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  • 1,2-Oxazines  (1)
  • Alkylation  (1)
  • [2 + 1] Cycloaddition
  • Oximes
  • Wiley-Blackwell  (3)
  • 1990-1994  (3)
  • 1991  (3)
Collection
Publisher
  • Wiley-Blackwell  (3)
Years
  • 1990-1994  (3)
Year
  • 1
    ISSN: 0170-2041
    Keywords: Amino acids ; GABA analogues ; Aminocyclopropanes ; Enamines, N-silylated ; [2 + 1] Cycloaddition ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An Efficient Route to GABA-Analogous Amino Acids: Cyclopropanation of N-Silylated Allylamines and EnaminesN-Silylated allylamines 1 are effectively transformed into methyl cyclopropanecarboxylates 2 by methyl diazoacetate under Rh2(OAc)4 catalysis. Derivatives 2a and 2b are smoothly converted into trans-substituted amino acids 6a and 6b, respectively, and to bicyclic γ-lactams 5a and 5b. The pharmacologically interesting γ-aminobutyric acid (GABA) analogue trans-6a is now available in few steps. Photochemical and thermal Fe(CO)5-induced hydrogen shift converts allylamine derivatives 1 into N-silylated enamines 7. While enamine (E)-7a can be cyclopropanated with methyl diazoacetate under Cu(acac)2 catalysis to afford the desired cyclopropane derivatives 8a in good yield, the other enamines are rather unreactive towards the carbenoid. Use of an optically active catalyst provides 8a with an ee of 56% (cis) and 20% (trans). Acid-induced ring cleavage of 8a gives the β-formyl ester 10a, and reduction of 8a followed by desilylation provides the aminocyclopropane 14 in good overall yield, thus demonstrating that cyclopropanes like 8a can serve as useful synthetic intermediates.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    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.
    Type of Medium: Electronic Resource
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