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  • Inorganic Chemistry  (22)
  • Regioselectivity  (3)
  • Transition structures  (3)
  • Amino acids  (2)
  • 11
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Hydrogenation, catalytic ; Amino alcohols ; γ-Amino acids ; Pyrroles ; γ-Lactams ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: While palladium-catalyzed hydrogenation of 3-phenyl-6H-1,2-oxazine 1 produces primary amine 5 in a nitrogen-transposition reaction, the reductions of the related 1,2-oxazines 2, 10, and the 1,2-oxazin-6-one 3 afford the expected amino alcohols 4, 11, and the γ-amino acid 6, respectively, with low diastereoselectivites. In the presence of acetic acid 3 is reductively converted into γ-keto carboxylic acid 9 and 1 into the γ-lactam derivative 12 probably by a ring contraction to a nitrone intermediate. Raney nickel as the catalyst is able to transform 1,2-oxazine 7 bearing an exo-methylene unit into 3,4-dihydro-2H-pyrrole 13. The reaction of 6H-1,2-oxazine 1 with aluminium amalgam produces pyrrole 14 in moderate yield. Treatment of 1 with sodium in 2-propanol brings about its transformation into pyrrolidine derivative 15 together with pyrrole 14 and amino alcohol 4 as minor products. The chemoselectivity and stereoselectivity of these reductions are discussed including mechanistic proposals for the multistep processes involved.
    Additional Material: 1 Tab.
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  • 12
    ISSN: 0009-2940
    Keywords: Cyclopropanecarboxylates, 2-silyloxy, methyl ; [2 + 1] Cycloaddition ; Enantioselectivity ; Asymmetric catalysis ; Copper-Schiff base complexes, chiral ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Optically Active Siloxycyclopropanes by Asymmetric Catalysis, I. - Influence of the Catalyst on the Cyclopropanation of (Z)-1-Phenyl-1-(trimethylsiloxy)prop-1-eneThe cyclopropanation of silyl enol ether 1a with methyl diazoacetate (2a) and diazoesters 2b-d in the presence of optically active copper salicylimine complexes 4-Cu was systematically studied. Up to 88% enantiomeric excess in products 3 were obtained by employing the appropriate reaction conditions and the optimal catalyst ligands. Thus, for the first time respectable optical yields were achieved in asymmetric cyclopropanations of silyl enol ethers with easily available copper-Schiff base catalysts. The ring opening of the separated diastereomers of 3a employing nBu4NF provided methyl γ-oxocarboxylate 6a in good optical purity. This demonstrates that this process occurs without racemization and also that 3a is formed with the same absolute configuration at C-1.
    Additional Material: 7 Tab.
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  • 13
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Bromination, radical ; SN2 reaction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,6-Dihydro-4H-1,2-oxazines 1a-b, 2, and 3 are easily brominated at C-4 with N-bromosuccinimide/dibenzoyl peroxide in tetrachloromethane. The bromo substituent is incorporated with surprisingly high diastereoselectivity trans to the substituent at C-6. 4-Bromo-5,6-dihydro-4H-1,2-oxazines are useful reagents for substitution reactions with N-nucleophiles such as primary amines and azide ions. Inversion of configuration at C-4 provides derivatives of 4-amino-1,2-oxazines with uniform relative configuration. As a minor byproduct the dibromo adduct 7 is obtained by bromination of 3. The dehydrohalogenation of this compound allows the synthesis of the 4-bromo-6H-1,2-oxazine 12. The presented reaction sequence thus constitutes an “umpolung” reaction that allows the introduction of nucleophiles into a position of the oxazine ring that so far was accessible only for electrophiles. The diastereoselectivity of the bromination reaction is discussed.
    Additional Material: 4 Tab.
