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  • 1
    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.
    Type of Medium: Electronic Resource
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  • 2
    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.
    Type of Medium: Electronic Resource
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