氧杂环丁烷的扩环反应

氧杂环丁烷是一类重要的小杂环化合物,也是重要有机合成中间体,在有机化学、药物化学和高分子化学中都有广泛的应用.作为具有较大环张力的小杂环化合物,氧杂环丁烷类化合物除可以发生开环反应外,也很容易发生扩环反应,构建含氧普通环到大环化合物.主要总结了氧杂环丁烷的扩环反应,包括重氮化合物作为卡宾前体与氧杂环丁烷的扩环反应、金属...     

Conformational effects in acid-mediated ring opening of epoxides: a prominent role of the oxirane Walsh orbital

A kinetic study has been made of the BF(3)-mediated ring opening of two rigid conformers (alpha and beta) of m- and p-dimethyl-substituted homobenzoquinone epoxides spiro-linked with a twist-boat dibenzocycloheptene ring as compared with the conformationally mobile epoxides bearing diphenyl groups. The rates of the regioselective ring opening were dependent on the topological alignment of the dibenzocycloheptene ring as well as the substitution pattern of the quinone dimethyl groups, indicating pi-aryl participated orbital interaction with the vacant Walsh orbital of the oxirane ring.     

Synthesis of Ethyl 8‐Aryl‐8‐hydroxy‐3‐oxo‐8H[1,2,4]oxadiazolo‐[3,4‐c][1,4]thiazine‐5‐carboxylates by Ring‐Expansion Rearrangement

The title compounds were prepared by the ring–ring interconversion of ethyl 5‐nitroso‐6‐arylimidazo[2,1‐b]thiazole‐3‐carboxylates with hydrochloric acid. The effect of electron‐withdrawing substituent in the thiazole ring on the general applicability of the ring–ring interconversion has been also evaluated.     

Stereoselective synthesis of piperidinone and quinolinone systems via ring opening reactions using TiCl4/silyl reagents

Titanium(IV) chloride and silyl reagents mediated regio- and chemoselective ring opening reactions of oxa-bridged piperidinone ring systems were demonstrated. This methodology interestingly undergoes the stereoselective ring opening at the C-O bond of oxa-bridged piperidinone ring systems. Study of TiCl₄ with hydride or non-hydride silyl reagents furnished the product with selectivity. This protocol is highly valuable to synthesize a range of stereoselective piperidinones, quinolinones ring systems.     

Computational evaluation of radical ring‐opening polymerization

The tendencies of ring‐opening processes in radical ring‐opening polymerizations were evaluated by AM1 and PM3 semi‐empirical calculations and 6‐31G*‐level calculations based on the density functional theory (DFT) B3LYP models. Sixteen cyclic monomers bearing vinyl or exomethylene groups were categorized into ring‐opening and no‐ring‐opening monomers by the evaluation of the differences of the internal energies and the lengths of the cleaving bonds between the ground states of the initial radicals and the activated states in the ring‐opening processes. Although the semi‐empirical calculations not parameterized to radical reactions resulted in the moderate categorization of the ring‐opening monomers, the DFT calculation clearly distinguished the ring‐opening and no‐ring‐opening monomers. The ring‐opening tendencies were also evaluated with the changes in the internal energies throughout the ring‐opening processes, but this method could not group the ring‐opening and no‐ring‐opening monomers clearly. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2827–2834, 2007     

Development of an approach to the synthesis of the ABC ring system of Hemibrevetoxin B

An efficient approach for the synthesis of a model of the ABC ring system of Hemibrevetoxin B is described. Key features include a ring expansion to yield the ring C oxepane, the reduction of a 2-furyl ketone with high levels of 1,3-stereocontrol, and an Achmatowicz oxidative ring expansion to yield the ring A tetrahydropyran. All seven stereogenic centers present in the model compound were controlled with high levels (>98:<2) of diastereoselectivity.     

Ring Expansion Strategies for the Synthesis of Medium Sized Ring and Macrocyclic Sulfonamides

Two new ring expansion strategies are reported for the synthesis of medium sized ring and macrocyclic sulfonamides. Both methods can be performed without using classical protecting groups, with the key ring expansion step initiated by nitro reduction and amine conjugate addition respectively. Each method can be used to make diversely functionalised cyclic sulfonamides in good to excellent yields, in a range of ring sizes. The ring size dependency of the synthetic reactions is in good agreement with the outcomes modelled by Density Functional Theory calculations.     

