bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal ion – biomolecule interactions. Part 14. Methylmercury and hydrogen ion catalysis of C(2)-H isotopic exchange in 1-methylhistidine
Authors: Buncel Erwin , Joly Helen A. , Yee Diane C.
Published: May 2006

The rate constants for detritiation from the C(2) position of 1-methyl[2-3H]histidine have been determined in a series of aqueous buffers at 85 °C. The resulting sigmoidal rate–pH profile was indicative of a mechanism involving hydroxide ion attack on the N(3)-protonated (4) and the amino-protonated (5) forms of 1-methylhistidine, and dissection of the kinetic data allowed the extraction of the second-order rate constants for the two pathways, k and k′. The unusually large value of k′ for a species not protonated at N(3) of the imidazole ring suggested the involvement of a kinetically equivalent zwitterionic form of the substrate (7). Comparison of the rate constant k with values determined previously for closely related substrates, such as histidine, 1-methylimidazole, and imidazole, led to the use of FMO theory to explain the effect of the various structural changes, e.g., the effect of methylation and a positively charged side chain on k and k′. The addition of MeHgNO3 resulted in a decrease in the pseudo-first-order rate constant for detritiation. The rate retardation was discussed in terms of two mechanisms (Schemes 2 and 3). Analysis of the data in terms of the various metal-ion-coordinated species present under the experimental conditions showed that the reactivity of the protonated substrate greatly exceeds that of the metal-coordinated species. The difference in the catalytic ability of H+ vs. MeHg+ is discussed in terms of the extent of positive charge developed on the ligating heteroatom in the ylide (carbenoid) reaction intermediate. Keywords: methylmercury, 1-methylhistidine, isotopic exchange, proton transfer, metal ion catalysis.
125Te and 19F NMR spectroscopic study of the hydrolysis of pentafluorotellurium chloride: evidence for trans- and cis-HOTeF4Cl and cis, mer-(HO)2TeF3Cl
Authors: Lawlor Lawrence J. , Martin Alban , Murchie Michael P. , Passmore Jack , Sanders Jeremy C. P.
Published: May 2006

The hydrolysis of TeF5Cl has been followed by 125Te and 19F NMR spectroscopy. The spectra show that the known series (HO)nTeF6−n(n = 2−6) are present. In addition the spectra are consistent with the presence of the previously unreported series (HO)nTeF5−nCl (n = 1–3) as well as unidentified fluorotellurium(IV) species. Evidence is presented for the new species trans- and cis-HOTeF4Cl and cis, mer-(HO)2TeF3Cl. The course of the hydrolysis of TeF5Cl is compared with that of SF5Cl. Keywords: trans-tetrafluorotellurium hydroxychloride, cis-tetrafluorotellurium hydroxychloride, cis,mer-tetrafluorotellurium dihydroxychloride, pentafluorotellurium chloride, hydrolysis.
The electron spin resonance spectra of dioxene radical cations
Authors: Davies Alwyn G. , Shields Charles J. , Evans Jeffrey C. , Rowlands Christopher C.
Published: May 2006

The electron spin resonance spectra of the radical cations of dioxene (1a), 2,3-dimethyldioxene (1b), 2,3-diphenyldioxene (1c), and benzodioxane (2) in fluid solution have been observed and analysed, with the help of ENDOR spectroscopy in the case of 1c. It is concluded that in 1c the molecule has overall C2 molecular symmetry. In all four compounds, the pseudo-axial and pseudo-equatorial protons of the methylene groups in the half-chair dioxene rings are distinguished by different hyperfine coupling constants, and simulation of the spectra over a range of temperatures has given the Arrhenius parameters for the ring inversion of 1a, 1b, and 2. Keywords: ESR spectra, radical cation, 1,4-dioxene, inversion barrier.
Ionization of sulfonic acids and dimethylsulfoxide basicity. A 13C nuclear magnetic resonance and calorimetric study
Authors: Benoit Robert L. , Fréchette Monique , Boulet Danielle
Published: May 2006

The ionization of trifluoromethane-, methane-, and benzene-sulfonic acids in Me2SO and in water has been studied by 13C nuclear magnetic resonance. While the protonation shifts of CF3SO3− and CH3SO3− in both solvents are too small to make a quantitative interpretation possible, the C6H5SO3− shift data in H2O–H2SO4 mixtures, when treated by the method of Cox and Yates, gave a pKa value of −2.5 for the ionization of C6H5SO3H. Our results are discussed in relation to those of other workers. Enthalpies of solution and of ionization of these acids and related compounds have been obtained in both solvents. Single ion enthalpies of transfer from water to Me2SO have been calculated from our data and from a transfer value for CF3SO3− (+1.9 kJ mol−1) based on the tetraphenylarsonium–tetraphenylborate extrathermodynamic assumption. The transfers are endothermic processes for the anions CH3SO3− (+29.6 kJ mol−1), C6H5SO3− (+ 20.2 kJ mol−1), and p-CH3C6H4SO3− (+ 21.8 kJ mol−1), and exothermic for H+(−27.4 kJ mol−1). These results are discussed in terms of the properties of the ions and solvents. The transfer value for H+ is a valuable measure of the basicity difference between bulk water and Me2SO. Keywords: sulfonic acids, ionization, dimethylsulfoxide, 13C NMR, calorimetry.
The mechanism of hydrolysis of 2-hydroxyethanesulfonyl chloride: the intermediacy of 1,2-oxathietane 2,2-dioxide (β-sultone)
Authors: King James Frederick , Khemani Kishan Chand
Published: May 2006

