摘要:

A study of carboxylic acid → diazoketone conversion was pursued employing the γ-carboxyl group of otherwise protectedL-glutamic acids. The Arndt–Eistert route employing carboxylic acid chloride intermediates was found best (52% yield,5b), performed at very low temperatures employing oxalyl chloride in dimethylformamide–tetrahydrofuran followed by diazomethane at −23 °C. Alternatively, substitution of a mixed carbonic anhydride for the acyl chloride led to very similar yields (57% of5b) of diazoketones (5). Among a series of active ester intermediates (7) examined, only the ODnp (7d) and SPfp (7f) esters were found to react (23–26% yield), at least partially, with diazomethane. The latter two reactions appear to represent the first such examples employing active esters.

ISSN:0008-4042    

DOI:10.1139/v86-346

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Dicumylhyponitrite (DCHN) was used as a thermal initiator of autoxidation of unsaturated phosphatidylcholines ino-dichlorobenzene and in mixed bilayers of dilinoleoylphosphatidylcholine (DLPC) with dimyristoylphosphatidylcholine (DMPC). Quantitative studies were made of the products, α,α-dimethylbenzyl alcohol (DMBA), acetophenone (AP), and dicumylperoxide (DCP) from DCHN. The ratio DMBA/Δ2DCHN (37 ± 6%) indicates the effectiveness of cumyloxy in hydrogen abstraction from the phospholipid in solution. Theoverallefficiency (e) of DCHN determined by the inhibitor method with α-tocopherol (α-T) was 84 ± 5%, close to the overall ratio (DMBA + AP)/Δ2DCHN (90 ± 6%), and indicates either significant initiation through the methylperoxy radical ino-dichlorobenzene solution or that α-tocopherol traps cumyloxy radical before β-scission occurs. The thermal decomposition rate constant of DCHN is greatly reduced in a medium of high viscosity; in DLPC to one-quarter, in Nujol to one-sixth that in isooctane and this is interpreted in terms of a one-bond scission of DCHN. In DLPC + DMPC mixed bilayers the effectiveness of the cumyloxy radical to initiate reaction varied from 30 to 19% depending on the amount of DLPC and in this medium there is little or no initiation by methylperoxy according to product analysis and α-T inhibition studies. In mixed bilayers, the rate of oxidation was directly related to the [DLPC] indicating that the classical rate law applies. The oxidizability of DLPC in mixed bilayers (0.046 M−1/2s−1/2) is comparable to that of neat DLPC.

ISSN:0008-4042    

DOI:10.1139/v86-347

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Reaction of 2′,3′-O-isopropylideneadenosine (1) withp-NCC6H4COCl in 1:6 Et3N–CH2Cl2afforded the cyanoimidazole nucleoside8(53%) and the 8,5′-O-cycloadenosine derivative7(29%). Treatment ofN6,N6-di-p-toluyl-2′,3′-O-isopropylideneadenosine (23) with ZnBr2inp-dioxane resulted in acyl migration fromN6toN7 to give the cycloadenosine derivative4. The coordination site of the zinc cation on the base moiety of adenosine derivatives was determined by15N nmr spectra.

ISSN:0008-4042    

DOI:10.1139/v86-348

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine,1, and its 3-isomer,2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated,1reacted faster than2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants,kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4and 47% H2SO4for1and2respectively. Activation parameters have also been determined. The pKavalues for the second protonation equilibria of1and2have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for1both the pyridinium and protonated azonium functions are involved whereas for2only the azonium function participates in charge delocalization.

ISSN:0008-4042    

DOI:10.1139/v86-349

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Enthalpies of solution of NaCl in aqueous solutions of isopropanol,s-butanol, 2-methoxyethanol, 2-ethoxyethanol, acetone, andN,N-dimethylformamide were measured. The results of enthalpy measurements were analyzed from the point of view of the effect of added nonelectrolyte on water structure, and enthalpic pair interaction coefficientshxy{(Na+ + Cl−)–nonelectrolyte} were calculated and compared with appropriate data for (Na+ + I−)–nonelectrolyte pairs. The group additivity concept appeared to be useful for the analysis of calculatedhxycoefficients. The correlations betweenhxyand the functions characterizing different properties of the solutes under study were examined. It was shown that the correlation with the heat capacity of transfer of the nonelectrolyte molecule from the vapour phase to high dilution in water was the most promising. The interpretation of observed correlations was proposed.

ISSN:0008-4042    

DOI:10.1139/v86-350

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The proton and carbon-13 resonance signals of carnosine (β-alanyl-L-histidine) were unambiguously assigned using a variety of nmr techniques including proton–carbon chemical shift correlations, titrations of nmr chemical shifts, coupling constants, and isotope shifts. From the13C nmr titration, carnosine's three pKavalues were estimated to be 2.7, 7.1, and 10.6, and it was found that the imidazole ring existed predominantly as the 3-H tautomer in basic solution. Conformational information about the Cα—Cβbond and about the N—Cαbond was deduced from observed3J(C,H) and3J(H,H) values. The13C nmr spectrum of carnosine in solution is also compared with that obtained for a solid sample.

