Showing papers on "Substituent published in 1998"

Selective Hydrogenation of α,β-Unsaturated Aldehydes

Abstract: The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry [1,2] and pharmaceuticals [3], involves the selective hydrogenation of unsaturated carbonyl intermediates as a critical step. The hydrogenation of α,β-unsaturated carbonyls into saturated carbonyls is comparatively easy to achieve because thermodynamics favor the hydro-genation of the C═C bonds; therefore, research efforts were more directed at improving the selectivity to unsaturated alcohols. When a substituent is present on the carbon atom of the carbonyl group (i.e. with ketones), there is no chance to hydrogenate selectively the C═O bond, and saturated ketones are obtained with a high yield. This review is thus mostly restricted to the hydrogenation of α, β-unsaturated aldehydes into the corresponding unsaturated alcohols.

Synthesis and Nonlinear Optical, Photophysical, and Electrochemical Properties of Subphthalocyanines

Abstract: Novel boron(III) subphthalocyanines (SubPcs) soluble in organic solvents containing a variety of donor and acceptor substituent groups have been synthesized by boron trihalide-induced cyclotrimerization of adequately substituted derivatives of phthalonitrile in 1-chloronaphthalene. The choice of the substituents on the 1,2-dicyanobenzene derivatives has been made taking into account the high reactivity of the Lewis acid BCl3 toward many functional groups. Considering this limitation, we set out to synthesize phthalodinitriles equipped with iodo, nitro, alkyl- or arylthio, alkyl- or arylsulfonyl groups that are sufficiently stable under the required reaction conditions and also provide an easily accessible set of acceptor/donor substituents. The quadratic and cubic hyperpolarizabilities of these compounds as well as their linear optical and electrochemical properties have been measured by several techniques, including EFISH (at two wavelengths), HRS, and THG, steady-state and time-resolved absorption and f...

Parametrization of substituents: effects of fluorine and other heteroatoms on OH, NH, and CH acidities

Abstract: Fluorine leaves nobody indifferent; it inflames emotion, be that affections or aversions. As a substituent it is rarely boring, always good for a surprise, but often completely unpredictable. It behaves nonconformingly even when fundamental properties such as ionic dissocn. are under consideration. Although fluorine scores highest in the Pauling scale of electronegativity, its Hammett consts. (sm + 0.34, sp + 0.06) are all but spectacular. Alanine and a-fluoroalanine have virtually the same pKa values, whereas trifluoroalanine proves to be a fairly strong acid. Apparently, the smallest halogen emits different kinds of electronic effects which, depending on the given situation, may counterbalance or amplify each other. To gain better insight, a new approach is undertaken. A comparison between thermodn. acidity (proton dissocn. in aq. medium and in the gas phase) and kinetic proton mobility (rates of base-catalyze hydrogen isotope exchange and stoichiometric hydrogen/metal interconversion processes) allows key issues to be addressed, in particular to identify medium effects on acidity and to probe the additivity of substituent effects. In this way fluorine, the hetero substituent par excellence, which can have a stronger impact on the reactivity in its vicinity than any other element, may serve as a crucial test of any model concerning the origin and transmission of electronic effects. There are, however, also practical implications. Organometallic compds. carrying one, two, or several fluorine atoms are versatile intermediates in org. synthesis; they allow access to a variety of pharmaceutically, agrochem., or tech. attractive products. For this reason, organofluorine chem. surely must welcome any information that helps to generate more members of this family of reactive species or aids the development of new methodical concepts, such as optional site selectivity ; 120 refs . [on SciFinder (R)]

New 1alpha,25-dihydroxy-19-norvitamin D3 compounds of high biological activity: synthesis and biological evaluation of 2-hydroxymethyl, 2-methyl, and 2-methylene analogues