The hydrolysis of 2-hydroxyethanesulfonyl chloride (1) has been investigated with the aid of kinetic and product analysis studies. The results are quantitatively consistent with the mechanism of hydrolysis shown in Scheme 1, the chief features of which are (a) formation of β-sultone (2) and its rapid further reaction (the major pathway), together with (b) a minor direct hydrolysis route. The kinetics of both the β β-sultone formation and the direct hydrolysis shows two terms, one first order in 1 alone, and the other first order in hydroxide as well; the rates of the first- and second-order reactions are lowered by added sodium chloride. It is suggested (a) that the unimolecular β-sultone formation involves 1 in a complex with water (as in 9) and that the water acts as a general base in the cyclization to 2, and (b) the hydroxide-promoted reaction proceeds by cyclization of the conjugate base of 1 (i.e., 10). The unimolecular direct hydrolysis is regarded as a conventional hydrolysis of a sulfonyl chloride with attack of the water with general base assistance from a second water molecule. The hydroxide-promoted direct reaction in D2O leads to no uptake of deuterium, showing that the reaction does not go by way of the sulfene, and a reaction by way of a six-membered cyclic transition state is tentatively proposed. Evidence is presented that the chloride ion rate suppression is not primarily due to reaction of β-sultone with Cl− to give back 1; the possible origins of the effect are discussed. Keywords: sulfonyl chlorides, 2-hydroxyethanesulfonyl chloride, β-sultone, kinetics of sulfonyl chloride hydrolysis, mechanisms of sulfonyl chloride hydrolysis.
Solvatation et complexation des métaux dans les mélanges eau – fluorure d'hydrogène. Sn, Cd, Pb et Tl
Authors: Beaudoin Réjean , Ménard Hugues
Published: May 2006

The behavior of tin and thallium has been studied in water–HF mixtures. The formation of thallium fluoride complexes was not observed. Tin Sn2+ forms two complexes, SnF2 and SnF+, in mixtures containing less than 60% in HF. SnF2 is strong enough to prevent the formation of chloride complexes. SnF62− and SnF5− have been determined for Sn4+ in the same solutions. The relatively high strength of the tin fluoride complexes is reflected in the R0 (M) function. In fact, all the metals studied, Sn, Cd, Pb, and Tl, are less solvated in water–HF mixtures than in water, except for the case where the fluoride complex is strong, i.e., tin. HF2− seems to be the complexing species at high HF concentrations. Keywords: polarography, thallium, tin, cadmium, lead, hydrogen fluoride. [Journal translation]
Structural studies of organoboron compounds. XXXVIII. 2,2-Diphenyl-4,4-tetramethylene-1,3,5-trioxa-3a-azonia-2-borata-2,3,4,5-tetrahydro-1H-cyclopenta[a]naphthalene
Authors: Kliegel Wolfgang , Tajerbashi Mahmood , Rettig Steven J. , Trotter James
Published: May 2006

Reaction of oxybis(diphenylborane) with the condensation product of salicylohydroxamic acid and cyclopentanone gives the title compound in good yield. Crystals of 2,2-diphenyl-4,4-tetramethylene-1,3,5-trioxa-3a-azonia-2-borata-2,3,4,5-tetrahydro-1H-cyclopenta[a]naphthalene are monoclinic, a = 12.521(1), b = 12.409(1), c = 12.975(1) Å, β = 93.784(8)°, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.043 and Rw = 0.044 for 1502 reflections with I ≥ 2σ(I). The molecule has a typical diphenylboron hydroxamate structure featuring a five-membered COBON chelate ring. The O,O-chelating ligand is weakly bound to the diphenylboron moiety. Bond lengths (corrected for libration) include: (N)O—B = 1.558(4), (C=)O—B = 1.578(4), and C—B = 1.589(5) and 1.603(5) Å. Keywords: crystal structure, boron compound, organoboron compound.
The total synthesis of (+)-ryanodol. Part III. Preparation of (+)-anhydroryanodol from a key pentacyclic intermediate
Authors: Deslongchamps Pierre , Bélanger André , Berney Daniel J. F. , Borschberg Hans-Juerg , Brousseau Robert , Doutheau Alain , Durand Robert , Katayama Hajime , Lapalme Richard , Leturc Dominique M. , Liao Chun-Chen , MacLachlan Frederick N. , Maffrand Jean-Pierre , Marazza Fabrizio , Martino Robert , Moreau Claude , Ruest Luc , Saint-Laurent Louiselle , Saintonge Roger , Soucy Pierre
Published: May 2006

This paper reports several studies that were carried out to learn how to transform the dextrorotatory pentacyclic key intermediate 5 into (+)-anhydroryanodol (6). This transformation requires 24 steps, and takes place via the following intermediates: 5 → 11 → 19 → 34 → 36 → 39 → 144 → 150 → 6. The preparation of the key pentacyclic intermediate 5 is reported in Parts I and II of this series. Keywords: anhydroryanodol, ryanodol, diterpene, organic synthesis, strategy.
The total synthesis of (+)-ryanodol. Part IV. Preparation of (+)-ryanodol from (+)-anhydroryanodol
Authors: Deslongchamps Pierre , Bélanger André , Berney Daniel J. F. , Borschberg Hans-Jürg , Brousseau Robert , Doutheau Alain , Durand Robert , Katayama Hajime , Lapalme Richard , Leturc Dominique M. , Liao Chun-Chen , MacLachlan Frederick N. , Maffrand Jean-Pierre , Marazza Fabrizio , Martino Robert , Moreau Claude , Ruest Luc , Saint-Laurent Louiselle , Saintonge Roger , Soucy Pierre
Published: May 2006

This paper reports the transformation of (+)-anhydroryanodol (6) to (+)-ryanodol (7) by reductive cyclization of lactone epoxide derivatives 9 and 20 (Scheme 4). Keywords: anhydroryanodol, ryanodol, diterpene, synthesis, reductive cyclization.
Characterization and spectroelectrochemical properties of a binuclear trans,trans-bis(ferrocenylidene)acetone [(C5H5)Fe(C5H4CHCH)]2CO
Authors: Harvey Pierre D. , Sharman Jeff G.
Published: May 2006

The binuclear compound trans,trans-bis(ferrocenylidene)acetone [(C5H5)Fe(C5H4CHCH)]2CO (dFca) has been synthesized in quantitative yield and characterized by FT-IR, 1H and 13C NMR, and mass spectroscopy. Cyclic voltammetry and coulometry measurements suggest that a reversible two-electron oxidative process takes place at 0.68 V vs. SSCE. FT-IR and UV–VIS spectroelectrochemical analyses show that this oxidative process occurs at the ferrocenyl centers. The compound is also found to be emissive in the solid state (λmax = 780 nm), and charge transfer assignments for the lowest singlet and triplet excited states are made. Keywords: ferrocenyl, spectroelectrochemistry, electronic charge transfer, emission spectroscopy.
Étude cinétique de la N-chloration de l'acide cyanurique en phase aqueuse
Authors: Matte Denise , Solastiouk Bernard , Merlin André , Deglise Xavier
Published: May 2006