Variable and stereoselective synthesis of azasugar analogues by a ruthenium-catalyzed ring rearrangement

A novel ruthenium-catalyzed ring opening/ring closing tandem metathesis reaction with a catalytic transfer of stereocenters from a ring to an olefinic chain is described. This ring rearrangement serves as the key step in the stereoselective synthesis of the new azasugar analogues 1 and 2.     

Thallium trinitrate mediated oxidation of 3-alkenols: ring contraction vs cyclization

The reaction of a series of six-membered ring 3-alkenols with thallium trinitrate (TTN) in three different experimental conditions was studied. Either cyclization products or ring contraction products were obtained, depending on the structure of the substrate as well as the nature of the solvent. The reaction of a seven-membered ring 3-alkenol with TTN led to the ring contraction product exclusively.     

Local angular momentum as ring strain descriptor

A new method is demonstrated to quantify local ring strain, which is based on the expectation value of orbital angular momentum along the internuclear axis. In contrast to energy based methods which provide overall ring strain, this method is able to identify the local strain in every part of the ring. The formalism is benchmarked on several cycloalkanes in which the presence of ring strain is well understood. The ring strain plays a decisive role in carbon nanotubes (CNTs) properties; for instance, the hydrogen storage capability of CNTs is related to their diameter, which in turn has a close relation to the ring strain in their C? C bonds. On this basis, the ring strain in five CNTs with different diameters is analyzed and the results reflected meaningful correlation between the CNTs diameter and ring strain. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Determination of the site of protonation of substituted benzenes in water chemical ionization mass spectrometry

The site of protonation of a substituted benzene may be determined using chemical onization mass spectrometry with D₂O as a reagent gas. The observation of extensive exchange of the ring hydrogens for deuteriums is linked to protonation on the benzene ring. The lack of this exchange coupled with the formation of cluster ions (the association of the protonated species with one or more D₂O molecules) is evidence of protonation on the substituent rather than the ring. Aniline, benzaldehyde and nitrobenzene are observed to protonate at the substituent while toluene, bromobenzene, biphenyl and iodobenzene protonate on the ring. The dimethylbenzenes protonate on the ring while the diaminobenzenes protonate at one of the substituents. The dihydroxybenzenes, as well as a number of other compounds in which an oxygen is attached directly to the ring, protonate predominantly at the substituent although a small amount of exchange of one ring hydrogen is observed.     

硼氢化钠还原潜手性酮反应机理的量子化学研究

用从头计算方法对硼氢化钠不对称还原对甲基环己酮的机理进行了研究. 探讨了四元环和六元环机理, 其中每种机理有三种途径, 主要的途径是经过过渡态的两步反应途径. 计算结果表明, 无溶剂参与反应的四元环过渡态机理虽然能解释其还原历程, 但计算所得的对映体组成与文献值相差很大. 而对质子性溶剂异丙醇参与的六元环机理的研究发现反应产物的对映体组成的计算结果与文献实验值吻合较好. 这表明该反应主要经历六元环过渡态的两步反应历程.     

Bromination of sydnones. I. Reaction with 3-arylsydnones containing electron-donors on the aryl ring

Bromination has been examined for a series of 3-arylsydnones (1) with electron donors (dimethyl to dimethoxy) on the aryl ring. In no example was exclusive aryl ring bromination observed, however, exclusive sydnone ring bromination could be realized in every case. For two dimethoxyphenyl examples both aryl and sydnone ring bromination occurred.     

非对称氮杂环丙烷的亲核开环反应及其区域选择性

本文系统地总结了各类亲核试剂对非对称氮杂环丙烷(吖丙啶)的亲核开环反应及开环的区域选择性.氮杂环丙烷亲核开环的区域选择性是一种空间效应和电子效应平衡的结果,非芳基和非烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷取代少的碳原子上,空间效应起主导作用;而芳基和烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷芳甲位和烯丙位的碳原子上,电子效应起主导作用,烯基取代的氮杂环丙烷的亲核开环还可以发生在烯基的β-碳原子上;分子内的亲核开环反应主要受成环时环大小的控制,成环时的倾向是五元环>六元环>七元环.对于亲核试剂,一般的亲核试剂也同时受电子效应和空间效应的影响; 而亲核性强的亲核试剂通常只受空间效应的影响.容易生成稳定自由基的亲核试剂容易发生单电子转移机理的开环反应,生成相当于亲核试剂进攻氮杂环丙烷中取代多的碳原子得到的开环产物.     

