摘要:

Someab-initiovalence-bond wave-functions are reported for the π-electrons of the ground-states of O3, NO2−, and CH2N2. Examination of these wave-functions provides further support for the hypothesis that, for the ground-states of many electron-excess molecules, important valence-bond structures are those that are compatible with the electroneutrality principle, i.e. they carry either small or zero formal charges on each of the atoms. For O3and CH2N2, the important valence-bond structures with zero atomic formal charges areEach of these structures has a 'long-bond' between non-adjacent atoms. The significance of 'long-bond' (or spin-paired diradical) structures for the electronic mechanism of 1,3-dipolar cycloaddition reactions is discussed and 'increased-valence' descriptions of the electronic structure of each molecule are presented. Some comments on the utility of 'increased-valence' structures are provided.

ISSN:0008-4042    

DOI:10.1139/v78-186

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The construction of the C/D ring system of the alkaloids chasmanine and napelline by modification and rearrangement of a common intermediate is explored on model compounds.The phenolic acid9was oxidized to the spirolactone10. Addition of benzyl vinyl ether to this material gave the adduct28in high yield. This central 'nordenudatine' type intermediate was then modified (in a very few stereospecific steps) to the 'napelline' model compound47and to the keto acetal38. Compound38is an intermediate of our photochemical route and we have already described its conversion to a chasmanine model derivative.The new syntheses of chasmanine and napelline foreshadowed by the present model studies are expected to be completely stereo- and regiospecific, almost10steps shorter and an order of magnitude more efficient than the total syntheses of these two alkaloids which we have reported in the recent past.Incidentally, with these model studies, the methoxy ketone20was prepared. The methoxyl in this compound is epimeric to the corresponding (C16) methoxyl of the aconite alkaloids. Some relevant observations on the nmr spectra and allylic rearrangement (pyro–isopyro rearrangement) of this material are also reported.

ISSN:0008-4042    

DOI:10.1139/v78-187

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Migration of the hydroperoxy group occurs during the reaction between base and several β-halohydroperoxides to give allylic hydroperoxides. An 86:14 ratio of 3-hydroperoxy-2-(4-methoxyphenyl)-3-methyl-1-butene-3-hydroperoxy-3-(4-rnethoxyphenyl)-2-methyl-1-butène was formed in 90% yield from 2-bromo-3-hydroperoxy-3-(4-methoxyphenyl)-2-methyibutane. From 1-chloro-, 1-bromo-, and 1-iodo-1-(1-hydroperoxy-1-methylethyl)cyclohexane there were obtained 1-(1-hydroperoxy-1-methylethyl)cyclohexene and 1-hydroperoxy-1-(2-propenyl)cyclohexane in ratios of 93:7, 94:6, and 94:6, respectively, in 98–100% yieid. Isopropylidenecyclohexane was converted with singlet oxygen to these allylic hydroperoxides in a 10:90 ratio. Only the latter allylic hydroperoxide was formed in the reaction between the alkene and triphenyl phosphite ozonide at −70 °C. Thep-nitrobenzoate esters of 1-hydroperoxy(1-chloro-, 1-bromo-, and 1-iodo-1-methylethyl)cyciopentane were converted quantitatively to 1-(1-hydroperoxy-1-methylethyl)cyclopentene and 1-hydroperoxy-1-(2-propenyl)-cyclopentane in 77:23, 73:27, and 70:30 ratios, respectively. Oxidation of isopropylidenecyclopentane with triphenyl phosphite ozonide at −70 °C and singlet oxygen produced the allylic hydroperoxides in 62:38 and 58:42 ratios, respectively. The reactions of the β-halohydroperoxides proceed exclusively via perepoxide intermediates. The reactions between the alkenes and singlet oxygen or triphenyl phosphite ozonide do not involve perepoxide inter mediates.