A complete kinetic study of the N-chlorination, in basic aqueous medium, of cyanuric acid by the stopflow spectrophotometric method is presented. Our experimental results can be interpreted by two pairs of kinetically indistinguishable mechanisms.[Formula: see text]or[Formula: see text]and[Formula: see text]or[Formula: see text]The rate constants and their corresponding activation energies were determined in the limiting case where one of the elementary steps in each pair is involved alone in the kinetics of reaction.[Formula: see text]The rate constants for hydrolysis of the monochlorinated cyanuric acid derivatives were derived from our experimental values. Keywords: kinetics, N-chlorination, 1,3,5-triazine-2,4,6-trione, aqueous medium, stopped flow.
Enantioselectivity of carbonic anhydrase catalyzed hydrolysis of mandelic methyl esters
Authors: Chênevert Robert , Létourneau Martin
Published: May 2006

We report the first enantioselective hydrolysis of esters catalyzed by carbonic anhydrase. We found that mandelic methyl esters are good substrates for carbonic anhydrase. The R enantiomers are better substrates and enantiomeric excess values are moderate (40–51%). Keywords: carbonic anhydrase, mandelic esters, enantioselectivity, hydrolysis.
The synthesis of protected 5-azido-5-deoxy-D-glucononitriles as precursors of glycosidase inhibitors
Authors: Bird P. , Dolphin D. H. , Withers S. G.
Published: May 2006

The successful syntheses of 5-azido-2,3,4,6-tetra-O-benzyl-5-deoxy-D-glucononitrile and 2,3,4,6-tetra-O-benzyl-5-deoxy-5-trifluoroacetamido-D-glucononitrile starting from D-glucose are described. Unsuccessful attempts were made to convert these two compounds into a protected 5-amino-5-deoxy-D-glucononitrile and to subsequently cyclize them to an amidine analogue of glucose as a possible glycosidase inhibitor. Keywords: synthesis, amino-sugars, glycosidase inhibitors.
Computations and measurements of the structures and conformations of 2- and 3-fluorobenzaldehyde in the gas and in solution
Authors: Schaefer Ted , Takeuchi Craig S.
Published: May 2006

Precise 1H and 19F nuclear magnetic resonance chemical shifts and spin–spin coupling constants are reported for 4 mol% solutions of 2-fluorobenzaldehyde (2FB) and 3-fluorobenzaldehyde (3FB) in CS2/C6D12/TMS/C6F6 and acetone-d6/TMS/C6F6 solvent mixtures at 300 K. A small amount of the O-cis conformer of 2FB is present even in the nonpolar solvent mixture, corresponding to a free energy difference of 7.6(3) kJ/mol between the planar O-cis and O-trans conformers. In the polar solvent, this number decreases to 4.5(2) kJ/mol. The O-cis and O-trans conformers of 3FB have very similar abundances in the two solvent mixtures, the former being favored by a free energy difference of 0.38(4) kJ/mol in the nonpolar medium and, unexpectedly, considering its highly polar nature, by only 0.27(4) kJ/mol in the polar environment. STO-3G MO computations, with geometry optimization, of the internal rotational potentials of 2FB and3FB confirm the planarity of the O-cis conformer of 2FB, that is, that the planar form is more stable than a somewhat twisted conformer. 6-31G MO calculations for the four planar conformers yield structures of potential use in the fitting of rotational spectra in the vapor. Extrapolation of the 1H nmr data for 2FB implies a free energy difference of 11.5 ± 0.6 kJ/mol at 300 K in the vapour favoring the O-trans form, midway between the two theoretical estimates. The present experiments and computations are compared with the latest rotational, vibrational, and electronic spectra and with other assessments of the relative conformer stabilities in the vapor and in solution. Keywords: NMR for 2- and 3-fluorobenzaldehyde; conformations in solution and vapor, for 2- and 3-fluorobenzaldehyde; MO calculations, 2- and 3-fluorobenzaldehyde.
Substituent effects in the bis(benzotriazolylmethylation) of aromatic amines
Authors: Katritzky Alan R. , Rachwal Stanislaw , Wu Jing
Published: May 2006

1-(Hydroxymethyl)benzotriazole and aromatic amines react to give an equilibrium mixture of N-mono- and N,N-bis(benzotriazolylmethyl)arylamines. Electron-releasing substituents on the arylamine ring shift the equilibrium towards the N,N-bis(benzotriazolylmethylated) products, and removal of water formed in the reaction leads to these bis derivatives in quantitative yields. These products were isolated as mixtures of three isomers, the benzotriazolyl-1,1′, -1,2′, and -2,2′ derivatives; these mixtures were characterized by full assignment of their 13CNMR spectra and by partial separation and enrichment. Electron-withdrawing substituents on the arylamine ring increase the isomer interconversion energy barriers and make N,N-bis(benzotriazolylmethyl)arylamines more resistant to hydrolysis. A strong influence of ortho alkyl groups on the stability and NMR spectral properties of N,N-bis(benzotriazolylmethyl)arylamines is interpreted on the basis of steric hindrance. Keywords: Mannich reaction, benzotriazole isomerization, substituent effects, anilines, bis(benzotriazolylmethyl)arylamines.
1,1-Dichlorocyclopropane derivatives of α and β ionones. Structure determination of 3R,3′S-bis[3-methyl-5-(1′,3′,3′-trimethylcyclohex-1′-en-2′-yl)-2(1H)-furanone]
Authors: Díaz E. , Toscano R. A. , Alvarez A. , Shoolery J. N. , Jankowski K.
Published: May 2006

1,1-Dihalocyclopropane derivatives and furanones were synthesized from ±α- and β-ionones using CHCl3, NaOH, and triethylbenzyl ammonium chloride (TEBAC). 2D NMR and single-crystal X-ray analyses of the products are reported. Keywords: addition of 1,1-dihalocarbenes to ionones, phase transfer addition of 1,1-dihalo systems, formation of cyclopropanes.
Novel Synthesis of monosulphated methyl α-D-galactopyranosides
Authors: Rashid Abdul , Mackie William , Colquhoun Ian J. , Lamba Doriano
Published: May 2006

