Cationic copolymerization of 2-methylene-5,5-dimethyl-1,3-dioxane with 2-methylene-1,3-dioxolane and 2-methylene-1,3-dioxane

Copolymers of the cyclic ketene acetals, 2-methylene-5,5-dimethyl-1,3-dioxane, 3 , (M₁) with 2-methylene-1,3-dioxolane, 4 , (M₂) or 2-methylene-1,3-dioxane, 5 , (M₂), were synthesized by cationic copolymerization. An experimental method was designed to study the reactivity of these very reactive and extremely acid sensitive cyclic ketene acetal monomers. The reactivity ratios, calculated using a computer program based on a nonlinear minimization algorithm, were r₁ = 6.36 and r₂ = 1.25 for the copolymerization of 3 with 4 , and r₁ = 1.56 and r₂ = 1.42 for the copolymerization of 3 with 5. FTIR and ¹H-NMR spectra when combined with the values of r₁ and r₂ showed that these copolymers were formed by a cationic 1,2-polymerization (ring-retained) route. Furthermore the tendency existed to form very short blocks of M₁ or M₂ within the copolymers. Cationic copolymerization of cyclic ketene acetals have the potential to be used for synthesis of novel polymers. © 1996 John Wiley & Sons, Inc.     

Structure of aldose condensation products with 2-hydroxyand 2-sulfanylbenzohydrazides

The structure of the condensation products of 2-hydroxy- and 2-sulfanylbenzohydrazides with a series of aldoses (L-arabinose, D-ribose, L-ramnose, D-galactose, D-glucose, D-mannose) was studied by ¹H and ¹³C NMR spectroscopy. The condensation products of monosaccharides with 2-hydroxybenzohydrazide in DMSO-d ₆ solution exist as equilibrium mixtures of linear hydrazone and cyclic pyranose and furanose forms, the cyclic tautomers being represented by two stereoisomers (α- and β-anomers). The aldose condensation products with 2-sulfanylbenzohydrazide in the crystalline state have cyclic 1,3,4-benzothiadiazepine structure, while in DMSO-d ₆ solution they undergo complete or partial isomerization into cyclic pyranose tautomer.     

S2 in Drug Synthesis

Abstract

Two convenient methods for the preparation of S₂ have been developed and a third, based on binaphthyl chemistry, Is delineated. Although S₂ additions to acyclic 1,3-dienes afford 1,2-dlthlin Dlels-Alder adducts, cyclic 1,3-dienes give bicycllc trisulfides. A [3,3] sigmatropic rearrangement Initiated from an additional S₂ addition to the Diels-Alder adduct produced with cyclic dienes is proposed to account for the latter result. S₂ also adds to strained olefins. However, it does not participate in the “ene” type reactions that plague singlet oxygen chemistry. Several of the 1,2-dithiln adducts, prepared from S₂ additions, have been found to have antimicrobial properties, and more importantly, some also to Inhibit the in vitro HIV infection of H9 type tissue cells.     

Synthesis,characterization, electrochemical and crystal structure investigation of bis(2-((2-aminoethylimino)methyl)-6-methoxyphenolato) cobalt(III)

The mononuclear cobalt(III) complex [Co(L)₂]Cl ·?H₂O (1) (where L is H₂N(CH₂)₂N=CC₆H₃(OMe)(O^?)) has been prepared and characterized by IR, UV-Vis spectroscopy, conductivity measurements, elemental analysis, TGA, cyclic voltammetry and an X-ray structure determination. The cobalt(III) coordination sphere in [Co(L)2] is cis-CoN₄O₂ with the NNO ligands. Electrochemical studies of 1 using cyclic voltammetry indicate an irreversible cathodic peak (E _pc, ca ?0.60 V) corresponding to reduction of cobalt(III) to cobalt(II).     