Abstract: New highly active isomers of the natural hormone 1alpha, 25-dihydroxyvitamin D3 possessing an exomethylene group at the 2-position were prepared in a convergent manner, starting with (-)-quinic acid and the corresponding (20R)- and (20S)-25-hydroxy Grundmann ketones. These 2-methylene-19-norvitamins were efficiently converted to the 2-methyl and 2-hydroxymethyl derivatives, some of which exhibited pronounced in vivo biological activity. Configurations of the A-ring substituents were determined by 1H NOE difference spectroscopy as well as by spin decoupling experiments. It was established that the bulky methyl and hydroxymethyl substituents at C-2, due to their large conformational free energies, occupy mainly equatorial positions. Additionally, hydroxylation of the C(10)-C(19) double bond in 1alpha,25-(OH)2D3 was performed, resulting in 1alpha,19,25-trihydroxy-10,19-dihydrovitamin D3 derivatives in which the hydroxymethyl substituent at C-10, for steric reasons, is forced to occupy an axial position. In consequence, the vitamin D3 analogues were synthesized in which the 1alpha-hydroxy group, required for biological activity, is almost exclusively axially or equatorially oriented because of stabilization of the single A-ring chair conformations. The relative ability of the synthesized analogues to bind the porcine intestinal vitamin D receptor was assessed and compared with that of the natural hormone. It was established that vitamins possessing the axial orientation of the 1alpha-hydroxy substituent exhibit a significantly increased receptor binding affinity. Compounds with a 2-methylene substituent showed selective calcemic activity profiles, being extremely effective on bone calcium mobilization. 2alpha-Methyl-substituted vitamins proved to be much more active in vivo than the corresponding epimers with 2beta-configuration. All of the 2-substituted vitamins exhibited pronounced HL-60 differentiating activity, those 2alpha-substituted in the 20S-series being especially potent. The present studies imply that the axial orientation of the 1alpha-hydroxy group is necessary for biological activity of vitamin D compounds.

Total Synthesis of Roseophilin

Abstract: The first total synthesis of the antitumor agent roseophilin 1 is reported. Its intricate macrotricyclic core 2 is obtained by means of a new palladium-catalyzed manifold for the formation of ansa-bridged pyrroles which proceeds via vinyl oxirane 8 and allyl lactone 11 as key intermediates. After conversion of the latter into pyrrolophane 14, a base-induced elimination of the sulfone group followed by the Michael addition of a zincate onto the resulting enone 18 installs the isopropyl substituent in a stereoselective manner. The pyrrolylfuran side chain 3 of roseophilin is prepared from 4-methoxy-2(5H)-furanone (24) and methyl 4-chloropyrrole-2-carboxylate (26) as the starting materials. The appropriate building blocks 25a and 28 derived thereof are combined via a sequence comprising a directed metal−halogen exchange reaction, transmetalation of the resulting lithiopyrrole to the corresponding organozinc compound, a palladium-catalyzed cross coupling of the latter, and a subsequent acid-catalyzed closure ...

N-(2-Benzoylphenyl)-l-tyrosine PPARγ Agonists. 3. Structure−Activity Relationship and Optimization of the N-Aryl Substituent

Abstract: 3-{4-[2-(Benzoxazol-2-ylmethylamino)ethoxy]phenyl}-(2S)-((2-benzoylphenyl)amino)propionic acid (1) and (2S)-((2-benzoylphenyl)amino)-3-{4-[2-(5-methyl-2-phenyloxazol-4-yl)ethoxy]phenyl}propionic acid (2) are peroxisome proliferator-activated receptor γ (PPARγ) agonists and have antidiabetic activity in rodent models of type 2 diabetes. As part of an effort to develop the SAR of the N-2-benzoylphenyl moiety of 1 and 2, a series of novel carboxylic acid analogues, 23−66, modified only in the N-2-benzoylphenyl moiety were synthesized from l-tyrosine and evaluated as PPARγ agonists. In general, only modest changes in the N-2-benzoylphenyl moiety of 1 and 2 are tolerated. More specifically, the best changes involve bioisosteric replacement of one of the two phenyl rings of this moiety. Addition of substituents to this moiety generally produced compounds that are less active in the cell-based functional assays of PPARγ activity although binding affinity to PPARγ may be maintained. A particularly promising set o...

