合成天然产物曲古抑菌素A的新方法

报道了一条组蛋白去乙酰化酶抑制剂曲古抑菌素A的有效合成路线. 通过L-脯氨酸催化对硝基苯甲醛与丙醛的羟醛缩合反应, 高立体选择性地构建了目标分子的手性中心, 羟醛缩合产物的ee值大于99%, anti∶syn＝16∶1. 随后的合成过程中无消旋化现象, 合成的曲古抑菌素A是单一R型异构体, ee值大于99%.     

A quadrupolar expansion for r

An expansion is given for the inverse interelectronic distance in terms of the coordinates of the two electrons. The terms in the expansion contain the coordinates of the first electron with respect to two centers A, B and those of the second electron with respect to two centers C, D.     

A new polymorph of physcion

The structure of the title compound, 7‐methoxy‐2‐methyl‐4,5‐dihydroxyanthracene‐9,10‐dione, C₁₆H₁₂O₅, was originally reported by Ulickýet al. [Acta Cryst. (1991). C 47 , 1879–1881] in the space group P2₁2₁2₁ [polymorph (Io)]. The new polymorph, (Im), crystallizes in the space group P2₁/c. The molecular structures are closely similar, with both –OH groups forming intramolecular hydrogen bonds to one of the neighbouring quinone O atoms, thus slightly lengthening this C=O bond; the pattern of C—C bond lengths in the ring system is consistent with some contribution from a resonance form with a negative charge at the hydrogen‐bonded quinone O atom and an aromatic region around its neighbouring C atoms. The packing of (Im) is simpler than the extensively crosslinked pattern of (Io), with molecular tapes connected by classical (but three‐centre) and `weak' hydrogen bonds, parallel to [20].     

A study on ester formylhydrazones

A series of ester formylhydrazones 2 were synthesized from the reaction of alkyl imidate hydrochlorides 1 with formylhydrazine. Treatment of 2 with hydrazine hydrate, ethyl carbazate and tert-butyl carbazate led to the formation of 3-alkyl-4-amino-, 3-alkyl-4-ethoxycarbonylamino- and 3-alkyl-4-tert-butoxycar-bonylamino-4H-1,2,4-triazoles 3–5 , respectively. Reaction of compounds 2 with formylhydrazine gave N,N'-diformylhydrazine 6 . Compounds 2 were reacted with 2,5-dimethoxytetrahydrofuran to afford 3-alkyl-4-(1H-pyrrol-1-yl)-4H-1,2,4-triazoles 8 .     

Arcyriaflavin A monohydrate

The asymmetric unit of the title compound comprises the monohydrated form of the natural product arcyriaflavin A [systematic name: 12,13‐dihydro‐5H‐indolo[2,3‐a]pyrrolo[3,4‐c]carbazole‐5,7(6H)‐dione monohydrate], C₂₀H₁₁N₃O₂·H₂O. Individual molecular units are engaged in hydrogen‐bonding interactions, forming two‐dimensional zigzag supramolecular layers parallel to the (02) plane. The close packing of the layers is mediated by strong co‐operative π–π stacking interactions, in tandem with interlayer hydrogen bonds involving the solvent water molecule.     

A Convenient Synthesis of Vinyl Sulfides

tert-Alkyl sulfides, with an α-(1H-benzotriazol-1-yl) group 6 and 13 , are readily prepared from N-[(aryl-thio)methyl]-1H-benzotriazoles 3 and N-( 11 ), respectively, by reaction with BuLi and then with the appropriate electrophile. The tert-alkyl sulfides 6 and 13 are smoothly converted by BF₃. OEt₂ into vinyl sulfides 5, 7 or 12 , respectively, in satisfactory yields.     

Catalysis of the cure reaction of bisphenol A dicyanate. A DSC study

The kinetics of the thermal cure reaction of Bisphenol A dicyanate (BACY) in presence of various transition metal acetyl acetonates and dibutyl tin dilaurate (DBTDL) was investigated using dynamic differential scanning calorimetry (DSC). The cure reaction involved a pregel stage corresponding to around 60% conversion and a postgel stage beyond that. Influence of nature and concentration of catalysts on the cure characteristics was examined and compared with the uncatalyzed thermal cure reaction. The activation energy (E), preexponential factor (A), and order of reaction (n) were computed by the Coats–Redfern method. A kinetic compensation correction was applied to the data in both stages to normalize the E values. The normalized activation energy showed a systematic decrease with increase in catalyst concentration. The exponential relationship between E and catalyst concentration substantiated the high propensity of the system for catalysis. At fixed concentration of the catalyst, the catalytic efficiency as measured by the decrease in E value showed dependency on the nature of the coordinated metal and stability of the acetyl acetonate complex. Among the acetyl acetonates, for a given oxidation state of the metal ions, E decreased with decrease in the stability of the complex. A linear relationship was found to exist between activation energy and the gel temperature for all the systems. Manganese and iron acetyl acetonates were identified as the most efficient catalysts. In comparison to DBTDL, ferric acetyl acetonate proved to be a more efficient catalyst. The activation parameters computed using the Coats–Redfern method agreed well with the results from two other well known integral equations. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1103–1114, 1999     