Three‐dimensional hydrogen‐bonded structures in the guanidinium salts of the monocyclic dicarboxylic acids rac‐trans‐cyclohexane‐1,2‐dicarboxylic acid (2:1, anhydrous), isophthalic acid (1:1, monohydrate) and terephthalic acid (2:1, trihydrate)

The structures of bis(guanidinium) rac‐trans‐cyclohexane‐1,2‐dicarboxylate, 2CH₆N₃⁺·C₈H₁₀O₄²⁻, (I), guanidinium 3‐carboxybenzoate monohydrate, CH₆N₃⁺·C₈H₅O₄⁻·H₂O, (II), and bis(guanidinium) benzene‐1,4‐dicarboxylate trihydrate, 2CH₆N₃⁺·C₈H₄O₄²⁻·3H₂O, (III), all reveal three‐dimensional hydrogen‐bonded framework structures. In anhydrous (I), both guanidinium cations form classic cyclic R₂²(8) N—H...O,O′_carboxylate and asymmetric cyclic R₂¹(6) hydrogen‐bonding interactions, while one cation forms an unusual enlarged cyclic interaction with O‐atom acceptors of separate ortho‐related carboxylate groups [graph set R₂²(11)]. Cations and anions also associate across inversion centres, giving cyclic R₄²(8) motifs. In the 1:1 guanidinium salt, (II), the cation forms two separate cyclic R₂¹(6) interactions, one with a carboxyl O‐atom acceptor and the other with the solvent water molecule. The structure is unusual in that both carboxyl groups form short interanion O...H...O contacts, one across a crystallographic inversion centre [O...O = 2.483 (2) Å] and the other about a twofold axis of rotation [O...O = 2.462 (2) Å], representing shared sites on these elements for the single acid H atom. The water molecule links the cation–anion ribbon structures into a three‐dimensional framework. In (III), the repeating molecular unit comprises a benzene‐1,4‐dicarboxylate dianion which lies across a crystallographic inversion centre, two guanidinium cations and two solvent water molecules (each set related by twofold rotational symmetry), and a single water molecule which lies on a twofold axis. Each guanidinium cation forms three types of cyclic interaction with the dianions: one R₂¹(6), the others R₃²(8) and R₃³(10) (both of these involving the water molecules), giving a three‐dimensional structure through bridges down the b‐cell direction. The water molecule at the general site also forms an unusual cyclic R₂²(4) homodimeric association across an inversion centre [O...O = 2.875 (2) Å]. The work described here provides further examples of the common cyclic guanidinium–carboxylate hydrogen‐bonding associations, as well as featuring other less common cyclic motifs.     

Cyclic Heptapeptides Axinastatin 2, 3, and 4: Conformational analysis and evaluation of the biological potential

The conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two-dimensional NMR spectroscopy in combination with distance-geometry (DG) and molecular-dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D₆)DMSO. Axinastatin 2 was also investigated in CD₃OH. In all structures, Pro² is in the i + 1 position of a βI turn and Pro⁶ occupies the i + 2 position of a βVIa turn about the cis amide bond between residue 5 and Pro^6. In all peptides, a bifurcated H-bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn I_g turn was found about Asn¹ and Pro^2. We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A β-bulge motif with two β turns and one bifurcated H-bond is found as the dominating backbone conformation of cyclic all-L-heptapeptides. Axinastatin 2, 3, and 4 can be characterized by six trans and one cis amide bond resulting in a β/βVI(a)-turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be inactive or of low activity.     

Electrocatalytic reduction of O2 at pyrolytic graphite electrode modified by a novel copper(II) complex with 2-[bis(2-aminoethyl)amino]ethanol and imidazole ligands

A new complex formed by Cu(II) with 2-[bis(2-aminoethyl)amino]ethanol and imidazole is prepared, and its electrochemical properties are studied. The electrochemical experiments are carried out in deaerated pH 7.0 buffer solution through cyclic voltammetry by scanning the potential from 0.1 to −0.5 V with this copper(II) complex-modified electrode as the working electrode. One redox process is observed, which could be assigned to Cu(II)/Cu(I). The formal potential E _0′ = (E _pa + E _pc)/2, where E _pa and E _pc are anodic and cathodic peak potentials, is −248 mV vs. SCE. A straight line, obtained from the plot of I _pc vs. v, indicated a surface-controlled reaction. The modified electrode is very stable and exhibits catalytic activity for oxygen reduction. The possible mechanism for the catalytic reduction of oxygen is studied by cyclic voltammetry and chronoamperometry. The results show that the dioxygen is reduced via a pathway of four-electron reduction to form water. Chronoamperometric measurements show the potentiality of the use of this working electrode as an amperometric sensor for dissolved dioxygen in aqueous media. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 8, pp. 975–979. The text was submitted by the authors in English.     

