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  • 1
    ISSN: 0170-2041
    Keywords: 1,2-Oxazines, transformations of ; Pyrroles ; Molybdenum, hexacarbonyl- ; 1,4-Dicarbonyl compounds ; Oximes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of 6-siloxy-substituted 5,6-dihydro-4H-1,2-oxazines (abbreviation: 1,2-oxazines) 1, 3 could be transformed into di- and trisubstituted pyrroles 2, 4 by means of molybdenum hexacarbonyl. The mechanism of this deoxygenating ring contraction is discussed. With two bicyclic 1,2-oxazines an acid-catalyzed fragmentation affording α-methylenecycloalkanones 7 has been observed, while other 1,2-oxazines rearrange in methanolic acid to give nitrones 9, 10. The desilylation of 6-siloxy-substituted 1,2-oxazines 1, 3 employing NEt3 · 3 HF is a very general and smooth process providing 6-hydroxy-1,2-oxazines 11, 12 or their corresponding acyclic tautomers 13, 14 in high yields. For two examples of 11 deoximations by use of formalin could be achieved with moderate efficiency giving 1,4-dicarbonyl compounds 15.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0170-2041
    Keywords: Pyrenophorin ; (Z)-Jasmone ; 1,2-Oxazine ; Pheromones ; Pyrroles ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methyl (E)-5-bromo-4-oxo-2-pentenoate (6) and the corresponding oxime 5 were prepared in high yields by starting from methyl levulinate (3). The novel nitrosoalkene 8, generated in situ from oxime 5, smoothly added to silyl enol ether 7 to give 1,2-oxazine 9 and nitrone 10 as byproduct. Methods are described transforming intermediate 9 into unsaturated diketo ester 11, or oximes 13, 15, and 18, which may serve as precursors of pyrenophorin. Reductive ring cleavage of 1,2-oxazine 9 with Raney nickel efficiently afforded the saturated diketo ester 19. Its intramolecular aldol reaction gave cyclopentenone derivative 20, which is a known precursor of (Z)-jasmone. Cycloaddition of methyl α-nitrosoacrylate to silyl enol ether 22 provided 1,2-oxazine 23. This compound was directly converted into the ant-trail pheromone 24 by hexacarbonylmolybdenum-promoted ring contraction.
    Type of Medium: Electronic Resource
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  • 3
    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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