- Intramolecular cycloaddition of O-tert-butyldimethylsilyloximes in the presence of BF3-OEt2
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Intramolecular cycloaddition of novel 1,3-dipoles, N-boranonitrones, was examined. Treatment of O-tert-butyldimethylsilyloximes 9-12 having olefin moieties with 2 equiv of BF3·OEt2 generated N-boranonitrones, which underwent intramolecular cycloaddition to afford N-nonsubstituted cycloadducts 16 (and/or 18) after extractive workup. Despite the Lewis-acidic conditions, the olefin geometry of the substrates was retained in the cycloadducts in the present cycloaddition. The electronic nature of the N-boranonitrones appeared to be electrophilic. In the case of substrate 11c, having an electron-donating methyl group at an internal position of the olefin moiety, the cycloaddition gave the bridged cycloadduct 18b. The cycloaddition proceeded at relatively low temperature, and the diastereoselectivity was high.
- Tamura, Osamu,Mitsuya, Takahiro,Huang, Xin,Tsutsumi, Yoshiyuki,Hattori, Sanae,Ishibashi, Hiroyuki
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p. 10720 - 10725
(2007/10/03)
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- Asymmetric photolysis of 2-phenylcycloalkanones with circularly polarized light: A kinetic model for magnetic field effects
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Magnetic field effects (MFE) on asymmetric photolysis involving a biradical intermediate have been investigated on a kinetic model where an intersystem crossing (ISC) of the intermediate is taken into account. Changes in the enantiomeric excesses (ee) of chiral substances with circularly polarized light (CPL) irradiation have been simulated, and the necessary conditions for observing the MFE were obtained. The asymmetric photolysis of racemic 2-phenylcycloalkanones (2-PCAs) with CPL has been carried out in both the presence and absence of a magnetic field. Since the anisotropy g factors of 2-PCAs are considerably large, the CPL-induced ee are achieved to a few percent after 90% decomposition though the MFE are not observed. The photolysis mechanism of 2-PCAs in an air-saturated solution has been also clarified. Triplet acyl-benzyl biradicals, formed via photochemical α-cleavage of 2-PCAs, react with O2 dissolved in the solution and result in the formation of acetophenone and alkenoic acids. The bimolecular reactions are diffusion-controlled and the rates are comparable to those of ISC to the singlet biradicals for all 2-PCAs. The recombination yields of the biradicals are sufficiently large. However, the biradical ISC rate shows little magnetic field dependence, which explains the absence of the MFE in this asymmetric photolysis.
- Kohtani, Shigeru,Sugiyama, Masahide,Fujiwara, Yoshihisa,Tanimoto, Yoshifumi,Nakagaki, Ryoichi
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p. 1223 - 1233
(2007/10/03)
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- Synthesis of 2-phenyl-2-cycloalkenones via palladium-catalyzed tandem epoxide isomerization-intramolecular aldol condensation
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We have extended the scope of our palladium-catalyzed tandem epoxide isomerization/aldol condensation reaction to encompass intramolecular condensations, which provide facile access to conjugated cycloalkenones from epoxy aldehydes or diepoxides. For example, reaction of 5,6-epoxy-6- phenylhexanal with Pd(OAc)2-PBu3 catalyst in the presence of NaHCO3 and 3A molecular sieves forms 2-phenyl-2-cyclopentenone (80%). Similarly, 1,2;5,6-diepoxy-1-phenylhexane affords 3-methyl-2-phenyl-2-cyclopentenone (72%). The observation of dicarbonyl intermediates (e.g., 1-phenyl-2,5- hexanedione in the latter case) demonstrates that the reaction proceeds via Pd-catalyzed isomerization of the diepoxide to the diketone, followed by base-catalyzed aldol condensation.
- Kim, Ji-Hyun,Kulawiec, Robert J.
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p. 3107 - 3110
(2007/10/03)
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- Generation of Acyl Radical Equivalents by Cathodic Reduction of Acyl Tributylphosphonium Ions
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The species generated by one-electron transfer to acyl tributylphosphonium ions has proved to be potent acyl radical equivalents by the results that unsubstituted and 6-phenyl- substituted 5-hexenoic acids were transformed into cyclopentanones by the electrolysis in the presence of Bu3P in an undivided cell, although the yields were not satisfactory due to the formation of 5-hexen-1-als via further one-electron reduction of the radicals before their cyclization.
- Maeda, Hatsuo,Maki, Toshihide,Ohmori, Hidenobu
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p. 249 - 250
(2007/10/02)
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- Oxidation of Styrene Derivatives by S2O82--CuII in Acetic Acid and Acetonitrile. Reaction Paths in Oxidations via Radical Cations
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β-Aryl carbonyl compounds are major products in the oxidation of a variety of styrene derivatives by S2O82--CuII.Evidence is presented that they arise via oxidation to a radical cation, nucleophilic addition of water to give a β-hydroxyalkyl radical, CuII oxidation to epoxide, and finally acid-catalyzed rearrangement.Data on oxidation of alkyl aromatics with additional functional groups are presented.With ether and amino groups, oxidation occurs at the functional group even when it is remote from the aryl nucleus.These and previous data are summarized to give a coherent picture of the various paths by which aryl side chains may be degraded via initial radical cation intermediates.
- Walling, Cheves,El-Taliawi, Gamil M.,Amarnath, Kalyani
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p. 7573 - 7578
(2007/10/02)
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