A Facile Method for the Epoxidation of Cyclic Enone Catalyzed by tert-BuNH2 with 30% Hydrogen Peroxide as Oxidant

It was found that tert-BuNH_2 is an efficient catalyst for epoxidation of α,β-unsaturated ketones with 30% H_2O_2under mild conditions.For most of the cyclic unsaturated ketones used,moderate to excellent yields (45%—93%)were obtained within 24 h period.     

High-level ab initio calculations on CH+2(2A1) + PO(2II) reactions

We present ab initio calculations carried out in the framework of the G 2 theory on the singlet and triplet potential energy surfaces corresponding to the gas-phase between CH⁺₂ and PO. The global minimum of both potential energy surfaces is a cyclic singlet-state cation. Oxygen attachment of PO to CH⁺₂ in a triplet configuration is accompanied by a P(SINGLEBOND)O bond fission, with the result that the corresponding global minimum is an ion-dipole complex between P⁺(³P) and formaldehyde. This is also consistent with the fact that our results predict the formation of formaldehyde to be highly exothermic, either as a neutral or as radical cation. Both charge-transfer processes yielding CH₂(³B₁) or CH₂(¹A₁) are also exothermic. The formation of other carbon and oxygen containing species are endothermic. © 1996 John Wiley & Sons, Inc.     

(L)2C2P2: Dicarbondiphosphide Stabilized by N-Heterocyclic Carbenes or Cyclic Diamido Carbenes

Carbon phosphides, C_nP_m, may have highly promising electronic, optical, and mechanical properties, but they are experimentally almost unexplored materials. Phosphaheteroallenes stabilized by N-heterocyclic carbenes undergo a one-electron reduction to yield compounds of the type (L)₂C₂P₂ with diverse structures. The use of imidazolylidenes as ligands L give complexes with a central four-membered ring C₂P₂, while more electrophilic cyclic diamidocarbenes (DAC) give a compound with an acyclic π-conjugated CP−PC unit. Cyclic C₂P₂ compounds are best described as non-Kekulé molecules that are stabilized by coordination to the NHC ligands NHC→(C₂P₂)←NHC. These species can be easily oxidized to give stable radical cations [(NHC)₂C₂P₂]^+.. The remarkably stable molecules with an acylic C₂P₂ core are best described with electron-sharing bonds (DAC)=C=P−P=C=(DAC).     

Theoretical investigation of the cyclic GaO2 and GaS2 molecules at DFT and correlated wave function levels

The geometries and the bonding properties have been predicted for cyclic GaO₂ and GaS₂ species at density functional theory (DFT), MPn (n=2,3,4 with different substitutions), QCISD(T), and CCSD(T) all‐electron correlation levels with 6‐311+G* basis set. The geometrical optimizations and the harmonic vibrational frequency analysis are performed using DFT and second‐order Møller–Plesset (MP2) methods. The relevant energy quantities are also calibrated at the high‐order electron correlation levels [MP3, MP4, quadratic configuration interaction (QCI), and coupled cluster (CC)]. Each species possesses a ²A₂ ground state with a higher energy level ²A₁ state. The corresponding state–state separations are about 32 kcal/mol for GaO₂ species and about 20 kcal/mol for GaS₂ species at the QCISD(T)/6‐311+G* level. The QCISD(T) and CCSD(T) calculations yield dissociation energies of 42.0 and 59.0 kcal/mol for two species, respectively, and other methods yield dissociation energies within ∼5 kcal/mol. Result analysis has indicated that the cyclic GaO₂ should be classified as superoxide and the GaS₂ species should be classified as supersulfide in their ground state, and those in the excited state (²A₁) should not be. However, the cyclic GaS₂ (²A₂) is less ionic than the GaO₂ (²A₂) and they are far less ionic than NaO₂. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 222–231, 2001     