The syntheses of the methyl α-D-galactopyranoside 2-, 3-, 4-, and 6-sulphates (11, 12, 16, and 21 respectively) have been achieved using trimethylamine – sulphur trioxide as the sulphating agent. The procedure used is high yielding and requires shorter reaction times than reported previously. Each compound was obtained in a crystalline form as the potassium salt. Improved syntheses are described of the 3,4-isopropylidene acetal 2 of the methyl α-D-galactopyranoside and methyl. 2-O-acetyl 4,6-O-benzylidene α-D-galactopyranoside 8, which are starting materials in the preparation of the 2- and 3-sulphates (11 and 12 respectively). An efficient route has been devised for the synthesis of methyl 2,3,6-tri-O-acetyl-α-D-galactopyranoside 14 using a base-catalysed transesterification (C4-OH → C6-OH). Keywords: synthesis, methyl α-D-galactopyranoside, monosulphates.
A theoretical study of silyl anions and radicals. Change of mechanism from vertex inversion to edge inversion with increasing substitution by halogens
Authors: Hopkinson A. C. , Rodriquez C. F. , Lien M. H.
Published: May 2006

Structures for trivalent silyl anions [Formula: see text] and [Formula: see text], where n takes values from 0 to 3, have been optimized at the HF/6-31 + +G* level and single point calculations made at the MP2/6-31 + +G* level (core included). SiH3− and ions containing one halogen invert by the vertex mechanism in which the lone-pair has π-symmetry, and the monosubstituted ions have high barriers (SiH2F− 45.2 kcal/mol and SiH2Cl− 44.0 kcal/mol). Further substitution by halogens results in a change to the edge inversion mechanism involving a T-shaped transition structure with the lone-pair coplanar with the ligands. Barriers (kcal/mol) at the MP2/6-31 + +G* level including ZPE are lower than for the monosubstituted ions and are SiHF2− 35.0, SiF3− 35.9, SiHCl2− 28.4, and SiCl3−32.5. In SiLi3− edge inversion is preferred, but the surface is much flatter and the barrier is low (9.8 kcal/mol). Trivalent silyl radicals SiHnF(3−n), SiHnCl(3−n) and SiFnCl(3−n) (with n having values 0 to 3) have also been examined with the 6-31 + +G* basis set, with optimization at the UHF level and single point calculations at the UMP2 level. Radicals SiH3, SiH2F, SiH2Cl, and SiHCl2 all invert by the vertex mechanism. Increased halogenation results in a change of mechanism and SiF3, SiCl3, SiF2Cl, and SiFCl2 invert by the edge mechanism. For radical SiHF2 the calculated barriers for the two mechanisms are almost identical with the higher level of theory slightly favouring edge inversion. Keywords: inversion mechanism, halogenated silyl radicals and anions.
Photochemistry of 2-azido-2-alkyl-3-oxobutanoic esters and amides
Authors: Edwards Oliver E. , Rank Werner
Published: May 2006

Photolysis of 2-alkyl-2-azido-3-oxobutanoic esters and amides gave products resulting from alkyl or acyl migration onto nitrogen. The resulting ketimimes could be isolated in moderate purity, but the N-acyl imines were rapidly converted into solvation products or enamides (N-acetyl dehydroamino acid esters) resulting from double bond migration. Mechanisms for formation of these products are discussed. The N-(2-azido-2-methyl-3-oxobutanoyl) derivative of the diketopiperazine from L-valyl-L-proline gave a stereoisomer of the cyclol tripeptide unit of ergovaline. Keywords: N-acyl imines, mechanism of 2-azido-3-keto ester photolysis, cyclol tripeptide synthesis, ergovaline.
A deuterium labelling study into the insertion of diphenylacetylenes into iron–aryl bonds
Authors: Butler Ian R. , Elliot Joanne L. , Houde Jr Jean
Published: May 2006

The mechanism of the reaction of dicarbonyl(η5-cyclopentadienyl)phenyliron, 1, with diphenylacetylene to give 2,3-diphenyl-[1,H]-inden-1-one has been investigated using a partially deuterated 1. The mechanistic pathway of the reaction is discussed in terms of the labelling of the indenones obtained. The reaction of phenylacetylene with 1 gives a mixture of products including 2,4,6-triphenyl-4′-methylbenzophenone and C-4,C-6a-dihydro-4-(4′-methylphenyl)-2-phenylpentalene(1-r-3aH)one. Keywords: indenones, acetylenes, iron, carbonyl.
The consecutive cyclization–elimination reaction of a radical generated from an epoxide: stereospecific formation of an exocyclic alkene
Authors: Lowinger Timothy B. , Weiler Larry
Published: May 2006

A titanium(III) mediated radical cyclization–elimination reaction is reported, which allows for the efficient generation of functionalized cyclopentane derivatives. The reaction is highly stereospecific, giving excellent control over the geometry of the resulting exocyclic alkene. In addition, functionality at both reacting termini is retained for subsequent transformations. Keywords: stereospecific, radical cyclization–elimination, exocyclic alkene.
The chemistry of thujone. XV. A versatile route to antifeedants of the polygodial family
Authors: Kutney James P. , Piotrowska Krystyna , Chen Yong-Huang , Cheng Kwok-Ping Norman , Gao Zhenyong , Rettig Steven J.
Published: May 2006

Utilizing thujone-derived intermediates, efficient routes to the natural polygodial family of antifeedants as well as novel analogues within this family are described. One of the key features of this study involves the insertion of an oxygen function into a tertiary carbon center adjacent to a cyclopropane ring system thus providing convenient intermediates for subsequent elaboration. An interesting ring expansion of five-membered to six-membered ring systems by means of tri-n-butyltin hydride is also described. Keywords: antifeedants, polygodial, thujone.
Isotope effects in solution: contrasts between muonium and hydrogen in reactions with acetone
Authors: Walker David C.
Published: May 2006