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  • 14
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Hetero Diels-Alder reactions ; Nitroso alkene cycloaddition ; exo: endo Selectivity ; Stereospecificity ; Silyl enol ethers, relative reactivity of ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mechanism of the hetero Diels-Alder reaction of nitroso alkenes 2 with silyl enol ethers and other olefins has been investigated. Using the bicyclic nitroso compound 2a a study of the exo/endo selectivity has demonstrated that the exo approach is preferred with the siloxyethene l a as dienophile. On the other hand, the siloxycyclopentene 1 c gives a mixture of cycloadducts 3 c with an excess of endo product (endo: exo = 82: 18). The stereospecificity of the nitroso alkene cycloaddition could be demonstrated with the stereochemically homogeneous silyl enol ethers 1 b and 1 d. Experiments with enol ethers 1 f and 1 g also occur stereospecifically. α-Nitrosostyrene 2b reveals surprisingly high kE/Z values when E/Z-isomeric olefins are compared in competition experiments. Also, a detailed reactivity scale of 2b including various structurally different silyl enol ethers and other typical dienophiles shows that a complex interplay of electronic and steric effects is operating. The large influence of steric effects is taken as evidence for a highly ordered transition state in the cycloaddition. All mechanistic details for the Diels-Alder reactions of nitroso alkenes 2 with (silyl) enol ethers are in strong accord with a concerted mechanism and exclude the involvement of zwitterions or diradicals as intermediates.
    Additional Material: 7 Tab.
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  • 15
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines, lithiated ; Deprotonation ; Alkylation ; Deuteration ; Regioselectivity ; Calculations, MNDO ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,6-Dihydro-5-methylene-4H-1,2-oxazine 1 is smoothly converted by n-butyllithium into 1-Li which reacts with electrophiles such as D2O, carbonyl compounds, dimethyl sulfide, or an azo diester to give the γ-adducts 4a-4f. On the other hand, alkylation of 1-Li occurs exclusively at C-4 of the heterocycle and provides the α-adducts 3g and 3h. These reactions require the activation of 1-Li by tetramethylethylenediamine. Treatment with allyl bromide and methyl acrylate affords mixtures of regioisomers 3 and 4. 1,2-Oxazine 5 with a conjugated C = C bond is less acidic than 1 but is also converted into 1-Li, whilst compound 6, lacking the 6-methoxy group, is not deprotonated under standard conditions. The dianion of 1,2-oxazine 7 is generated by employing an excess of n-butyllithium. This dianion displays a similar regiochemical behavior as 1-Li. Deuterium is exclusively incorporated into the γ-position to give product 8, while methylation occurs at C-4 to produce 9. 1,2-Oxazine 3g with an additional 4-methyl group can also be metalated and affords γ-adducts 10 and 11 upon reaction with D2O or acetone. Treatment with methyl iodide gives a 3:1 mixture of regioisomers 12 and 13. Deuteration of 1,2-oxazines 14 and 16 bearing a 3-CF3 or 3-CO2Et substituent requires more severe deprotonation conditions to provide the γ-adducts 15 and 17 in moderate yields. MNDO calculations of neutral 1,2-oxazines, the corresponding carbanions, and the lithium compounds allow an insight into the structure and charge distribution of these species, and also an estimation of the relative acidities. The regioselectivity of reactions of 1-Li is discussed on the basis of these semiempirical calculations and comparison with related ambident nucleophiles.
    Additional Material: 1 Ill.
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  • 16
    ISSN: 0009-2940
    Keywords: Chromium complexes ; Carbene complexes ; 1,3-Dienes, electron-deficient ; [2 + 1] Cycloadditions ; Vinylcyclopropanes, donor-acceptor-substituted ; Diastereoselectivity ; Regioselectivity ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermal reactions of Fischer carbene complex 1 with trisubstituted electron-deficient 1,3-dienes 7, 8, 23, 25, 32, and 35 provided highly substituted vinylcyclopropanes in good yields. The carbene transfer proceeds highly regioselectively favouring cyclopropanation of the double bond not bearing the ester function. In addition, the diastereoselectivity is generally fairly high in preference of cyclopropanes with the methoxy group cis-positioned with respect to the olefin moiety. The reaction of methylcarbene complex 2 with diene ester 8 displays inversed regioselectivity. These observations are discussed together with solvent effects, and a mechanistic rationale is presented.
    Additional Material: 5 Tab.