热环合反应速率的量子理论

应用多声子无辐射跃迁理论研究热环合反应, 给出热环合反应速率的一般公式。在高温条件下得出反应速率和活化能公式。所得公式可适用于多个环合点的热环合反应。     

Theoretical Calculations of β-Lactam Antibiotics. Part VI. AM1 calculations of alkaline hydrolysis of clavulanic acid

The gas-phase basic hydrolysis of clavulanic acid ( a ) was studied by using the AM1 semi-empirical method. The results obtained show that the hydroxyethylidene side chain at C(2) is pivotal to the stability of the different reaction products involved. The products with an open oxazolidine ring are more stable than those with a closed ring fused to the β-lactam ring. This behaviour differs from that of penicillins and cephalosporins where the most stable degradation products are those with an intact thiazolidine or dihydrothiazine ring, respectively, fused to the β-lactam ring. The different chemical reactivity of clavulanic acid relative to penicillins and cephalosporins could explain the disparate behaviour of the latter two types of compound towards β-lactamases. Once the acyl-enzyme intermediate of clavulanic acid has been formed, it can evolve with cleavage of the oxazolidine ring to form a difficult to deacylate compound.     

硅杂四元环化合物的合成和反应

硅杂四元环化合物在有机硅化学中是一类非常重要的小分子环系化合物, 广泛应用于有机化学、金属有机化学以及材料化学. 环上只含有一个硅原子的硅杂环丁烷可以通过γ-卤代丙基硅烷的Grignard反应、Si＝C键与烯烃的 [2＋2]环加成反应以及硅杂环丙烷的扩环反应合成, 环上只含有一个硅原子的硅杂环丁烯可以通过格氏试剂或锂试剂参与的Si—C键的关环反应、硅杂环丁烷的转化反应、硅卡宾对C—H键的插入反应、Si＝C键与炔烃的[2＋2]环加成反应以及二炔基硅烷的分子内成环反应等途径合成. 硅杂环丁烷和硅杂环丁烯由于存在环张力和具有一定的Lewis酸性, 能够通过扩环反应生成五元和六元含硅杂环化合物, 也能够通过开环反应生成不同结构的有机硅分子和聚合物, 抑或实现有机反应在温和条件下的转化.     

Crystal Structure of 8-Demethoxyrunanine

A new hasubanane-type alkaloid, 8-demethoxyrunanine, was isolated from Sino- menium acutum and characterized by melting point, HREIMS, 1H NMR, and X-ray diffraction analysis. X-ray diffraction reveals that the title compound crystallizes in the orthorhombic system, space group P212121 with a = 7.308（1）, b = 21.742（5）, c = 22.893（4）A, V = 3637.5（11） A^3, Z = 8, Dx = 1.254 g/cm^3, F（000） = 1472, μ（MoKα） = 0.087 mm^-1, the final R = 0.0438 and wR = 0.0575 for 4497 independent reflections with Rint = 0.0192 and 2091 observed reflections with I 〉 2σ（I）. Four rings （ring A： one benzene ring, ring B： one hexagon carbon ring in a half-chair conformation, ring C： one hexagon carbon ring with α,β-unsaturated ketone segment （-CR2=CR1-C=O） in a screw-boat conformation, and ring D： one nonplanar tetrahydropyrrole） form a hasubanane-type alkaloid.     

若干杂环模型化合物的热氧化稳定性

<正> 自五十年代以来,耐热的杂环高分子取得了很多进展。曾报道聚酰亚胺、聚苯并咪唑、聚苯并噻唑、聚苯并噁唑、聚喹噁林及聚苯并噁嗪酮等都是性能较好的耐热高分子,而有些在工业上已得到应用。我们在进行芳杂环二腈聚合的工作中,曾将上述杂环结构引入二腈,以期获得性能优良的耐热高分子,所得结果已陆续报道。     

Remarkable synthesis and structure of allene type zerumbone

The ring expansion of zerumbone to a 12-membered ring was studied via a ring opening system or a ring closure system of zerumbone. We succeeded in the synthesis of a zerumbone derivative with 12-membered ring, an allene type zerumbone. For the first time, a Doering-LaFlamme allene synthesis method was adopted and the structure was confirmed by monocrystal X-ray diffraction. It was obtained in total 27.7% yield from zerumbone. We believe that this compound is not only an important building block in synthesizing the BC ring of paclitaxel, but also plays an important role in a novel structure formation and a reactive discovery.