Ring-Closing Metathesis in Methanol and Water

Abstract: The ring-closing metathesis (RCM) of acyclic dienes in both methanol and water has been achieved through the use of water-soluble ruthenium alkylidenes. These alkylidenes react readily with acyclic olefins in protic solvents, but they do not cyclize α,ω-dienes because of the instability of the resulting methylidene. Successful cyclization has been achieved through simple substrate modificationincorporation of an olefin substituent allows cyclization to proceed in good yield. A methyl-substituted substrate was cyclized in 60% conversion in methanol, and the incorporation of a phenyl substituent resulted in nearly quantitative cyclization. Phenyl-substituted substrates are best suited for the reaction, as a more stable, active catalyst is regenerated upon each catalyst turnover. Using this methodology, 90% conversion of a water-soluble substrate to a substituted cyclopentene has been achieved in aqueous solution. This methodology has also been successfully applied to increase RCM yields in organic solvents.

4-Bromo or 4-iodo phenylamino benzhydroxamic acid derivatives and their use as MEK inhibitors

Abstract: Phenylamino benzhydroxamic acid derivatives of formula (I) where R1, R2, R3, R4, R5, and R6 are hydrogen or substituent groups such as alkyl, and where R7 is hydrogen or an organic radical, are potent inhibitors of MEK and, as such, are effective in treating cancer and other proliferative diseases such as psoriasis and restenosis.

Catalytic Asymmetric Oxidation of Aryl Sulfides with a Ti/H2O/(R,R)-Diphenylethane-1,2-diol Complex: a Versatile and Highly Enantioselective Oxidation Protocol

Abstract: A new catalytic procedure for the asymmetric oxidation of aryl alkyl and aryl benzyl sulfides to optically active sulfoxides by hydroperoxides is described. This oxidation of sulfides is mediated by a chiral Ti complex formed in situ by reacting Ti(i-PrO)4, (R,R)-diphenylethane-1,2-diol (1), and water. The conditions of the reaction (stoichiometric composition of the catalyst, temperature, and the presence of additives and solvent) have been determined in order to reach the highest enantioselectivity and avoid the intervention of a kinetic resolution process. The oxidation protocol described herein is quite versatile as the values of chemical yields (60−73%) and of enantioselectivity (ee 70−80%) achieved for aryl alkyl sulfides are almost independent of the nature of the aryl substituent and of the size of the alkyl group. Notably, aryl benzyl sulfides, which are poor substrates for the Ti/DET catalyzed oxidations, afforded very high ee's (92−99%) with this oxidation system.

Aryloxyarylsulfonylamino hydroxamic acid derivatives

Abstract: A compound of formula (I) or the pharmaceutically acceptable salts thereof, wherein R1 is (C?1?-C6)alkyl; R?2? is (C?1?-C6)alkyl; or R?1 and R2? taken together with the carbon atom to which they are attached form a ring selected from (C?5?-C7)cycloalkyl, 4-tetrahydropyranyl and 4-piperidinyl; R?3? is hydrogen or (C?1?-C6)alkyl; and Y is a substituent of any of the carbon atoms of the phenyl ring capable of supporting an additional bond, preferably from 1 to 2 substituents (more preferably one substituent, most preferably one substituent in the 4-position) on the phenyl ring, independently selected from hydrogen, fluoro, chloro, trifluoromethyl, (C1-C6)alkoxy, trifluoromethoxy, difluoromethoxy and (C1-C6)alkyl.