Electrochemical Reduction of Cyclohex-2-enones

The electrochemical reduction of the cyclohex-2-enones 1a–1e (mercury cathode, CH₃CN, Bu₄NBF₄) was studied by means of cyclic voltammetry, d.c. polarography, coulometry and chemical product analysis. Compounds 1a–1c give a mixture of the hydrodimers 4 and 5 via formation of the radical anion 2 by an irreversible one electron transfer, followed by protonation and dimerization of the allylic radical 3 . The 6-halocyclohex-2-enones 1d and 1e exhibit two distinct reduction waves. The first corresponds to an irreversible two electron transfer with formation of the halide anion and the enolate anion 6 which gives 1b by protonation. The second wave corresponds to a quasi-reversible one electron transfer to 6 to afford the radical dianion 7 (Scheme 2).     

Comparison of the Complexation of Open-Chain and Cyclic N2S2 Ligands with Cu+ and Cu2+

The complexation properties of the open-chain N₂S₂ ligands 1–4 are described and compared to those of analogous N₂S₂ macrocycles 5–7 . With Cu²⁺, the open-chain ligands give complexes with the stoichiometry CuL²⁺ and CuLOH⁺, the stabilities and absorption spectra of which have been determined. The ligand field exerted by these ligands is relatively constant and independent of the length of the chain. With Cu⁺, the species CuLH, CuLH²⁺, and CuL⁺ were identified and their stabilities measured. The redox potentials calculated from the equilibrium constants and measured by cyclic voltammetry agree and lie between 250 and 280 mV against SHE. The comparison between open-chain and cyclic ligands shows that (1) a macrocyclic effect is found for Cu²⁺ but not for Cu⁺, (2) the ligand-field strength is very different for the two types of ligands, and (3) the redox potentials span a larger interval for the macrocyclic than for the open-chain complexes.     

Synthesis and Characterization of Diorganotin Compounds {[R2Sn（ON =CHC6H5）]2O}2 and Crystal Structure of {[（C6H5CH2）2Sn（ON=CHC6H5）]2O}2

Introduction Dimeric tetraorganodistannoxanes are a kind of in-teresting organotin oxo clusters and have attracted con-siderable attention during the last several decades, in view of their unique structural features1-5 as well as their applications as biocides6,7 and in homogenous cataly-sis.8,9 In the solid state, they contain characteristic Sn4O2X2Y2 structural motifts with staircase or ladder arrangements, a planar four-membered Sn2O2 ring and, generally, penta-coordination around the tin…     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

Reexamination of the C2H4F potential surface. the status of the classical 2-fluoroethyl cation: a local minimum or transition structure?

According to ab initio molecular orbital calculations carried out with full geometry optimization at the MP2/6–31G^** level, the classical 2-fluoroethyl cation, FCH₂CH₂⁺, is a transition structure for H-scrambling in CH₃CHF⁺. Single point MP4/6–31G^** calculations at the optimized geometries predict the cyclic ethylene fluoronium ion to lie 24.2 kcal mol⁻¹ above CH₃CHF⁺ and 5.4 kcal mol⁻¹ below the 2-fluoroethyl cation. ΔG‡ for ring opening of the cyclic fluoronium ion at -60° is estimated to be ca 15 kcal mol⁻¹. This barrier is largely attributable to the powerful negative fluorine hyperconjugation in the transition state as described by Hoffmann and coworkers. When electron correlation effects are ignored a qualitatively different potential surface is obtained on which the 2-fluoroethyl cation is calculated to be a local minimum separated from the stable 1-fluoroethyl cation by an H-bridged transition state.     