Munonium atoms are hydrogen isotopes with positive muons rather than protons, deuterons, or tritons as nuclei. Thus, they have one ninth the mass of 1H, and microsecond lifetimes. By using nuclear physics counting techniques, muonium can be studied in a wide variety of media and its chemical and physical properties used to appraise hydrogen. Results are described for the interaction of muonium with acetone, showing two types of kinetic isotope effects, formation of free radicals, evidence for intermolecular "muonium bonding", and micelle-induced enhancements of reaction rate constants. Keywords: isotope effects, muonium atoms, muonium bonding, thiyl radicals, micelles.
Structural studies of organoboron compounds. XLI. (3-Hydroxy-2-methyl-1-(p-tolyl)-4-pyridinonato)diphenylboron
Authors: Orvig Chris , Rettig Steven J. , Trotter James , Zhang Zaihui
Published: May 2006

(3-Hydroxy-2-methyl-1-(p-tolyl)-4-pyridinonato)diphenylboron has been prepared in high yield by the reaction of 3-hydroxy-2-methyl-1-(p-tolyl)-4-pyridinone with diphenylborinic acid. Crystals of the hemi-acetone solvate are triclinic, a = 15.119(2), b = 16.528(1), c = 9.5802(9) Å, α = 98.721(6), β = 93.344(8), γ = 102.443(6)°, Z = 4, space group [Formula: see text]. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.050 and Rw = 0.065 for 4330 reflections with I ≥ 3σ(I). The five-membered chelate rings in both of the two crystallographically independent molecules have flattened B-envelope conformations (maximum deviations from the mean planes = 0.114(4) and 0.065(5) Å). Mean bond lengths O—B = 1.553 and C—B = 1.599 Å are consistent with intermediate strength binding of the diphenylboron moiety by the O,O-chelating ligand. Keywords: crystal structure, organoboron compound.
Metal ion catalysis in nucleophilic displacement reactions at carbon, phosphorus, and sulfur centers. II. Metal ion catalysis in the reaction of p-nitrophenyl diphenylphosphinate with alkali metal phenoxides in ethanol
Authors: Dunn Edward J. , Moir Robert Y. , Buncel Erwin , Purdon J. Garfield , Bannard Robert A. B.
Published: May 2006

The reactions of p-nitrophenyl diphenylphosphinate (1) with lithium, sodium, potassium, and benzyltrimethylammonium phenoxides (BTMAOPh) have been studied by spectrophotometric techniques in anhydrous ethanol at 25 °C. The reactivity (kobs) of the alkali metal phenoxides increases in the order BTMAOPh < KOPh < NaOPh < LiOPh. The rate of reaction of 1 with LiOPh is enhanced when lithium salts (LiSCN, LiNO3, LiClO4, LiOAc) are added to the reaction media. The addition of the alkali metal complexing agents dicyclohexyl-18-crown-6 ether or [2.2.2]cryptand for Na+, and [2.1.1]cryptand for Li+, to each of the alkali metal phenoxide reactions resulted in a decrease in rate, indicating catalysis by the alkali metal ions. The kinetic data are analyzed to obtain specific rate coefficients of reactions of phenoxide and ethoxide as the dissociated ions and as alkali metal – phenoxide ion pairs. Reactivities follow the order [Formula: see text]; [Formula: see text]; [Formula: see text]; [Formula: see text]. A mechanism is proposed in which the ion-paired metal phenoxide is more reactive towards the substrate than is the dissociated phenoxide. Analysis of the data in terms of initial state and transition state interactions with metal ions indicates that the increased reactivity of the ion-paired species results from greater stabilization of the negatively charged transition state relative to stabilization of the ion-paired nucleophile. Keywords: nucleophilic displacement at phosphorus by phenoxide, alkali-metal-ion catalysis.
An efficient route to vinylporphyrins
Authors: Yon-Hin Paul , Wijesekera Tilak , Dolphin David
Published: May 2006

The use of 1-bromo-19-methylbiladienes-ac in the synthesis of vinylporphyrins is described. The effect of the vinyl precursors on the reactions of the dipyrromethene intermediates and the construction of the linear tetrapyrrole are discussed. High-yield syntheses of a methylvinylporphyrin, an H-vinyl analogue, as well as two A,C-divinylporphyrins are presented. Keywords: pyrrole, dipyrromethene, acetoxyethylpyrrole, chloroethylpyrrole, biladiene-ac, porphyrin, vinylporphryin, benzoporphyrin, decarboxylation, transesterification.
Synthesis of 14-hydroxy steroids. Total synthesis of methyl 14β-hydroxy-1,7,17-trioxo-5β,8-androstene-10β-oate and related compounds
Authors: Ruel Réjean , Deslongchamps Pierre
Published: May 2006

The total synthesis of the title compound 22 and methyl 14α-hydroxy-5β,13α,8-androstene-1,7,17-trioxo-10β-oate 21 isomer is reported. We also describe the 1,6-Michael addition of 2-methyl-1,3-cyclopentanedione on dienone 14 and the protic ammonium salt catalyzed intramolecular Michael addition of cyclic β-ketoester on the conjugated acetylenic ketone 13. Keywords: cardenolides, steroid synthesis, aldol, Michael addition.
Studies of electronic excited states of substituted norbornenes by UV absorption, electron energy loss, and HeI photoelectron spectroscopy
Authors: Wen A. T. , Hitchcock A. P. , Werstiuk N. H. , Nguyen N. , Leigh W. J.
Published: May 2006