Reactivity of Phosphate Diesters Doubly Coordinated to a Dinuclear Cobalt(III) Complex: Dependence of the Reactivity on the Basicity of the Leaving Group

Abstract: Reactivities of eight phosphate diesters each coordinated to a dinuclear Co(III) complex were investigated ([Co2(tacn)2(OH)2{O2P(OR)2}]3+; tacn = 1,4,7-triazacyclononane). Four of the complexes were coordinated by substituted phenyl methyl phosphates (substituent m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); unsubstituted (1d)) and two by substituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsubstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phosphate (4)) were also investigated. Hydrolysis of the phosphate diesters in 1a to 1d takes place by intramolecular oxide attack on the bridging phosphate while hydrolysis of 3 principally takes place by intermolecular hydroxide attack on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by intramolecular transesterification. Dimethyl phosphate dissociates from 4 without any observable cleavage of the diester. T...

Molecular recognition of sec-alcohol enantiomers by Candida antarctica lipase B

Abstract: A model to explain the enantioselectivity of Candida antarctica lipase B towards sec -alcohols based on structure activity and molecular modelling is presented. The origin of the enantioselectivity was found to be due to different modes of binding for the enantiomers. The fast enantiomer places its medium substituent in a site of limited size, the stereoselectivity pocket, whereas the slow enantiomer has to position the large substituent in that same pocket. Our model is in agreement with the 24 different substrates tested. Only substituents smaller than n -propyl can be accommodated by the stereoselectivity pocket. Moreover, important unfavourable electrostatic interactions are involved between this region and halogenated substituents. The former requirement entails a high enantiomeric ratio ( E ) for sec -alcohols with a medium group smaller than n -propyl and a large group larger than n -propyl. The latter requirement allows high E only for short chain vic -halogenated alcohols.

Dynamics of Anilinium Radical α-Heterolytic Fragmentation Processes. Electrofugal Group, Substituent, and Medium Effects on Desilylation, Decarboxylation, and Retro-Aldol Cleavage Pathways

Abstract: A single electron transfer (SET) photosensitization technique in conjunction with time-resolved, laser spectroscopy has been employed to generate and kinetically analyze decay processes of anilinium radicals derived by one-electron oxidation of α-anilinocarboxylates, β-anilinoalcohols, and α-anilinosilanes. In this manner, the rates of unimolecular decarboxylation of aniliniumcarboxylate radicals were determined to be in the range 106−107 s-1 and dependent upon solvent polarity, the nature of the metal cation, and substituents on the aniline ring, nitrogen, and α-carbon. In addition, kinetic analysis of base-induced retro-aldol fragmentations of cation radicals arising by SET oxidation of β-anilinoalcohols has shown that they occur with bimolecular rate constants which vary from 104 to 105 M-1 s1. These values are close to those for α-deprotonation reactions of related N,N-dialkylanilinium radicals. The retro-aldol fragmentation rates, like those for α-decarboxylation, also vary in a patterned way with ch...

A Novel Electrostatic Approach to Substituent Constants: Doubly Substituted Benzenes

Abstract: The most negative-valued molecular electrostatic potential (MESP) minimum (Vmin) observed over the benzene ring is proposed as a sensitive quantity for the analysis of the electronic perturbations ...

Effects of the substituent groups at the 4- and 7-positions on the fluorescence characteristics of benzofurazan compounds

Abstract: To develop new fluorogenic reagents having the benzofurazan structure, we investigated the effects of the substituent groups at the 4- and 7-positions of the benzofurazan skeleton on the fluorescence characteristics (fluorescence intensity, maximum excitation wavelength and maximum emission wavelength). Seventy benzofurazan compounds substituted at the 4- and 7-positions were obtained for this purpose. The Hammett substituent constant (σp) was adopted as a parameter for electronic effects by substituent groups. The study using the sum and the difference of the Hammett substituent constants (σp) at the 4- and 7-positions revealed that the highly fluorescent benzofurazan compounds were classified into two groups and that singlet excitation energies, calculated by the maximum excitation and emission wavelengths, of the benzofurazan compounds were different between these two groups. The fluorescence characteristics of benzofurazan compounds substituted at the 4- and 7-positions were empirically predictable by these relationships and σp values. A new fluorogenic reagent, 4-phenylaminosulfonyl-7-fluoro-2,1,3-benzoxadiazole, for amines was developed based on this method and applied to the amino acids analysis.