Synthesis and Oxidation of N‐Aminoglyconolactams: A Synthesis of Mannostatin A

The N‐amino‐ribono‐1,5‐lactam 4 was prepared in two high‐yielding steps from the known methanesulfonate 2 . Oxidation of 4 with t‐BuOCl in the presence of 2,6‐lutidine afforded the tetrazene 6 (63%). Oxidation with MnO₂ gave the deaminated lactam 7 (40%), which was also obtained, together with the lactone 8 , upon oxidation of 4 with PhSeO₂H. Oxidation with Mn(OAc)₃/Cu(OAc)₂ provided the lactam 7 as the major and the dimer 9 as the minor product. Oxidation of 4 with 3 equiv. of Pb(OAc)₄ in toluene at room temperature gave two cyclopentanes, viz. the acetoxy epoxide 10 and the diazo ketone 11 in a combined yield of 78%. Oxidation with Pb(OBz)₄ provided 11 and the crystalline benzoyloxy epoxide 12 . The crystal structure of 12 was established by X‐ray analysis. The N‐amino‐glyconolactams 41, 46 , and 51 were prepared similarly to 4 . Their oxidation with Pb(OAc)₄ provided the diazo ketones 56, 57 , and 58 as the only isolable products. Oxidation of the N‐amino‐mannono‐1,5‐lactam 55 with Pb(OAc)₄ in the presence of DMSO gave the sulfoximine 59 . Mannostatin A, a strong α‐mannosidase inhibitor, was synthesized from the acetoxy epoxide 10 (obtained in 48% from 4 ) in seven steps and in an overall yield of 45%.     

Ascorbigen A—NMR identification

The connectivities of all atoms in ascorbigen A, an important metabolite, were determined unambiguously for the first time. The connectivity between carbon atoms was established by 2D INADEQUATE, and one-bond ¹³C–¹³C coupling constants were determined for all pairs of directly connected carbon atoms except for two strongly coupled carbon pairs. The ¹³C–¹³C coupling in one of the pairs was proved by a modification of standard INADEQUATE; however, the signals from the other pair were too weak to be observed. The connectivity within the two strongly coupled C–C pairs was confirmed by a combination of COSY and gHSQC; the latter experiment also identified all C–H bonds. The proton nuclear magnetic resonance (¹H NMR) spectra in dry dimethyl sulfoxide allowed identification and assignment of the signals due to NH and OH protons. The derived structure, 3-((1H-indol-3-yl)methyl)-3,3a,6-trihydroxytetrahydrofuro[3,2-b]furan-2(5H)-one, agrees with the structure suggested for ascorbigen A in 1966. The density functional theory (DFT) calculations showed that among 16 possible stereoisomers, only two complied with the almost zero value of the measured ³J(H6–H6a). Of the two stereoisomers, 3S,3aS,6S,6aR and 3R,3aR,6R,6aS, the latter was excluded on synthetic grounds. The nuclear Overhauser effect measurements unveiled close proximity between H2′ proton of the indole and the H6a proton of the tetrahydrofuro[3,2-b]furan part. Detailed structural interpretation of the measured NMR parameters by means of DFT NMR was hampered by rotational flexibility of the indole and tetrahydrofuro[3,2-b]furan parts and inadequacy of Polarizable Continuum Model (PCM) solvent model.     

t-Si64: A Novel Silicon Allotrope

Utilizing first principle calculations, a novel Si₆₄ silicon allotrope in the I4₁/amd space group with tetragonal symmetry (denoted as t-Si₆₄ below) is proposed in this work. In addition, also its structural, anisotropic mechanical, and electronic properties along with its minimum thermal conductivity κ_min were predicted. The mechanical and thermodynamic stability of t-Si₆₄ were evaluated by means of elastic constants and phonon spectra. The electronic band structure indicates that t-Si₆₄ is an indirect band gap semiconductor with a band gap: 0.67 eV (primitive cell) compared to a direct band gap of 0.70 eV with respect to a conventional cell. The minimum thermal conductivity of t-Si₆₄ (0.74 W cm⁻¹ K⁻¹) is much smaller than that of diamond silicon (1.13 W cm⁻¹ K⁻¹). Therefore, Si−Ge alloys in the I4₁/amd space group are potential thermoelectric materials.     