Valence and inner-shell (C1s) electronic excitation spectra of norbornene, 2-methylnorbornene, and 2-trifluoromethylnorbornene have been measured by electron energy loss spectroscopy. These data have been complemented with gas- and solution-phase UV absorption spectra in the 5–6.7 eV range, HeI UV photoelectron spectra, and AM1 semi-empirical calculations for the three compounds. The valence shell energy loss and UV absorption spectra show marked variations throughout the series of compounds, suggesting the presence of at least three low-lying electronic excited states. The spectral bands have been assigned as due to π,R(3s), π,π*, and π, R(3p) excitations on the basis of their term values and the differences between their intensities in the solution and gas phase absorption spectra. The valence shell spectra indicate that while the vertically-excited π,R(3s) state is substantially lower in energy than the (vertically-excited) π,π* state in norbornene and the 2-methyl derivative, they are nearly isoenergetic in 2-trifluoromethylnorbornene. This difference in excited state manifolds is reflected in profound differences between the solution phase photochemistry of norbornene and the 2-trifluoromethyl derivative.The carbon 1s spectra are even more sensitive to substituent than the valence shell energy loss spectra. An interpretation is proposed which reflects the spatially localized character of inner-shell excitation and a "group orbitals" picture of the unoccupied MO's as accessed by C1s excitation. Keywords: Rydberg, electron energy loss, UV absorption, UV photoelectron spectroscopy, alkene, gas-phase.
Cyclobutene photochemistry. Substituent and wavelength effects on the photochemical ring opening of monocyclic alkylcyclobutenes
Authors: Leigh William J. , Zheng Kangcheng , Clark K. Brady
Published: May 2006

The photochemical ring opening of cis- and trans-3,4-dimethyl-, 1,3,4-trimethyl-, and 1,2,3,4-tetramethylcyclobutene (1, 3, and 4, respectively) has been investigated in hydrocarbon solution with 193 nm and 214 nm light sources. Ring opening is non-stereospecific in all cases at both wavelengths. The ratio of dienes formed by the formally allowed to formally forbidden pathways in the photolysis of these compounds is highest (ca. 2) for the trimethylcyclobutenes, and approximately 1 for both cis and trans isomers of the di- and tetramethylcyclobutenes with 193 nm excitation. The diene distributions from photolysis of all compounds but cis-3 show slight wavelength dependence. Gas- and solution-phase UV absorption spectra are reported for 3 and 4, and indicate that there are at least three singlet excited states accessible in the 185–230 nm region in these molecules. The π,R(3s) state is the lowest energy state in the gas phase in 3 and 4. The results verify that orbital symmetry factors do not play a role (or a consistent one, at least) in controlling the stereochemistry of the reaction, but they do not allow a firm assignment of the excited state(s) responsible for ring opening. Direct photolysis of these compounds also results in fragmentation to yield Z-2-butene (from cis-3 and 4) or E-2-butene (from trans-3 and 4) in addition to propyne or 2-butyne. The 2-butenes are formed with greater than 90% stereospecificity in all cases. The structures of the four 3-methyl-2,4-hexadiene isomers obtained from photolysis of 3 have been assigned on the basis of 1H NMR spectroscopy and the results of thermolysis of the two cyclobutene isomers. Keywords: cyclobutene, photolysis, Rydberg, orbital symmetry, far-UV, solution phase, UV spectra.
Lithium aluminium hydride reduction of some triarylvinyl bromides and acetates catalyzed by some transition metal chlorides
Authors: Obafemi Craig A. , Lee Choi Chuck
Published: May 2006

A number of triarylvinyl halides and acetates were reduced with lithium aluminium hydride using various transition metal chlorides as catalysts. The vinylic halides were reduced to the corresponding alkenes while the vinylic acetates were reduced to mixtures of triarylketones and alcohols. The reduction of labeled vinylic halides did not result in any scrambling of the label from C-2 to C-1. The reactions took place under mild conditions and relatively fast reaction times. Keywords: triarylvinyl chlorides, triarylvinyl acetates, lithium aluminium hydride reduction, 1,2,2-triarylethanol, 1,1,2-tri-p-tolyl[1-13C]ethane.
Synthesis and biological activity of (R)-2′-fluorocarminomycin
Authors: Baer Hans H. , Mateo Fernando Hernández
Published: May 2006

Methyl 3-amino-2,3,6-trideoxy-2-fluoro-(β-L-talopyranoside was hydrolyzed to the free sugar, (R)-2-fluorodaunosamine hydrochloride, which was converted into the α-1,4-di-O-acetyl-N-trifluoroacetyl derivative. The latter was condensed with carminomycinone by use of trimethylsilyl triflate as the activating agent, and the product was deprotected to give the title compound. Cytotoxicity of the new fluoroanthracycline against a number of tumor cell lines in vitro equalled that of parent carminomycin, and activities of the two compounds against P-388 murine leukemia in vivo were the same, although the fluoro derivative was fourfold less potent and appeared to be somewhat less toxic. Keywords: (R)-2-fluorodaunosamine hydrochloride, synthesis; (R)-2′-fluorocarminomycm, synthesis and biological activity.
The chlorination of propiophenone; determination of pKa value and of the course of the reaction
Authors: Guthrie J. Peter , Cossar John
Published: May 2006

Chlorination of propiophenone in alkaline aqueous solution leads to formation of α-hydroxypropiophenone as the first detectable intermediate; this undergoes slower oxidation to aromatic acids without any accumulation of further intermediates. From the rates of the initial chlorination we have been able to determine the pKa value as 17.56 ± 0.51. Keywords: propiophenone, chlorination, hydrolysis, rearrangement, pKa.
Stabilities in the gas phase of the hydrogen bonded complexes, YC6H4OH-X−, of substituted phenols, YC6E4OH, with the halide anions X− (Cl−, Br−, I−)
Authors: Paul Gary J. C. , Kebarle Paul
Published: May 2006

The equilibria, YPhOH + Br− = YPhOH-Br−, involving 26 differently substituted phenols, were determined with a pulsed high pressure mass spectrometer. The −ΔG0 evaluated from the equilibrium constants represent the hydrogen bond free energies in YPhOH-Br−. These data and data for X− = Cl− and I−, determined previously in this laboratory, are used to examine the substituent effects on the hydrogen bonding. It was found that the hydrogen bond energies in YPhOH-X− increase approximately linearly with the gas phase acidities of the phenols, YPhOH. This is in agreement with earlier observations that showed the bond energies in AH-B−, where AH were oxygen and nitrogen acids and B− closed shell anions, increase with increasing acidity of AH.A detailed analysis of the substituent effects, which is possible for YPhOH-X−, shows that the relationship with the acidity of AH can be divided into two parts. One is the increasing extent of actual proton transfer from AH on formation of the hydrogen bonded complex. Such proton transfer occurs in YPhOH-X− only for the series X− = Cl−. The second effect, which occurs for Cl− and is dominant for Br− and I−, is not directly related to the acidity of the phenols (or AH in general) but depends on a similarity of the substituent effects on the acidity and the stabilization of YPhOH-X− (or AH-B− in general). The dominant contribution to YPhOH-X− stabilization in this case is due to the field effects of the substituents, i.e., π delocalization plays only a small part. Therefore, the correlation with the acidity of YPhOH, where π delocalization is important, is not very close. Keywords: hydrogen bonding, substituent effects, ion–molecule equilibria, stability constants, thermochemistry.
Stereocontrolled construction of A.B.C.[6.6.6] tricycle via transannular Diels–Alder reaction of 14-membered macrocycle with acetylene as dienophile
Authors: Xu Yao-Chang , Cantin Michel , Deslongchamps Pierre
Published: May 2006