Catalytic CN Bond Formation by Metal-Imide-Mediated Imine Metathesis

Abstract: Molybdenum bis(imide) complexes of the general formula (DME)Cl2Mo(NR)2 catalytically metathesize acyclic imine substrates. This CN bond-forming reaction has significant implications for the synthesis of small molecules by ring-closing metathesis and polymers by ring-opening metathesis. A series of closely related NR transfer reactions were examined in an effort to fully understand the process. (RO)2Mo(CHR‘)(NAr) (1a, Ar = 2,6-diisopropylphenyl in all cases; R = C(CF3)2CH3) underwent alkylidene/imine exchange with a variety of imines in situ to give a mixed bis(imide) product and an olefin. The reactivity decreased if the alkoxide substituents were changed for more electron donating ones. (DME)Cl2Mo(NR)2 (3a, R = 2,6-diisopropylphenyl; 4, R = t-Bu) reacted with imines in an imide/imine metathesis to give mixed bis(imide) complexes and new imines. The reaction rates depended strongly on the steric demands of the imide NR substituent. Replacement of the chlorides with more electron donating alkoxide ligands ...

Linear Free Energy Substituent Effect on Flavin Redox Chemistry

Abstract: A systematic study on the effect of various substituents at the 7- and/or 8-position on the redox properties of isoalloxazines (flavins) is reported. The redox properties of these flavin derivative...

Design of novel, potent, noncovalent inhibitors of thrombin with nonbasic P-1 substructures: rapid structure-activity studies by solid-phase synthesis.

Abstract: Study of surface representations of the inhibitor-bound thrombin P-1 pocket revealed a lipophilic recess in this pocket which is not occupied by any known inhibitor. Solid-phase synthesis was used to generate benzylamides of D-diphenylAlaPro by aminolysis of Boc dipeptide Kaiser resin. The resulting amides inhibited thrombin in the range IC50 = 3-13,000 nM, and the structure-activity relationships and molecular modeling suggest a unique fit of the benzyl side chain into P-1 with the meta substituent occupying the recess.

Inclusion complexation of (cyclo)alkanes and (cyclo) alkanols with 6-O-modified cyclodextrins

Abstract: Novel α- and/or β-cyclodextrin benzoates (2α, 2β), methyl phthalate (3β), tethered benzamide (4β) and 2-naphthoate (5β) have been synthesized. The complex stability constants (Ks) of these cyclodextrin derivatives with a series of acyclic and cyclic hydrocarbons, and alcohols have been determined in water to reveal the role of the hydrophilic group in the guest molecule, and to evaluate the individual contribution of weak interactions involved in inclusion complexation by cyclodextrin. The free energy of complexation (–ΔG°) increases linearly to a certain limit with extending chain length or ring size (NC) of the guests, giving unit increments per methylene (–dΔG°/dNC). Interestingly, the unit increment obtained is independent of the host’s size or substituent introduced, but is a critical function of the guest type. Thus, a remarkably large –dΔG°/dNC value of 5.4 kJ mol–1 has been obtained for the cycloalkane series, whereas much smaller, but conventional, values have been recorded for cycloalkanols (3.3 kJ mol–1), alkanes (3.1 kJ mol–1) and alkanols (2.7 kJ mol–1). No significant isotope effects on Ks are observed when deuterated cyclohexane is complexed with the same cyclodextrins. Similarly, there is no significant effect when deuterated water is used as solvent. Moderate enantioselectivities of up to 2.0 are obtained with some chiral guests.