A calorimetric study of acrylate photopolymerization

The kinetics of the photoinitiated polymerization of lauryl acrylate (LA), 1,6-hexanedioldiacrylate (HDDA) and pentaerythritol tetraacrylate (PET₄A) have been investigated using differential scanning calorimetry (DSC). An autoacceleration phenomenon is observed with the multifunctional acrylates, but not with lauryl acrylate. The empirical dependences of reaction rate on such parameters as incident light intensity, initiator concentration, and temperature have been established and are in general found to vary with monomer conversion. Apparent activation energies for the photopolymerizations have been determined from rate versus temperature data. The multifunctional acrylates show an increasing activation energy with monomer conversion, whereas the apparent activation energy for lauryl acrylate not only decreases with conversion, but becomes negative at conversions greater than about 30%. The ratio k_p/k is calculated from rate versus conversion data under constant illumination and the (independently determined) initiation rate. Analysis of rate versus time data under nonsteady-state conditions (light turned off) yields the ratio k_t/k_p. With these two ratios the rate constants for propagation (k_p) and termination (k_t) may be separated and their respective values calculated. Both k_p and k_t are found to decrease substantially with monomer conversion, indicating a significant change in the rates of both the propagation and termination steps as the polymerization advances. These observations are explained in terms of a radical isolation phenomenon and diffusion control of the propagation step.     

Polyisobutylene: A most unusual polymer

The influence of molecular weight, M, on the fragility and fast dynamics in polyisobutylene (PIB) was studied using dielectric and mechanical relaxation spectroscopies, calorimetry, and Raman spectroscopy. The measurements indicate a decrease in fragility with increasing M for shorter chains, in the range of M where T_g is M‐dependent. Such behavior is not observed for other polymers and is at odds with traditional theoretical models that predict an increase in fragility with chain length. These results confirm the unusual character of PIB, as evident in various properties including extremely low gas permeability, a low fragility, and a segmental relaxation spectrum much broader than expected for a low‐fragility material. The reason for this anomalous behavior remains unclear, but might be related to the symmetric structure of the PIB repeat unit, together with comparable flexibility of both structural components, the backbone and side groups. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1390–1399, 2008     

A Study of the Reactivity of Secondary Phosphanes with Radical Sources: A New Dehydrocoupling Reaction

The reactions of secondary phosphanes with radical sources have been investigated. A stoichiometric dehydrocoupling of Ph₂PH with 1,1′‐azobis[cyclohexane‐1‐carbonitrile] (VAZO^® 88) affords tetraphenyldiphosphane in good yields, whilst reduction of the nitrosyl function was observed upon using 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO). Dialkylphosphane–borane adducts also undergo a dehydrocoupling reaction in the presence of VAZO^® 88 to form R₄P₂.     

Polyimidines: A new class of polymers. III. A comparison of isomers of polypyromellitimidines

The cis (3,3,5,5-), trans (3,3,7,7-), oxo, and thio analogs of tetraphenylpyromellitide were polymerized with 1, 6-hexane diamine, p-phenylene diamine, and p,p'-diaminodiphenyl ether under various conditions. A comparison was then made of reactivity of the isomers and of the properties of the polymers. In general the thio monomers were more soluble and reactive than the oxo. They also gave more thermally stable polymers. The cis isomers of the monomers were more soluble than the trans, but the trans were more reactive. The least stable of the 12 polymers prepared was that from the cis–oxo monomer and 1,6-hexane diamine. It gave a 10% weight loss at 300°C in air and 340°C in nitrogen by TGA. The most stable polymer was from the reaction of the cis–thio pyromellitide with p,p'-diaminodiphenyl ether, which showed 10% weight losses by TGA at 560 and 650°C in air and nitrogen, respectively. The polymers were stable in hot dilute hydrochloric acid and sodium hydroxide. They were all soluble in chloroform, dimethylformamide, and sulfuric acid. Polymers that contained sulfur were also soluble in carbon tetrachloride, benzene, xylene, and toluene. Brittle films could be cast from solution or melt-pressed.     

