气相色谱-质谱法研究3-氧代戊酸酯烯醇-酮互变体质谱断裂途径

以3-氧代戊酸甲酯为反应物,在气相色谱上在线制备了一些β-二酮酯类化合物,用HP-5色谱柱分离了烯醇式和酮式互变体,并讨论了3-氧代戊酸酯化合物"纯"的烯醇式和酮式互变体的质谱图断裂机理.酮式和烯醇互变体的质谱图显示了两者的质谱断裂途径有明显的差异,而这种差异和两者的结构密切相关.烯醇式互变体的分子离子比酮式稳定.酮式和烯醇式互变体容易发生α断裂和麦氏重排,但烯醇式互变体发生麦氏重排后的碎片离子,会进一步发生断裂,产生有别于酮式的离子碎片.实验表明,烯醇式互变体容易丢失中性醇,而酮式不能.在质谱上表现出来的种种差异,表明3-氧代戊酸酯的酮式和烯醇式互变体在离子源中不会快速地相互转变.     

β-二酮酯化合物在气相色谱上的酯交换反应及其互变异构体分离

在气相色谱上,乙酰乙酸甲酯、乙酰乙酸乙酯、3-氧代戊酸甲酯与醇在无任何催化剂存在下,发生酯交换反应。酯交换反应受气相色谱的进样口温度、柱温、载气流速影响。该反应为气相色谱在线制备纯的直链β-二酮酯类化合物提供了一种简便的方法。当采用HP-5毛细管色谱柱时,这些直链β-二酮酯化合物的烯醇式和酮式互变体能够得到很好的分离。     

2,6-硫代黄嘌呤质子互变异构反应机理的理论研究

在密度泛函B3LYP/6-311G~(**)理论水平上,对气相和水相中2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构体及其过渡态进行几何构型全自由度优化,获得它们在气相和水相中的几何结构和电子结构,PCM反应场溶剂模型用于水相计算.结果显示在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但是没有改变各异构体的稳定性顺序,其顺序为W1＞W3＞W2.进一步研究了2,6-硫代黄嘌呤各烯醇式与酮式水助质子互变异构的反应机理,提出了2,6-硫代黄嘌呤各烯醇式与酮式互变异构质子迁移的反应为平面六元环的过渡态结构.探讨了溶剂化效应对互变异构体的几何结构、能量、电荷分布以及互变异构反应活化能的影响等.     

碲氢化钠存在下3—氧化硫代羧酸酯的一步合成法

３－氧代硫代羧酸酯可由α－溴代酮与碲氢化钠作用形成的烯醇盐与二硫代碳酸二乙酯进行乙氧硫代羰基化反应制得，３－氧代硫代羧酸酯的烯醇含量可由核磁振谱测得。     

碲氢化钠存在下3－氧代硫代羧酸酯的一步合成法

３－氧代硫代羧酸酯可由α－溴代酮与碲氢化钠作用形成的烯醇盐与二硫代碳酸二乙酯进行乙氧硫代羰基化反应制得。３－氧代硫代羧酸酯的烯醇含量可由核磁共振谱测得     

几种β-二酮化合物互变异构体的光谱性质研究

合成了5种不同结构的β-二酮化合物,采用电喷雾质谱、紫外光谱、红外光谱和核磁共振法对其结构进行表征,并讨论了它们的酮式-烯醇式互变异构现象.结果表明,β-二酮分子内酮式-烯醇式的互变异构平衡明显受取代基影响,它们的酮式-烯醇式互变异构体含量完全不同.在固态和CDCl3溶液中,其中2种β-二酮化合物(a1,a2)中主要以烯醇形式存在,仅含有少量的酮式结构;而2种α-取代β-二酮(b1,b2)由于烯醇异构体的空间位阻效应,不能以分子内氢键形成烯醇六元环,全部以酮式结构形式存在;另外,由于强吸电子基团-CF3的作用,使得β-二酮(c)全部以烯醇形式存在.     

三种烯丙基取代的β-环糊精作为毛细管气相色谱固定相的研究

本文通过将β-环糊精的2,6-羟基取代为烯丙基,3-羟基用3种不同链长的酰基(戊酰基、庚酰基、辛酰基)酰化合成了3种新的环糊精衍生物,并研究其作为CGC手性固定相的色谱性能和手性分离性能.色谱分离测试结果表明,此固定相不仅对一些苯的二取代位置异构体具有良好的分离能力,还具有一定的对映体分离能力,可以将反式-二氯菊酸甲酯、顺式-菊酸甲酯、丙烯醇酮乙酸酯、丙炔醇酮乙酸酯、顺式-2,3-环氧-1-己醇乙酸酯和顺式-2,3-环氧-1-戊醇乙酸酯等手性药物很好地分开.     