Study of Substituent Effects on the Photoconductivity of Soluble 2,(3)- and 1,(4)-Substituted Phthalocyaninato- and Naphthalocyaninatotitanium(IV) Oxides

Abstract: Soluble alkyl (II, 8a,b), fluoroalkyl (4a), and fluoroalkoxy (4b,c, 8c) 1,(4)- or 2,(3)-substituted phthalocyaninato- and linear 2,(3)- and angular 1,(2)-annulated naphthalocyaninatotitanium(IV) oxides 10, 12, and 14 were synthesized and characterized with regard to their spectroscopic, photophysical, and photochemical properties. While alkyl- and fluoroalkoxy-substituted compounds are highly soluble in nonpolar solvents, e.g., hexane, fluoroalkyl-substituted compounds are better soluble in polar aprotic solvents such as acetone. The stability against photooxidation in solution is enhanced on going from alkylated phthalocyanines 1,(4)-(C5H11)8PcTiO (8a), 1,(4)-(C6H13)8PcTiO (8b), and 2,(3)-(C4H9)8PcTiO (II) to fluorinated phthalocyanines 2,(3)-(CF3)4PcTiO (4a), 2,(3)-(CF3CH2O)4PcTiO (4b), and 2,(3)-(CF3CH2O)8PcTiO (4c), from phthalocyanines to naphthalocyanines (tert-butyl)4-2,(3)-NcTiO (10), 1,(2)-NcTiO (12), and (tert-butyl)4-1,(2)-NcTiO (14), and on going from 2,(3)-substituted 4a−c to 1,(4)-substitute...

Cation Stabilities, Electrophilicities, and “Carbene Analogue” Character of Low Coordinate Phosphorus Cations

Abstract: The stabilities of low coordinated phosphorus cations can be expressed in the frame of the HSAB concept by the transferred charge density Δq(N) which a cation receives upon formation of a donor-acceptor adduct with a Lewis base N. This concept allows to differentiate between relative stabilities towards different reaction partners, and to compare the electrophilicities of phosphenium ions to those of isoelectronic carbenes and silylenes. An analysis of substituent influences on Δq(H) in cations [P(R)2]+suggests an increasing stabilizing power of substituents in the series R = Cl < CH3 < OH, SH < NH2. The same ordering was derived from isodesmic hydride transfer reactions. Interpretation of population analyses suggests that the individual substituent contributions to cation stabilities result from a balance between π-donation into the empty p(P) orbital and electrostatic stabilization by polar P–R σ-bonds. A further stabilizing effect, which is of similar magnitude as in isoelectronic carbenes or silylenes, may arise from cyclic π-conjugation between a diaminophosphenium fragment and an adjacent double bond. Substituent effects influence further the nature of the frontier orbitals of phosphenium ions, resulting in orbital sequences which resemble those of carbenes, allyl anions, or phospholides, respectively. The absence of frontier orbital related changes in reactivity patterns suggests that in all reactions, including metal complex formation, phosphenium ions behave as purely electrophilic rather than ambiphilic species.

New activator system for metallocene compounds

Abstract: Supported olefin polymerization catalyst compositions prepared by contacting a support comprising a solid compound which is one of pure aluminium oxide, a mixed aluminium oxide, an aluminium salt, a magnesium halide, or a C1-C8 alkoxy magnesium halide, in any order with at least a) an organometallic compound of the general formula (1): RlMXv-1, wherein each R is the same or different and is a C1-C10alkyl group; M is a metal of Group 1, 2, 12 or 13 of the Periodic Table; each X is the same or different and one of a halogen atom, a hydrogen atom, a hydroxyl radical or a C1-C8 hydrocarbyloxy group; l is 1, 2 or 3; v is the oxidation number of the metal M, b) a metallocene of the general formula (2): (CpY)mM'X'nZo, wherein each CpY is one of a mono- or polysubstituted, fused or non-fused, homo- or heterocylic cyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl, or octahydrofluorenyl ligand, which ligand is substituted at its cyclopentadienyl ring with at least one substituent Y which is one of a -OR', -SR', -NR'2, -C(H or R')=, or -PR'2 radical, each R' being one of a C1-C16 hydrocarbyl group, a tri-C1-C8 hydrocarbyl silyl group or a tri-C1-C8 hydrocarbyloxy silyl group; M' is a transition metal of Group 4 of the Periodic Table and bound to the ligand CpY at least in an θ5 bonding mode, each X' is one of a hydrogen atom, a halogen atom, a C1-C8 hydrocarbyl group, a C1-C8 hydrocarbylheteroatom group or a tri-C1-C8 hydrocarbylsilyl group or two X' form a ring with each other; Z is a bridge atom or group between two CpY ligands or one CpY ligand and the transition metal M'; m is 1 or 2; o is 0 or 1; and n is 4-m if Z is a bridge between two CyP ligands or n is a 4-m-o if Z is a bridge between one CpY ligand and the transition metal M', and c) an aluminoxane of general formula (3).