ISSN:0008-4042    

DOI:10.1139/v86-351

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Volumetric heat capacities for binary mixtures ofn-octane withn-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane were measured at 298.15 K in a Picker flow microcalorimeter. The results were combined with previously published excess molar volumes to obtain excess molar isobaric heat capacities. Interpretation of the results in terms of the Flory theory of mixtures is discussed.

ISSN:0008-4042    

DOI:10.1139/v86-352

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The conformational and dynamic properties of 1,5-benzodithiepin (1), its 3,3-dimethyl (2), 3-methyl (3), and 3-methoxy (4) derivatives have been investigated by high-field1H and13C dynamic nuclear magnetic resonance methods. Analyses of the spectra at low temperatures indicate that, in CS2, both1and2exist as a mixture of C and TB forms (97:3 for1; 63:37 for2). In contrast, both3and4exist as a mixture of three equilibrating conformations Ce, Ca, and TB in ratios of 55:25:20 and 88:8:4 respectively. Comparison with analogues derived from 1,5-benzodioxepins (1a–4a) indicates that the replacement of O by S decreases the amount of the TB form for all four compounds studied. The observation that Ceis the most abundant form for4while it is not observed for4acan be explained by a repulsive S/Ogaucheinteraction in4compared to an attractive O/O interaction in4a.

ISSN:0008-4042    

DOI:10.1139/v86-353

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The visible absorption spectrum of 1,2-cyclobutanedione has been measured in the gas phase at wavelengths between 4000 and 5100 Å. The absorption is attributed to the allowed π* ← n+,1B1 ← 1A1transition corresponding to the first excited singlet state. The spectrum shows a complex well-resolved vibrational structure which has been analysed, with some 125 bands measured and assigned. The bands at the longer wavelengths show sharp rotational fine structure, not yet analysed. The strongest band in the spectrum at 4933 Å has been assigned as the 0–0 band, while a band almost as strong at 4820 Å is attributed to excitation of one quantum of, thea2out-of-plane carbonyl bending vibration, and it is suggested that this band owes its intensity to vibronic coupling. A number of symmetric vibrations are also excited in the spectrum, but with no long progressions. Sequence bands running to the blue with an interval of about 72 cm−1are prominent throughout the spectrum, and are assigned tov13, thea2ring-twisting vibration. Other hot bands were also observed involvingv13which permitted estimation of energy levels for this vibration both in the ground state and the excited state. The infrared spectrum was also measured and analysed in the gas phase between 600 and 4000 cm−1, and 14 bands were assigned to fundamental vibrations; some of these assignments, at the lower frequencies, are uncertain.

ISSN:0008-4042    

DOI:10.1139/v86-354

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

13C,19F and19F,19F nuclear spin–spin coupling constants overnformal bonds,n = 1–9, are reported for 4-fluorobiphenyl, 4,4′-difluorobiphenyl, 4,4′-difluoro-2,2′,6,6′-tetramethylbiphenyl, 2,7-difluorofluorene, 2-fluoro-9-fluorenone, and 2,7-difluoro-9-fluorenone in acetone solutions. The signs of many of the coupling constants are deduced from second-order spectral phenomena caused by differential13C isotope effects on theI9F nmr chemical shifts. Theoretical potentials, based on geometry-optimized STO 3G MO computations for 4-fluorobiphenyl and 4,4′-difluorobiphenyl, yield expectation values for the torsion angles about the exocyclic C—C linkage that are very close to those deduced from electron diffraction patterns. These potentials and INDO MO FPT computations of the long-range coupling constants allow a discussion of the coupling mechanisms. In Hz,9J(F,F) = 1.3(1) cos2 θ, where θ is zero for a planar biphenyl, while8J(C,F) = 0.8(1) cos2 θ and7J(C,F) = −0.43(5) cos2 θ.6J(C,F) is a composite of σ–π and π electron coupling components and is written in Hz as 0.57(1) + 0.29(1) sin2 θ. The corresponding coupling constants in the fluorene and 9-fluorenone derivatives are enhanced in magnitude relative to a hypothetical planar biphenyl derivative. It is tentatively suggested that5J(C,F) consists of three coupling components, one negative and proportional to cos2θ, the other two positive and independent of θ.4J(C,F) is suggested to consist of a σ component of −1.0 Hz and a π component proportional to the atom–atom polarizability for the parent hydrocarbon.

ISSN:0008-4042    

DOI:10.1139/v86-355

出版商:NRC Research Press    

年代:1986

数据来源: NRC