ISSN:0008-4042    

DOI:10.1139/v78-188

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The first and second dissociation constants of the ten homologous a, α,ω-amino-acidshave been determined by a potentiometric method at different temperatures, and hence the thermodynamic parameters ΔG0, ΔH0, and ΔS0were computed. Calculation by Kirkwood–Westheimer theory of ΔG10for the first (carboxyl) dissociation, assuming the conformation of the polymethylene chain to be intermediate between the random-coil and the fully extended conformation, gives results in good agreement with experiment for the longer chains. A similar calculation of ΔG20for the second (ammonium) dissociation gives results in fair agreement with theory for the very long chains (n = 7–10), but in disagreement for shorter chains. A tentative explanation based on electrostatic effects on hydration of the ammonium group is advanced.

ISSN:0008-4042    

DOI:10.1139/v78-189

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

Acid dissociation constants of twocis-3- and of fourtrans-4-substituted cyclohexanecarboxylic acids in water – dimethyl sulfoxide mixtures of varying composition were determined by a potentiometric method. Results have been analyzed according to the model of Kirkwood and Westheimer, and shown to differ from calculated values because of certain oversimplifications built into the model. Dissociation constant differences between cyclohexanecarboxylic acid and the compounds studied vary only slightly with changing solvent composition.

ISSN:0008-4042    

DOI:10.1139/v78-190

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The syntheses of 4,4,6-trimethyl-5-methylene-2-norbornanone (5a), 6,6-dimethyl-2-methyl- ene-2-norbornanone (5b), 5-methylene-2-norbornanone (5c), and 6-methylene-2-norbornanone (11) are described. An unusual extended Grob-type cleavage of 6-ethylenedioxy-exo-2-methyl-endo-2-norbornanol (13d) to 2-chioroethyl (3-methyl-3-cyclopentenyl)acetate (17) is documented.

ISSN:0008-4042    

DOI:10.1139/v78-191

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

We report the rates of base-catalyzed hydrogen isotope exchange of theexoandendoprotons in a series of 2-norbornanones. Theexo:enclorate ratios for 4-methyl-substituted 2-norbornanones3and6are less than the rate ratios of the 4-demethyl analogs2and5. Therefore, a methyl group is not a useful torsional-effect probe. This is supported by an analysis which established that the small difference in the torsional strain (0.45 kcal) between eclipsed vicinal hydrogens and an eclipsed vicinal methyl and hydrogen would only resuit in a maximum twofoldincreasein the rate ratio. The decrease in theexo:endorate ratio is a consequence of the electron-donating effect of a methyl group which affects theexorate to a greater degree. Introduction of a strain-inducingexomethylene group at C(5) and C(6) in the norbornyl system does not produce a significant change in the exchange rates. The average increase of 7.9 and 2.6 in theexoandendorates, respectively, in enones4,5, and6over the ketones1,2, and3are explicable by an inductive effect. Thus, homoallylic stabilization of the developing carbanion is not important.Through application of a new mechanistic criterion which involves a log–log plot of theexoandendoexchange rate constants, we establish, for the bicyclic ketones, that charge transfer to oxygen is greater in the transition state forendodeprotonation.

ISSN:0008-4042    

DOI:10.1139/v78-192

出版商:NRC Research Press    

年代:1978

数据来源: NRC

摘要:

The rate enhancements relative to the monoketones, for base-catalyzedexoandendohydrogen isotope exchange in 2,5-norbornanedione (2), 3,3-dimethyl-2,5-norbornanedione (3), and 1,3,3-trimethyl-2,5-norbornanedione (4) have been determined at 25 ± 0.05 °C in 60% dioxane–40% D2O. After the increase in rate due to the inductive and strain effects of the second carbonyl group is taken into account, the homoenolic-assistance factors are extracted from the overall rate enhancements: forexoexchange in2,3, and4, the factors are 13, 39, and 32; forendoexchange in2,3, and4, the factors are 26, 64, and 71, respectively.

ISSN:0008-4042    

DOI:10.1139/v78-193

出版商:NRC Research Press    

年代:1978

数据来源: NRC