8-O-Sialylation of Neuraminic Acid

Abstract: Synthesis of Neu5Acα(2−3)Galβ(1−4)Glc and Neu5Acα(2−8)Neu5Ac derivatives 12α and 14α, from lactose derivative 6 and 2,3-dehydro-Neu5Ac derivative 7, respectively, with anchimerically assisted Neu5Ac donor 3e, 3f, or 3g has been studied. Reaction of halogenose 3e, having a 3-phenoxythiocarbonyloxy moiety as the assisting group, afforded with 6 and 7 in the presence of equimolar amounts of AgOTf as promoter α-sialosides 11α and 13α which were readily deoxygenated to afford 12α and 14α, respectively. Reaction of phosphite 3f, having a 3-bromo substituent as the potentially anchimeric assisting group, furnished in the presence of catalytic amounts of TMSOTf with 6 as acceptor α-sialoside 15α and with 7 as acceptor β-sialoside 17β, which gave on debromination 12α and 14β, respectively. Reaction of readily available phosphite 3g, having a 3-thiobenzoyloxy group as the anchimeric assisting group, afforded with 6 and 7 in the presence of catalytic amounts of TMSOTf in very high yields α-linked sialosides 21α and ...

Kinetics and Mechanism of the Pyridinolysis of Phenyl Chloroformates in Acetonitrile

Abstract: Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Bronsted plots are linear with enhanced substituent constants, σp-, and basicities, pKa-, for strong para π-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C6H5 than of CH3. The proposed mechanism is supported by a relatively small βX (≅0.3) and by the lower ΔH⧧ (6.7 kcal mol-1) and ΔS⧧ (−44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para π-acceptor (X = p-CN) in pyridine.

New homogeneous olefin polymerization catalyst composition

Abstract: Homogeneous olefin polymerization catalyst compositions prepared by contacting a) a metallocene of the general formula (1): (CpYq)mMXnZo, wherein Cp is one of a mono- or polysubstituted, fused or non-fused, homo- (= iso-) or heterocyclic cyclopentadienyl ligand, Y is a substituent at the cyclopentadienyl ring and is one of an -OR, -SR, -NR2, -C(H or R)=, or -PR2 radical, M is a transition metal of Group 4 of the Periodic Table and bound to the ligand or ligands Cp in at least an θ5 bonding mode; X is bound to M and is one of a hydrogen, a halogen, a C?1?-C8 hydrocarbyl group, a C1-C8 hydrocarbylheteroatom (O, S, N, P) group or a tri-C1-C8 hydrocarbyl silyl group or two X form together with M a C4-C20 metallocyclic ring structure; Z is a bridge atom or group between two Cp ligands or between one Cp ligand and the transition metal M; q is 0-5; m is 1 or 2; m.q ≥ 1; o is 0 or 1; and n is 4-m-o, except when there is a bridge Z between two Cp ligands, in which case n is 4-m, and b) an aluminoxane of one of formulas (2), (OAlR')p (2 general), wherein each R' is the same or different and is a C2-C10 alkyl group; and p is an integer between 1 and 40.