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  • 17
    ISSN: 0009-2940
    Keywords: β-Formyl esters ; Titanium enolates ; Lithium enolates ; Silyl enol ethers ; Chelate control ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aldol-type additions of metal enolates 2-Met derived from pinacolone to the chiral β-formyl carboxylate 1a were optimized. The highest trans:cis ratio (86:14) of the products 3 was obtained when the trichlorotitanium enolate 2-TiCl3 was combined with 1a precomplexed with one equivalent of TiCl4. The lithium enolate 2-Li is rather unselective. The simple diastereoselectivity of prochiral enolates 4-Met was first examined with achiral β-formyl carboxylates 1b and 1c. Appropriate reagents made products with high anti or with high syn selectivity available when the unbranched aldehyde 1b was the electrophile. In contrast, the sterically more hindered aldehyde 1c provided syn products with all enolates 4-Met employed. Finally, chiral aldehyde 1a was combined with prochiral enolates 4-Met. Conditions could be found which furnished either the trans/anti or the trans/syn product 7 with good selectivity. The results are discussed and compared with reactions of related metal enolates with aldehydes capable of chelate formation.
    Additional Material: 2 Tab.
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  • 18
    ISSN: 0009-2940
    Keywords: Chromium carbene complexes ; Acrylonitrile derivatives, donor-substituted ; Insertion reaction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: A Facile Synthesis of Novel Alkenyl-Substituted Chromium Carbene ComplexesChromium carbene complex 1 adds to β-donor-substituted acrylonitrile derivatives 3 and 5 giving the novel alkenyl-substituted carbene complexes 4 and 6, respectively. The structures of these insertion products were determined spectroscopically. Structural data obtained for 4 by X-ray analysis are compared to those of related carbene complexes.
    Notes: Der Chromcarben-Komplex 1 reagiert mit den β-donorsubstituierten Acrylnitrilderivaten 3 und 5 zu neuartigen alkenylsubstituierten Carben-Komplexen 4 und 6. Die Struktur dieser Insertionsprodukte wurde spektroskopisch geklärt. Die von 4 durch Kristallstrukturanalyse erhaltenen Daten werden mit denen verwandter Carben-Komplexe verglichen.
    Additional Material: 1 Ill.
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  • 19
    ISSN: 0009-2940
    Keywords: Diels-Alder reaction, intramolecular / 1,7,9-Decatrien-3-ones / Octalone derivatives / Siloxycyclopropanes ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselectivity of Intramolecular Diels-Alder Reactions of 1,7,Q-Decatrien-3-ones to Octalone Derivatives2-Alkenyl-substituted 2-siloxycyclopropanes 1 and 2 serve as protected enone equivalents and are converted into key building blocks 5-8 in excellent yields. After desilylation and ring cleavage these compounds provide 1,7,9-decatrien-3-ones 9-12, which undergo smooth intramolecular Diels-Alder reaction at room temperature or approximately 100°C. The resulting octalone derivatives 13-16 are formed with exclusive or preferred cis connection of the rings. The relative configurations of the cycloadducts are secured by equilibration experiments and NMR spectroscopy (selective proton decouplings). For 11 → 15 and 12 → 16 the cis selectivitiy can remarkably be enhanced by trifluoroacetic acid catalysis. This high noninduced diastereoselectivity is caused by the endo approach of the reactive molecular sites, whereas the configuration of the third stereogenic centre (C-2) is determined by the folding of the linking carbon chain. Preference of a boat conformation with equatorial ester group endo-Beq may be assumed and appears to be general for intramolecular cycloadditions of such trienones. This effect as well as the influence of further substituents at the diene or dienophile portion on stereoselectivity and the reactions of other model compounds are discussed.
    Additional Material: 8 Tab.
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  • 20
    ISSN: 0009-2940
    Keywords: Epoxidation ; Enamines, N-silylated ; Indoles ; Epoxides, disilylamino- ; Ketones, α-amino ; Indolinones ; Dimethyldioxirane ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The oxygen transfer to the enamines 1a-f by dimethyldioxirane (DMD) in acetone solution leads to the α-amino epoxides 2a-f. The stability of the α-amino epoxides 2a-f depends only on the type of substitution at the nitrogen atom. Thus, while the epoxides 2a, b could be characterized spetroscopically, the epoxidation of the enamines 1c-f resulted in the corresponding α-amino ketone 3c (hydride shift) and the amides 4d-f (alkyl shift). The α-[bis(trimethylsilyl)]amino epoxides 2a, b represent the first observable enamine oxides and emphasize the value of stabilizing such labile epoxides through disilylation of the enamine nitrogen atom.
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