The syntheses of the acyclic dienynes 14a (cis-trans-acetylene) and 14b (trans-trans-acetylene) are described. The tandem macrocyclization and transannular Diels–Alder reaction of the allylic chloride 14b was conducted in the presence of Cs2CO3 at 85 °C in one-pot to provide tricyclic product 16, with the two hydrogens in ring B being cis. On the other hand, treatment of the allylic chloride 14a under the same conditions afforded 14-membered macrocycle 17, which could be transformed at 250 °C into a similar tricyclic product 18 with the two hydrogens in ring B being trans, along with an interesting by-product 19. The mechanism for the formation of 19 is also discussed. Keywords: stereocontrolled synthesis, macrocycle, tricyclic compound, transannular Diels–Alder reaction.
Nouveaux modèles siliciés dérivés de la quinoléine
Authors: Grignon-Dubois Micheline , Fialeix Michelle , Rezzonico Bernadette
Published: May 2006

Silylation of quinoline with Me3SiCl/Li/THF reagent has been studied. We obtained new tri-, tetra-, and hexasilylated derivatives whose stereochemistry has been established using NMR data. Comparison of these results to those previously obtained from naphthalene shows the effect of nitrogen on the silylation process. Keywords: silylation, quinoline, polysilyl-hydro-quinolines.
Modelling complex solution equilibria. II. Systems involving ligand substitutions and uncertainties in equilibrium constants derived from formation constants
Authors: Potvin Pierre G.
Published: May 2006

A general formalism is presented that expresses equilibrium constants K as combinations of pseudo-formation constants ψ. For equilibria involving complexes as reactants that undergo ligand substitutions, this formalism allows a reduction in the number of equations needed to model the system for faster calculation of species concentrations and refinement of unknown parameters. Gauss–Newton refinement of log ψ parameters is shown to be equivalent to that of log K parameters. Equations are developed for the calculation of uncertainties in log K and correlation coefficients between them from the corresponding values obtained for log ψ parameters. These equations can also be used to calculate the same quantities for constants (such as Ka's) derived from true formation constants. Keywords: Gauss–Newton, ligand substitution, equilibrium constants, formation constants.
The photochemistry of cylopentadienylmolybdenum complexes including the mechanism of the cis-trans photoisomerism of (η5-C5H5)Mo(CO)2LX, X = Cl Br, I; L = PPh3, PBu3
Authors: Debad Jeff D. , Hill Ross H.
Published: May 2006

Photolysis of trans-(η5-C5H5)Mo(CO)2LX (L = PPh3, PBu3; X = Br, I), at 12 K in 1,2-epoxyethylbenzene, results in CO loss. The product is formed as a specific isomer of (η5-C5H5)Mo(CO)LX, isomer 1. This isomer, upon warming, yields only trans-(η5-C5H5)Mo(CO)2LX in its reaction with CO. Photolysis of cis-(η5-C5H5)Mo(CO)2LX (L = PPh3, PBu3; X = Cl, Br, I) under similar conditions also results in CO loss to generate the unsaturated intermediate, (η5-C5H5)Mo(CO)LX as a different geometric isomer, isomer 2. The unsaturated intermediate, isomer 2, reacts thermally with CO to generate only cis-(η5-C5H5)Mo(CO)2LX. The geometry of these unsaturated intermediates and their interaction with solvent is discussed. Keywords: mechanism, isomerism, photochemistry, molybdenum.
Synthesis of 6-O-octyl-β-D-galactopyranosyl-(1 → 5)-L-arabinose and comparative 1-deoxyglycitolation of Nα-Z-L-lysine with amphiphilic lipodisaccharides
Authors: Cabaret Daniel , Wakselman Michel
Published: May 2006

A lipodisaccharide possessing a reactive aldopentose unit, 6-O-octyl-β-D-galactopyranosyl-(1 → 5)-L-arabinose 6, was obtained by glycoside synthesis. To avoid a possible intramolecular acyl transfer, benzoyl protecting groups and mild conditions of detritylation were used in the preparation of the furanosyl acceptor. The reductive alkylation of Nα-Z-L-lysine was then studied and compared to that of previously prepared liposaccharides. In this reaction, amphiphilic five-membered hemiacetals are generally more reactive than their six-membered analogues. The newly prepared disaccharide is the most reactive of the series and also the easiest to prepare. Therefore this reagent has been selected for a future study on the chemical modification of enzymes and the use in organic solvents of the biocatalysts obtained. Keywords: liposaccharides, reductive alkylation.
Étude chimique et cinétique de l'oxydation homogène en phase gazeuse d'alcanes légers. II. Propane et mécanisme généralisé
Authors: Vogin B. , Baronnet F. , Scacchi G.
Published: May 2006

An experimental study of the homogeneous gas phase oxidation of propane at 350 °C and subatmospheric pressure has been performed in order to identify and to measure the major primary products of the reaction. The experimental results have been interpreted by a chain radical mechanism, deduced from these results and from estimates of the rate constants for the elementary steps obtained by the methods of Thermochemical Kinetics. The proposed elementary steps are discussed and compared with the experimental observations. The results that we have obtained and their interpretation are compared with a similar detailed investigation performed on the oxidation of isobutane. As in the case of isobutane, two parallel reaction pathways appear, a dominant one leading to the conjugated alkene (propylene) and another one leading to the epoxide of this olefin (here propylene oxide). The oxidation of isobutane and that of propane appear to be quite similar, which corroborates the results that we have obtained. Key words: oxidation, kinetics, reaction mechanism, propane, thermochemical kinetics.
Controlled diastereoselection in 2-lithio-1,3-dithiane additions onto α-substituted γ-lactols. Model studies toward bryostatins from (R)-pantolactone
Authors: Roy René , Rey Allan W.
Published: May 2006