Calculating the Thermodynamics of Weakly Hydrogen-Bonded Complexes from Heteronuclear NMR Data: Base-pairing stabilities of a 5-methyl(15N2)[O2, O4-17O2]uridine (= (15N2)[O2,O4-17O2]ribosylthymine) derivative,and structural implications

2′,3′-O-Isopropylidene-5-methyl(¹⁵N₂)[O²,O⁴-¹⁷O₂]uridine (= 2′,3′-O-isopropylidene (¹⁵N₂)[O²,O⁴-¹⁷O₂]-ribosylthymine; 1 ) was analyzed by ¹⁵N-and ¹⁷O-NMR spectroscopy. The ¹⁵N and ¹⁷O chemical shifts revealed, in the absence and presence of unlabelled 2′,3′-O-isopropylideneadenosine ( 2 ), the formation of thymine-thymine and thymine-adenine base pairs in CHCl₃. As expected, cyclic complexes stabilized by two H-bonds occurred at low temperatures, but at elevated temperatures, the data suggest that open complexes involving only one H-bond prevailed. The ¹⁷O-NMR data showed the cyclic thymine-adenine pair in a reverse base pair geometry. The open base pair involved contacts to the urea-derived carbonyl O-atom of thymine. The thermodynamics of complex formation of the cyclic and open forms in both homo and hetero pairs were calculated from the temperature and concentration dependence of the ¹⁵N-NMR data using a new method. It involves a fitting procedure onto the experimental isotherms using a theoretically derived function with the standard Gibbs free energy as a parameter to be optimized. ΔH° and ΔS° were derived from a linear regression of ΔG°(T) vs. T. The fitting procedure circumvents the baseline problem and could be automated and used to calculate correct thermodynamics from UV-monitored melting curves of oligonucleotides. Since titrations are not involved, this dilution method should also be a useful alternative for stability studies of supramolecular complexes in H₂O and in organic solvents.     

(2S)‐1‐Carbamoylpyrrolidine‐2‐carboxylic acid

In the title compound, also known as N‐carbamoyl‐l ‐proline, C₆H₁₀N₂O₃, the pyrrolidine ring adopts a half‐chair conformation, whereas the carboxyl group and the mean plane of the ureide group form an angle of 80.1 (2)°. Molecules are joined by N—H...O and O—H...O hydrogen bonds into cyclic structures with graph‐set R²₂(8), forming chains in the b‐axis direction that are further connected via N—H...O hydrogen bonds into a three‐dimensional network.     

Internal conrotation and disrotation in H2BCH2BH2 and diborylmethane 1,3 H exchange

The paths of correlated internal disrotation (barrier less than 0.4 kcal/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH₂ groups in H₂BCH₂BH₂ have been computed employing ab initio [MP2(full)/6–31G**] and density functional theory (Becke3LYP/6–311+G**) methods. Two B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative interactions stabilize the C_s symmetric H₂BCH₂BH₂ isomer ( 1 ). The B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative stabilization, evaluated by homodesmotic reactions and using the orbital deletion procedure (which “deactivates” the “vacant” born p orbital), is less than 6 kcal/mol in diborylmethane. The B(SINGLE BOND)C(DOTTED BOND)B(p) stabilization is shown to be remarkably large in C₄B₆H₁₀ (Td). At MP2(fu)/6–31G**, disproportionation into 1 and methane is only 5.6 kcal/mol exothermic. The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds via a doubly bridged cyclic intermediate with 9.3 kcal/mol barrier. Structures with “planar tetracoordinate” carbon are stabilized considerably by BH₂ substituents, but they are still high in energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1792–1803, 1997     

Microsynthesis and mass spectral study of Chemical Weapons Convention related 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides

Chemical Weapons Convention (CWC)‐related compounds where the phosphorus atom is part of a ring have very limited representation in mass spectral libraries and the open literature. Here we report electron ionization (EI), chemical ionization (CI) and electrospray ionization tandem mass spectrometry (ESI‐MSⁿ) spectra and retention indices for 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides (alkyl C₁ to C₃) which are new cyclic chemicals covered under the CWC. The EI mass spectra show a pattern of ion fragmentation that is similar to that of other cyclic phosphonates in that loss of the alkylphosphonic acid as a neutral loss is more important than the presence of the protonated alkylphosphonic acid. In contrast to other cyclic phosphonates, the 2‐alkyl‐1,3,6,2‐dioxathiaphosphocane‐2‐oxides show almost no protonated alkylphosphonic acid and as a result the spectra do not carry the same distinctive signature of the phosphorus–carbon bond that is required for the chemical to be covered under the CWC. Copyright © 2008 John Wiley & Sons, Ltd.