A chiral furocoumarin

The furocoumarin 1,2‐di­hydro‐2‐(1,2‐di­hydroxy­prop‐2‐yl)‐8H‐furo­[2,3‐h]­benzo­pyran‐8‐one crystallizes from methanol–water as the monohydrate C₁₄H₁₄O₅·H₂O. Both chiral centers have the S configuration. Both OH groups and both H atoms of the water mol­ecule form intermolecular hydrogen bonds with O?O distances in the range 2.7686 (18)–2.8717 (18) Å.     

A comment on SCF energy partitioning schemes

Starting from the symmetry groupG × SU(2) for the special case of negligible spin-orbit coupling, a general character formula is derived for them-tuplet representation, which is realized in the space of state functions ofn-electron systems in fields with symmetryG. Apart from the characters of the initial representation for the single electron space function, the formula only contains the number of particlesn and the multiplicitym.

Herrn Professor Dr. Hermann Hartmann zu seinem 60. Geburtstag gewidmet.     

A Density Functional Investigation of Fluorinated Silicon Clusters

Density functional calculations have been carried out on a series of fluorinated empty cages X_nF_n(n=2–20) with X?Si, Ge, and Sn. It indicates that the fullerene‐like cage structure with pentagons turns out to be the most stable with n increasing, and the stability of the X_nF_n isomers increases with the number of five‐membered rings. The HOMO‐LUMO gap for Ge (n=6, 10) cages is found to be even larger than the values for Si cages, though in bulk Ge has a smaller band gap than Si. Moreover, calculation of the Gibbs free energy of oligomerization reaction of SiF→1/n (SiF)_n showed that this reaction is exothermic even at 900 K, indicating the favorability of their formation from the SiF monomer.     

A microcomputer-controlled microcalorimeter calibration circuit

Summary A Microcomputer-Controlled Microcalorimeter Calibration Circuit 3. Enthalpies of Solution of Linear and Cyclic Alkanes in Glacial Acetic Acid The molar enthalpy of solution ofn-alkanes and cycloalkanes has been studied in an isoperibol calorimeter at 25.0 °C. Reasoning from a purely volumetric effect of intrusion of solute molecules into an associated solvent, we might expect compact cycloalkane molecules to disturb the solvent structure less than linear alkanes and to have a smaller exothermic enthalpy of solution than then-alkanes. That is not the case, evidently, as the cyclic compounds mix with aH _s that is exothermic in the same degree as their linear counterparts. These results are discussed in terms of a model in which each alkane molecule occupies a cavity in the structured solvent that is large with respect to itself. Enthalpy of solution of saturated alkanes in glacial acetic acid does not depend on the geometry of the alkane for the two homologous series studied thus far.     

A Liquid-Crystal Study of Heptalene

The chiral structure of heptalene is characterised, for a given configuration, by helicities, which are opposite in the direction (x) of the central C(5a)-C(10a) bond and in the direction (y) of a line perpendicular to it, passing through the C(3)-and C(8)-atoms. These two helicities can be experimentally established and differentiated by the handedness of the cholesteric mesophases induced in a biphenyl-type liquid crystal (LC) by chiral heptalenes with substituents favouring the x or y orientation along the nematic director of the LC.     

A New Synthesis of Benzylidene Acetals

Aryl-halo-diazirines react under basic conditions with 1,3-cis-, 1,2-cisand 1,2-trans-diols to give acetals. Yields are high. Diastereoselectivities depend upon the diol and upon the reaction conditions. Thus, reaction of the 1,3-cis-diol 1 (Scheme 1) with 2 gave 3 as a single diastereoisomer. The 1,2-cis-diols 4 and 7 led to the endo- and exo-acetals 5 / 6 (93:7) and 8 / 9 (ca.10:1), respectively, The 1,2-trans-diols 10 , 16 , and 19 reacted with 2 to afford 11 / 12 (90:10), 17 / 18 (1:1), and 20 / 21 (6:1), respectively. Reaction of the (4-nitrophenyl)diazirine 13 with 10 at higher temperatures yielded 14 / 15 (6:4). The uracil moiety, the acetamido group, and the enol-ether moiety are compatible with the reaction conditions. The diastereoselectivity is rationalized on the basis of a reaction sequence involving alkoxy-halogen exchange, which is regioselective or not, thermolysis of the ensuing alkoxydiazirine(s), protonation of the alkoxycarbene to form an (E)-configurated oxycarbenium ion, and attack of the neighboring oxy or hydroxy group, which is only possible for a limited range of conformers.