某些胆酸及去氧胆酸肟类化合物的合成

从胆酸和去氧胆酸出发,经过不同反应,将甾核以及支链上的取代基分别转化为羰基及酯基,然后和盐酸羟胺反应引进肟基,合成了六个新的甾体肟类化合物:7-羟基-12-氧代-3-肟基胆酸甲酯(5)、7,12-二氧代-3-肟基胆酸甲酯(6)、12-氧代-3,7-二肟基胆酸甲酯(7)、3,7,12-三肟基胆酸甲酯(8)、12-氧代-3-肟基去氧胆酸甲酯(12)、3,12-二肟基去氧胆酸甲酯(13)。通过NMR和IR对合成物5~10的结构进行了表征。     

BPOB的绝对不对称合成机理及固体CD光谱

以苯乙酮与间苯二甲酸甲酯为原料经由Claisen缩合制备了1,3-二(3-苯基-3-氧代丙烯醇)苯(BPOB). 用元素分析、MS、1H NMR、UV吸收光谱、X射线单晶衍射和固体CD光谱等对其进行了表征. 晶体结构数据显示, BPOB属于Sohncke空间群P212121, UV和1H NMR谱表明, BPOB在溶液中主要以烯醇式存在, 固体CD光谱和重结晶实验证明, BPOB手性晶体的形成是结晶诱导的绝对不对称合成.     

在线裂解气相色谱/质谱联用法研究4-氧代-β-大马酮的热裂解行为

用在线裂解气相色谱/质谱法(PyGC/MS)研究了4-氧代-β-大马酮的热裂解行为。在氦气氛围中,将4-氧代-β-大马酮分别在350,450,550,650,700和750 ℃下进行热裂解,并以GC/MS对其裂解产物进行定性和半定量分析。结果表明,不同的裂解温度直接影响生成产物的类型和相对含量。4-氧代-β-大马酮可裂解出β-大马酮、4-氧代-β-紫罗兰酮、3,4,4-三甲基-环己-2-烯-1-酮和2,5,5-三甲基-环己-3-烯-1-酮等54种裂解产物。在550 ℃以下时,只有少量4-氧代-β-大马酮发生裂解; 在750 ℃时,几乎完全裂解,转移率达99.74%, 裂解产物达45种之多。随着裂解温度的升高,裂解产物越来越复杂,并出现有害物质如苯、甲苯、蒽和菲等。根据4-氧代-β-大马酮裂解产物相对含量的变化规律,对其裂解产物的形成机理进行了探讨,认为4-氧代-β-大马酮可能按照4种途径发生裂解。     

Keto forms of salicylaldehyde Schiff bases: structural and theoretical aspects

Twelve Schiff bases of methoxy-substituted salicylaldehyde have been examined by crystallographic and spectroscopic methods, as well as by DFT theoretical calculations in order to investigate the effect of the substituent's position on the keto-enol equilibrium in the crystalline state. Four out of the 10 structurally characterized compounds with methoxy substitution on the para and/or ortho positions with respect to the aldimine bridge and deriving from aliphatic amines or alkylarylamines are found as cis-keto tautomers and form dimers. In contrast, the five pure enol tautomers derive either from aliphatic or alkylarylamines and are meta substituted or from aniline or benzylamine and are para and/or ortho methoxy substituted. The DFT calculations support the crystallographic results and, moreover, they have shown that keto and enol tautomers are affected differently by the relative arrangement of the monomers. Overall, the DFT calculations point to a plausible hypothesis for the stabilization of the keto form in the crystalline state: In cases with a sufficiently low enol-keto energy difference of the isolated monomers, as when the methoxy group is at ortho and/or para positions with respect to the aldimino group, extra stabilization of the keto form is derived from molecular association, thus leading to its crystallization.     

Treatment of Naphthols with B(C(6)F(5))(3): Formation and Characterization of the Lewis Acid Adducts of Their Keto Isomers

With the strong Lewis acid B(C(6)F(5))(3), the keto tautomers from a variety of naphthol derivatives are obtained (e.g. alpha-naphthol, see scheme). The adducts of the tautomers were characterized by X-ray structure analysis, and the first attempts at hydrozirconation of the adducts were made.     

