-methylamide, including the transition-state structures allowing the conformational interconversion among the low-energy forms, was analyzed in this study. Our results offer new insights into the influence of polar side chains on the conformational preferences of peptide structures.     

X-ray diffraction analysis of nonequilibrium states in crystals: observation of an unstable conformer in flash-cooled crystals

Some molecules with a molecular skeleton similar to that of stilbenes and azobenzenes show orientational disorder in the crystals. If the disorder is dynamic, a conformational interconversion takes place through a pedal motion. In this study X-ray diffraction analyses of (E)-stilbene (1) and azobenzene (2) were carried out at various temperatures. We succeeded in observing thermodynamic nonequilibrium states that were generated by fast freezing of the conformational interconversion at low temperatures. The populations of the two conformers in crystals of 1 at 90 K varied with the cooling rate. Flash cooling of a crystal of 2 from room temperature to 90 K made it possible to observe the minor unstable conformer that does not exist in the equilibrium state at the same temperature.     

用分子动力学模拟方法研究磺酰脲化合物在溶液中构象的变化

农药化合物通常在溶液中起作用 ,由于溶剂的作用 ,其结构与在固体状态下有很大不同 ,因此研究农药化合物分子在溶液中结构的变化 ,对于了解农药作用机理以及与受体的相互作用是非常有意义的 .磺酰脲是一种超高效低毒 ,有着重要用途的除草剂 ,因而得到了广泛的应用与研究 .我们曾对其结构与性能的关系进行了系统的研究 ,并在此基础上建立了一个能解释若干实验规律的初级作用模型 [1] .但是以往有关结构的研究都是针对晶体和真空中的自由构象进行的 .由于在实际溶液中的结构情况会有所不同 ,因此 ,研究分子在不同溶液中的行为具有十分重要的意…