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ABSTRACT: A representative azulene formation from an active troponoid precursor (2-methoxytropone) and an active methylene compound (malononitrile) has been analyzed both experimentally and theoretically. (2)H-Tracer experiments using 2-methoxy[3,5,7-(2)H(3)]tropone (2-d(3)) and malononitrile anion give 2-amino-1,3-dicyano[4,6,8-(2)H(3)]azulene (1-d(3)) in quantitative yield. New and stable (2)H-incorporated reaction intermediates have been isolated, and main intermediates have been detected by careful low-temperature NMR measurements. The detection has been guided by mechanistic considerations and B3LYP/6-31(+)G(d) calculations. The facile and quantitative one-pot formation of azulene 1 has been found to consist of a number of consecutive elementary processes: (a) The troponoid substrate, 2-methoxytropone (2), is subject to a nucleophilic substitution by the attack of malononitrile anion (HC(CN)(2)(-)) to form a Meisenheimer-type complex 3, which is rapidly converted to 2-troponylmalononitrile anion (5). (b) The anion 5 is converted to an isolable intermediate, 2-imino-2H-cyclohepta[b]furan-3-carbonitrile (6), by the first ring closure in the reaction. (c) A nucleophilic addition of the second HC(CN)(2)(-) toward the imine 6 at the C-8a position produces the second Meisenheimer-type adduct 7. (d) The second ring closure leads to 1-carbamoyl-1,3-dicyano-2-imino-2,3-dihydroazulene (11). A base attacks the imine 11, which results in generation of a conjugate base 12 of the final product, azulene 1.
The Journal of Organic Chemistry 05/2012; 77(12):5318-30. · 4.45 Impact Factor
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ABSTRACT: Through variable-temperature solution-state NMR and molten- and solid-state CP/MAS (13)C NMR spectra, thiotropolone is found to exist as two rapidly equilibrated tautomeric structures, thione and enethiol, even in the solid state far below the melting point. The crystal structure shows an almost perpendicular packing, suggesting that the intramolecular hydrogen bond is dominant.
The Journal of Organic Chemistry 07/2011; 76(13):5457-60. · 4.45 Impact Factor
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ABSTRACT: The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.
Journal of the American Chemical Society 02/2006; 128(1):44-5. · 9.91 Impact Factor
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ABSTRACT: Six new bisabolane-type (1-3) and santalane-type (4-6) sesquiterpenoids, together with (+)-alpha-nuciferol, (+)-citronellol, and geraniol, were isolated from the heartwood of Santalum album of Indian origin. Their structures, including two bisabolol diastereomers (1, 2), were established on the basis of spectroscopic data interpretation.
Journal of Natural Products 01/2006; 68(12):1805-8. · 3.13 Impact Factor
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ABSTRACT: A phytochemical investigation of the polar constituents in the heartwood of Indian Santalum album L. resulted in the isolation of three new neolignans (1-3) and a new aromatic ester (4), along with 14 known components. The structures of the new compounds (1-4) were established using spectroscopic methods.
CHEMICAL & PHARMACEUTICAL BULLETIN 07/2005; 53(6):641-4. · 1.59 Impact Factor
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ABSTRACT: Tropone (1) reacts with ketenes 2 to yield [8+2] cycloadducts, the γ-lactones 3. The concerted [8+2] cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful low-temperature NMR (1H, 13C, and 19F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a β-lactone intermediates 5b,c and novel norcaradiene species 6b,c in head-to-head configurations. The [2+2] cycloadducts 5b,c equilibrated with the norcaradienes 6b,c. The β-lactones 5b and 5c were converted to the γ-lactones 3b and 3c, respectively, in quantitative yields. The DFT calculations showed that the concerted [8+2] cycloaddition is unfavorable. The first step of the calculated reaction 1+2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2+2] cycloadduct is isomerized to the β-lactone 5cvia the first zwitterionic intermediate. The β-lactone 5c is further isomerized to the product γ-lactone 3cvia the second zwitterion intermediate. Thus, 3c is not formed via the well-established two-step mechanism including zwitterionic intermediates but via a five-step mechanism composed of a [2+2] cycloaddition and subsequent isomerization (Scheme 12).
Helvetica Chimica Acta 06/2005; 88(6):1519 - 1539. · 1.48 Impact Factor
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ABSTRACT: This paper describes a new mechanistic feature for the Staudinger ketene-alkene cycloaddition reactions to give cyclobutanones. Low-temperature NMR (13C, 19F, and 1H) monitoring of a reaction between bis(trifluoromethyl)ketene (1) and ethyl vinyl ether (2) has shown that the Staudinger reaction proceeds to form initially and exclusively an alpha-methyleneoxetane (3) by [2 + 2](C=O) cycloaddition across the ketene C=O bond. The initial intermediate 3 undergoes ring cleavage to produce a 1,4-zwitterion (4), which is converted to the final [2 + 2](C=C)-type product, cyclobutanone (5). The key intermediate 3 has been isolated in its pure form and was found to be converted to the final products 5 on warming, via the 1,4-zwitterion 4. The alpha-methyleneoxetane 3 is so reactive that it reacts with methanol rapidly even at -80 degrees C via solvolysis to afford an adduct 7. The ion 4 derived from the pure isolated oxetane 3 was intercepted with acetone by a 1,4-dipolar cycloaddition to give a 1,3-dioxane 8. An open-chain alpha,beta-enone (6) has been also obtained from 3. We conclude that the (1 + 2) reaction proceeds in a new three-step mechanism; formation of an alpha-methyleneoxetane 3, a [2 + 2]-type cycloadduct across the C=O bond of ketene, followed by ring cleavage to give the zwitterion 4 and by recombination to form the final product, cyclobutanone 5. The zwitterion 4 is not equilibrating with reactants 1 and 2 but comes from the alpha-methyleneoxetane 3. Exclusive formation of another oxetane 12 has been observed in a reaction between diphenylketene (9) and methyl isopropenyl ether (11). The selectivity of initial formation of cyclobutanone or oxetane has been generalized with aid of frontier-orbital theory and ab initio calculations.
Journal of the American Chemical Society 12/2003; 125(47):14446-8. · 9.91 Impact Factor
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ABSTRACT: The mechanism of ketene−diene reactions has been studied both experimentally and theoretically. Careful experiments of the reactions of diphenylketene (1) with cyclic (s-cis) 1,3-dienes [cyclopentadiene (2) and cyclohexa-1,3-diene (3)] lead to the first direct detection of the Diels−Alder cycloadducts (10 and 11) by low-temperature NMR spectroscopy. The initially formed cycloadducts are converted to the final Staudinger products, cyclobutanones (6 and 7), by [3,3] sigmatropic (Claisen) rearrangements. In contrast, ketene 1 reacts with open-chain 1,3-dienes [2,3-dimethyl-1,3-butadiene (4) and 1-methoxy-1,3-butadiene (5)] to afford initially both the Staudinger-type (8, 9) and Diels−Alder-type cycloadducts (12, 13). The Staudinger cycloadducts (8, 9) are converted eventually to Diels−Alder products (12, 13) by the retro-Claisen rearrangement. Thus, ketene recognizes dienes in cycloadditions as ketenophiles different from olefins. [4 + 2] and [2 + 2] cycloadducts are generated and can be intermediates or products flexibly according to diene structures.
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