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  • 1,2-Oxazines  (6)
  • Asymmetric catalysis  (3)
  • 2-Siloxycyclopropanecarboxylates  (2)
  • 1
    ISSN: 0947-3440
    Keywords: Cyclopropanecarboxylates, 2-siloxy ; Silyl enol ethers ; Diazoacetates ; [2+1] Cycloadditions ; Asymmetric catalysis ; Copper ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By variation of the aryl substituents of silyl enol ethers 1a-e and 1f-k, the influence of their nucleophilicity on the stereochemical outcome of cyclopropanation reactions was studied. Using the neutral Schiff-base catalyst 2 · Cu(OAc)2, there was only a weak effect. On the other hand, with the cationic bisoxazoline complex 3· CuOTf a remarkable increase in the enantioselectivity was observed with more strongly electron-attracting substituents such as a trifluoromethyl or nitro group.
    Additional Material: 6 Tab.
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  • 2
    ISSN: 1434-193X
    Keywords: Natural products ; Intramolecular Diels-Alder reactions ; 2-Siloxycyclopropanecarboxylates ; 1,7,9-Decatrien-3-ones ; Octalones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Starting from enones 7 and 8 siloxydienes 9 and 12 were synthesized. Cyclopropanation and subsequent alkylation with sorbyl bromide provided tetrasubstituted siloxycyclopropanes 15 and 16 in good overall efficiency. Their ring cleavage with fluoride reagents gave 1,7,9-decatrienones 5 and 17; the latter was converted into the protected compound 6. The thermal intramolecular Diels-Alder reactions of 5 and 6 were compared with those of related trienones 3 and 4. Octalones 1, 2, 21, and 22 were formed as mixtures of three or four diastereomers. Thus, for the stereoselective construction of natural products such as α-eudesmol or dihydromevinolin alternative reaction conditions for the cycloaddition step have to be developed.
    Additional Material: 5 Tab.
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  • 3
    ISSN: 0170-2041
    Keywords: 1,2-Oxazines ; Aziridines ; Hydride reagents ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 6H-1,2-Oxazines 1 and 3 are converted into aziridines 2 and 4, respectively, by reduction with LiAlH4. Reduction of 1,2-oxazine 5 lacking the 6-alkoxy substituent leads to phenyl ketone 6, 6-Alkoxy-substituted 1,2-oxazine-3-carboxylates 7, 9, and 11 are reduced with NaBH4 to give the corresponding 3-hydroxymethylated compounds 8, 10, and 12 in good yields.
    Additional Material: 1 Tab.
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  • 4
    ISSN: 0170-2041
    Keywords: 1,2-Oxazines ; Oximes ; Pyrrolidines, 1-hydroxy- ; Beckmann rearrangement, reductive ; Hydrogenolysis, catalytic ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 6-Siloxy-substituted 1,2-oxazines 1 are transformed into 4-hydroxy ketoximes 2 by reduction with NaBH4 in ethanol. Reductive Beckmann rearrangement converts the oxime 2a into the 1,4-amino alcohol 7. Diisobutylaluminum hydride (DIBAH) induces a novel reductive ring contraction of 1 to provide either N-hydroxypyrrolidine derivatives 8 or nitrones 9. Other 1,2-oxazines lacking the 6-siloxy substituent are also studied under these reaction conditions. Catalytic hydrogenolysis either gives the acyclic amine 16 or it stops at the stage of the proline derivative 21. Mechanistic features of these synthetically valuable transformations are discussed.
    Additional Material: 5 Tab.
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  • 5
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Deprotonation, stereoselective ; Substitution with retention of configuration ; Ring opening, reductive ; Amino alcohols ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 6-(trimethylsilyl)methyl-substituted 1,2-oxazine 1 can smoothly be deprotonated with n-butyllithium at C-4 to give a lithiated species which reacts with a variety of electrophiles to provide the new 1,2-oxazines 5 - 16 in good yields. Besides the preparative aspect of these transformations, the high stereoselectivity of many reactions is also interesting from a mechanistic point of view. By deprotonation of the 4-deuterated compound 5a it has been proven that n-butyllithium removes exclusively the proton (or deuteron) cis to the 6-CH2SiMe3 group. Also, in most cases the reaction of lithiated 1 with electrophiles occurs with overall retention of configuration to afford preferentially cis-1,2-oxazines (series a). A mechanistic proposal for this highly stereoselective deprotonation process, which seems to be governed by the 1,2-oxazine oxygen, is discussed including a comparison with a recently reported ab initio calculation dealing with oxime ethers. Similar deprotonation/substitution reactions are described for 1,2-oxazines 14, 2, 3, and 4. Possibly due to a differing carbanion structure a deviating behavior is observed in some cases. Several acidinduced and reductive ring-opening reactions of 1, 6a, 8a, and 14a demonstrate the potential of 4-substituted 1,2-oxazines for the stereoselective synthesis of polyfunctionalized compounds.
    Additional Material: 11 Tab.
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  • 6
    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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  • 7
    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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  • 8
    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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  • 9
    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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  • 10
    ISSN: 0009-2940
    Keywords: 2-Siloxycyclopropanecarboxylates ; Silyl enol ethers ; [2 + 1] Cycloaddition ; Asymmetric catalysis ; (Schiff base)copper 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, II[1]. - Influence of the Silyl Enol Ether Structure on the Enantioselective Cyclopropanation with Methyl DiazoacetateOn the basis of reactions of (Z)-1-phenyl-1-(trimethylsiloxy)-1-propene (1a) with methyl diazoacetate (2) in the presence of different chiral (Schiff base) copper catalysts 4-Cu we extended our study to the silyl enol ethers 1b-1j. The highest enantiomeric excesses were obtained in most cases with catalyst 4b-Cu. Whereas cyclopropanations of 1-aryl- and 1-alkenyl-substituted silyl enol ethers 1b-1h gave moderate to good enantioselectivities (up to 80%ee), alkyl-substituted olefins 1i and 1j provided less satisfactory results. By deprotonation and alkylation of 3c, 3h, and 3i it could be established that for these cyclopropanes the predominating absolute configuration of cis- and trans-cyclopropanes is equal at C-1, but consequently opposite at C-2. The influence of the silyl enol ether structure on the enantioselectivities achieved is briefly discussed.
    Additional Material: 4 Tab.
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