22-hydroxybenziporphyrin: switching of antiaromaticity by phenol-keto tautomerization

22-hydroxybenziporphyrin, a porphyrin analogue containing a phenol moiety, has been shown to exist as an equilibrium mixture of two distinctly different tautomers. One of them actually contains the hydroxy group and shows the local [6]annulene aromaticity in the phenol fragment. The other tautomer contains a keto group and exhibits a [20]annulenoid structure characterized by macrocyclic antiaromaticity. The tautomerization process has been investigated in detail using variable-temperature 1H NMR spectroscopy. The process is very fast, with an estimated activation energy of ca. 30 kJ/mol. Further insight into the energetics of the tautomerization is obtained from density functional (DFT) calculations. Surprisingly, the estimated energy of the antiaromatic keto species is 3-5 kcal/mol lower than the energy of the phenolic tautomer. The geometric and magnetic manifestations of aromaticity and antiaromaticity in the two tautomers are probed using a number of computational devices, including Wiberg bond indices, resonace weights derived from the harmonic oscillator model, and nucleus-independent chemical shifts. It is shown that mixing of phenolic and keto contributions in both tautomers is stronger than that in related tautomers of phenol. This effect is caused by extensive conjugation with the tripyrrolic unit of 22-hydroxybenziporphyrin and, to a lesser extent, by intramolecular hydrogen bonding.     

Disentangling intrinsic ultrafast excited-state dynamics of cytosine tautomers

Gas-phase ultrafast excited-state dynamics of cytosine, 1-methylcytosine, and 5-fluorocytosine were investigated in molecular beams using femtosecond pump-probe photoionization spectroscopy to identify the intrinsic dynamics of the major cytosine tautomers. The results indicate that, upon photoexcitation in the first absorption band, the cytosine enol tautomer exhibits a significantly longer excited-state lifetime than its keto and imino counterparts. The initially excited states of the cytosine keto and imino tautomers decay with sub-picosecond dynamics for excitation wavelengths shorter than 300 nm, whereas that of the cytosine enol tautomer decays with time constants ranging from 3 to 45 ps for excitation between 260 and 285 nm.     

Conjugated macrocycles related to the porphyrins. Part 2. Further synthetic and spectroscopic studies on difuryl analogs of the oxophlorins

The synthesis of three difuryl analogs of the oxophlorins is described. In common with the tetrapyrrolic oxophlorins, these compounds favor the keto tautomers and little, if any, of the enol tautomers are present in neutral solutions. Unlike the tetrapyrrolic systems, the difuryl oxophlorins give well resolved nmr spectra. Attempts to extend these studies to the synthesis of dithienyl oxophlorin analogs were unsuccessful.     

Correlated ab initio study of nucleic acid bases and their tautomers in the gas phase,in a microhydrated environment and in aqueous solution. guanine: surprising stabilization of rare tautomers in aqueous solution

Altogether eight keto and enol tautomers of guanine were studied theoretically in the gas phase, in a microhydrated environment (1 and 2 water molecules) and in bulk water. The structures of isolated, as well as mono- and dihydrated tautomers were determined by means of the RI-MP2 method using the extended TZVPP (5s3p2d1f/3s2p1d) basis set. The relative energies of isolated tautomers included the correction to higher correlation energy terms evaluated at the CCSD(T)/aug-cc-pVDZ level. The relative enthalpies at 0 K and relative free energies at 298 K were based on the above-mentioned relative energies and zero-point vibration energies, temperature-dependent enthalpy terms and entropies evaluated at the MP2/6-31G level. The keto form having hydrogen atom at N7 is the global minimum while the canonical form having hydrogen atom at N9 represents the first local minimum at all theoretical levels in vacuo and in the presence of 1 and 2 water molecules. All three unusual rare tautomers having hydrogens at N3 and N7, at N3 and N9, and also at N9 and N7 are systematically considerably less stable and can be hardly detected in the gas phase. The theoretical predictions fully agree with existing theoretical as well as experimental results. The effect of bulk solvent on the relative stability of guanine tautomers was studied by self-consistent reaction field and molecular dynamics free energy calculations using the thermodynamic integration method. Bulk solvent, surprisingly, strongly favored these three rare tautomers over all remaining low-energy tautomers and probably only these forms can exist in water phase. The global minimum (tautomer with hydrogens at N3 and N7) is by 13 kcal/mol more stable than the canonical form (3rd local minimum). Addition of one or two water molecules does not change the relative stability order of isolated guanine tautomers but the respective trend clearly supports the surprising stabilization of three rare forms.     

