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  • 1
    ISSN: 0170-2041
    Keywords: 6H-1,2-Oxazines ; Methoxyallene ; Nitroso alkene ; Hetero Diels-Alder reaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective hetero Diels-Alder reactions of in-situ generated nitroso alkenes 2-4 with methoxyallene derivatives 1 provide 4H-1,2-oxazines 5-7 with an exo-methylene group at C-5. These primary cycloadducts are smoothly transformed into conjugated 6H-1,2-oxazines 11-13 by base or acid catalysis. The bicylic nitroso alkene 17 and methoxyallene (1a) combine to give the tricyclic 1,2-oxazine 18, thus demonstrating that an exo-transition state is favoured in this [4+2] cycloaddition. The reaction of the sterically hindered methoxyallene 1h with nitrosostyrene affords the cycloadduct 22 in low yield. 22 is very likely formed by a two-step Diels-Alder reaction via a zwitterion. Allene derivatives lacking a methoxy group are not sufficiently reactive towards nitroso alkenes and do not provide 1,2-oxazines.
    Additional Material: 8 Tab.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0170-2041
    Keywords: 1,2-Oxazine, siloxy-substituted ; Nitroso alkene ; Silyl enol ether ; Hetero Diels-Alder reaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of 5,6-dihydro-4H-1,2-oxazines 3 and 4 is prepared in good yields from silyl enol ethers 1 and nitroso alkenes 2a or 2b, respectively. A systematic variation of substituents reveals preparative scope and limitations of this hetero Diels-Alder reaction with inverse electron demand. The cycloaddition is rather sensitive with regard to steric effects - in particular, large groups at C-1 of the silyl enol ethers 1 completely prevent the reaction. On the other hand, excellent diastereo-selectivities are observed employing appropriately substituted olefins. The presented method for the synthesis of 1,2-oxazines also displays high regio- and periselectivity as is demonstrated by transformations 1r→3r and 13→14.
    Additional Material: 10 Tab.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0947-3440
    Keywords: Electrocyclic reactions ; Hetero Diels-Alder reaction ; Thionitroso compounds ; Nitroso compounds ; N,S-Heterocycles ; N,O-Heterocycles ; Transition structures ; Calculations, ab initio, semiempirical ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structure and reactivity of the title compounds 1 were examined by ab initio methods at the post Hartree-Fock level of theory. Both compounds are expected to undergo electrocyclic ring closure to 4H-1,2-oxazete (4O) and 4H-1,2-thiazete (4S), respectively. The alternative electrocyclic reaction affording 2H-azirine 1-oxide (3O) and 2H-azirine 1-thioxide (3S) is definitely less favoured. (Thionitroso)ethylene (1S) is more reactive than the oxygen-containing congener. The same holds for the cycloaddition reaction with ethylene. [4 + 2] Hetero Diels-Alder reactions furnishing 4H-1,2-oxazine (5O) or 4H-1,2-thiazine (5S) are favoured over the [3 + 2] cycloaddition reactions providing 2H-pyrrole 1-oxide (2O) and 2H-pyrrole 1-thioxide (2S). The results of RMP2/6-31G* calculations and results obtained at lower levels of theory (RHF, PM3) are compared and discussed with respect to the experimental material available. Some additional single-point calculations at the computationally more demanding QC1SD/6-31G* level confirmed conclusions drawn from RMP2 calculations. Some test calculations also showed that the theoretical results are less affected by consideration of the temperature by statistic thermodynamics and by inclusion of solvent effects by a self-consistent reaction-field method.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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