Homochiral α-substituted γ-lactols 3 and 4 derived from (R)-pantolactone 1 were used in 2-lithio-1,3-dithiane additions to afford very high controls in diastereoselectivities arising from 1,2-asymmetric inductions. Thus non-chelation controlled nucleophilic addition on 3 gave the anti diastereomer 5 as the major product (92% de), while the chelation controlled addition on 4 furnished the syn diastereomer 7 (96% de) as the almost exclusive product. The stereochemical outcomes of these reactions were proven unambiguously by locking the conformation of the syn- and anti-triol adducts 7 and 8 through their respective acetonides and by nuclear Overhauser enhancement measurements. The lack of 1,3-dioxolane formation in the case of the anti-triol 8 was taken as a further confirmation of the absolute configuration at the newly created stereocenter. Key words: byrostatin, pantolactone, α-hydroxylactol, dithiane.
A stereocontrolled intramolecular cycloaddition – sigmatropic rearrangement approach to a tricyclo[7.4.01,6.01,10]tridecane precursor for (±) gascardic acid
Authors: Bérubé Gervais , Fallis Alex G.
Published: May 2006

A general intramolecular Diels–Alder anionic oxy-Cope strategy for the synthesis of tricyclic skeletons and its application to the preparation of the gascardic acid precursor 21 is described. In situ generation of the appropriate cyclopentadiene 9 by isomerization (Et3N) afforded the [4+2] adduct directly or with EtAlCl2 as catalyst. The requisite potassium salt of the 1,5 diene 16 rearranged with concomitant epimerization at 67 °C to the fused ring ketone 18. Selective allylic oxidation and reduction provided 21. Key words: Diels–Alder, oxy-Cope, sesterterpene, synthesis, cycloaddition.
Bimetallic titanium alkoxides: synthesis and structure of Cp2Ti(Ph)(μ-OCH2C(CH3)2CH2O)(Ph)TiCp2; a precursor to macrocyclic metallocene derivatives
Authors: Nadasdi T. Timothy , Stephan Douglas W.
Published: May 2006

The reaction of Cp2TiPh2 with 2,2-dimethylpropane-1,3-diol proceeds with the evolution of benzene affording the bimetallic titanocene alkoxide species Cp2Ti(Ph)(μ-OCH2C(CH3)2CH2O)(Ph)TiCp2, 1. Complex 1 crystallizes in the monclinic space group Cc with a = 25.448(7) Å, b = 8.483(s) Å, c = 14.512(3) Å, β = 101.52(2)°, Z = 4, and V = 3070(1) Å3. The structure was refined employing 1471 data with Fo > 3σFo and 138 variables to R values of R = 0.0588 and Rw = 0.0828. the dimeric nature of 1 is confirmed. The coordination spheres of the two Ti centres are pseudo-tetrahedral. The Ti—C σ bonds were found to be 2.21(1) and 2.25(1) Å. Although the phenyl substituents are directed towards each other, steric effects appear to be overcome by the approximately perpendicular orientation of the phenyl rings (in fact the angle between the planes is 81.9°). The Ti … Ti separation in 1 is 6.725 Å. The implications of the preparation of 1 regarding the mechanism of formation of early metal macrocyclic compounds are discussed. In addition, the ramifications for the synthesis of dissymmetric early metal macrocycles are considered. Key words: titanium alkoxides, alkoxide bridged, structure.
Sedum alkaloids. XI. Synthesis of sedinone and sedacrine by application of anodic oxidation
Authors: Driessens Frank , Hootelé ClaUDE
Published: May 2006

α-Methoxycarbamate 12, synthetic precursor of the 2,6-disubstituted alkaloids of Sedum acre, was obtained in high yield from 2-phenacylpiperidine 5; the key step of the synthesis rests on the anodic methoxylation, which allows the functionalization of carbon 6. Nucleophilic substitution of the methoxy group by an acetonyl chain leads, after the required transformations, to sedinone 23. Bromomethoxylation of the enecarbamate 19 followed by dehydrohalogenation and nucleophilic substitution of the methoxy group leads to sedacrine 33. In both cases, the nucleophilic substitution of the methoxy group leads to a cis 2,6-disubstituted piperidine derivative. Key words: synthesis, piperidine alkaloids, anodic methoxylation
1,n-Radical ions. Photosensitized (electron transfer) carbon–carbon bond cleavage. Formation of 1,6-radical cations
Authors: Arnold Donald R. , Lamont Laurie J. , Perrott Allyson L.
Published: May 2006

The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-1,4-dioxaspiro[4.5]decane (8), methyl cis- and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-1,4-dioxaspiro[4.5]decane (10), generated by photosensitized (electron transfer) irradiation, has been studied. Solutions of the ethers and acetals in acetonitrile–methanol (3:1), with 1,4-dicyanobenzene (2) serving as the electron acceptor, were irradiated with a medium-pressure mercury vapour lamp through Pyrex. The diphenyl derivatives 7 and 8 were reactive; 7 gave 6,6-diphenylhexanal dimethyl acetal (11) and 8 gave 2-methoxy-2-(5,5-diphenylpentyl)-1,3-dioxolane (12). These are the products expected from the intermediate 1,6-radical cation, formed upon carbon–carbon bond cleavage of the cyclic radical cation. The monophenyl derivatives 9cis and trans and 10 were stable under these irradiation conditions. The mechanism for the carbon–carbon bond cleavage and for the cis–trans isomerization is discussed. An explanation, based upon conformation, is offered for the lack of reactivity of 9 and 10. Molecular mechanics (MM2) calculations were used to determine the preferred conformation of 9cis and trans, and 10. Key words: photosensitization, electron transfer, radical cation, carbon–carbon bond cleavage, conformation.