Ultrafast branching of reaction pathways in 2-(2'-hydroxyphenyl)benzothiazole in polar acetonitrile solution

In a combined study on the photophysics of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in polar acetonitrile utilizing ultrafast infrared spectroscopy and quantum chemical calculations, we show that a branching of reaction pathways occurs on femtosecond time scales. Apart from the excited-state intramolecular hydrogen transfer (ESIHT) converting electronically excited enol tautomer into the keto tautomer, known to be the dominating mechanism of HBT in nonpolar solvents such as cyclohexane and tetrachloroethene, in acetonitrile solution twisting also occurs around the central C-C bond connecting the hydroxyphenyl and benzothiazole units in both electronically excited enol and keto tautomers. The solvent-induced intramolecular twisting enables efficient internal conversion pathways to both enol and keto tautomers in the electronic ground state. Whereas relaxation to the most stable enol tautomer with twisting angle Θ = 0° implies full ground state recovery, a small fraction of HBT molecules persists as the keto twisting conformer with the twisting angle Θ = 180° for delay times extending beyond 120 ps.     

A theoretical study of solvent effects on tautomerism and electronic absorption spectra of 3-hydroxy-2-mercaptopyridine and 2,3-dihydroxypyridine

The tautomerization equilibria of 3-hydroxy-2-mercaptopyridine (HMP) and 2,3-dihydroxypyridine (DHP) in vacuo and in ethanol solution have been studied using the density functional theory (DFT) at B3LYP/6-31Gd level. The results indicate that the thione form of HMP and the keto form of DHP are the most stable tautomers in the equilibrium, and the energy barrier for the thiol-thione and enol-keto proton transfer decreases significantly when the tautomerism is mediated by a specific ethanol molecule in solution. The time-dependent density functional theory--polarizable continuum model (TDDFT-PCM) calculations on all tautomers of HMP and DHP in vacuo and in ethanol have assigned the lowest pi --> pi* excitations of thione and keto tautomers to the observed absorption bands of HMP and DHP in solutions. The solvation is predicted to have relatively small effect on these pi --> pi* excitations in ethanol.     

A thermochemistry and kinetic study on the thermal decomposition of ethoxyquinoline and ethoxyisoquinoline

Quantum chemical calculations were used to study the production of ethylene and keto/enol tautomers from ethoxyquinoline (2‐EQ) and ethoxyisoquinoline (1‐EisoQ and 3‐EisoQ) in the gas phase and ethanol at the MP2/6‐311++G(2d,2p)//BMK/6‐31+G(d,p) level. The obtained data indicate that the elimination of ethylene from 1‐EisoQ and 2‐EQ is slightly more favorable than from 3‐EisoQ. Formation of quinolone and isoquinolone (2‐EQO, 1‐EisoQO, and 3‐EisoQO) is kinetically favored compared to their enols. Decomposition of 2‐EQ and 1‐EisoQ to ethylene and keto forms is thermodynamically and kinetically preferable more stable than the corresponding enols. However, the hydroxy form of 3‐EisoQ is more stable than its keto tautomer in the gas phase and ethanol. The enol tautomers cost less energy when formed from their keto forms rather than from the parent ethoxyquinolone and ethoxyisoquinoline.     

2-硫代黄嘌呤互变异构体的密度泛函理论计算

在密度泛函B3LYP/6-311G**水平上,对14种气相和水相中可能存在的2-硫代黄嘌呤互变异构体进行了几何构型全自由度优化,并计算出它们的总能量、焓、熵、吉布斯自由能.Onsager反应场溶剂模型用于水相的计算.计算结果表明,2-硫代黄嘌呤在气相和水相中主要以酮式结构形式存在,与已有实验结果一致.在气相和水相中,酮式结构—N(7)(H)均比酮式结构—N(9)(H)更稳定.2-硫代黄嘌呤互变异构的熵效应小,对互变异构平衡没有显著的影响,而焓变对互变异构却产生了主要的影响.水溶剂化自由能与异构体的气相偶极矩存在相关性.另外,较详细地考察了2-硫代黄嘌呤与6-硫代黄嘌呤的相对稳定性.