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  • 1,2-Oxazines  (6)
  • Regioselectivity  (3)
  • Transition structures  (3)
  • Amino acids  (2)
  • Amino alcohols  (2)
  • 1
    ISSN: 1434-193X
    Keywords: Rearrangements ; Density functional calculations ; Substituent effects ; Transition structures ; Small-ring compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Two possible types of competing [1,3] rearrangements of divinylcyclopropane derivatives and of the hetero-analogous compounds vinylcyclopropanecarbaldehyde and vinylcyclopropanecarbthioaldehyde to five-membered ring compounds have been investigated theoretically by (U)DFT (Becke3LYP/6-31G*) methodology. Although both of the rearrangement pathways correspond to a reaction of the monosubstituted cyclopropane substructure, i.e. a reaction of vinylcyclopropane or of cyclopropanecarb(thio)aldehyde, new features in the reactivity are seen due to the combination of two substituents. Reaction parameters such as reaction and activation energies of the [1,3]-sigmatropic rearrangements have been calculated and compared. Substituent effects of hydroxy and formyl (donor and acceptor) groups in specific positions have also been considered. On the basis of these calculations, the [1,3] rearrangement of vinylcyclopropanecarbaldehydes to dihydrofurans can be expected to be kinetically favored over their conversion to cyclopentenes, while formation of the latter should be favored thermodynamically. This prediction is borne out by the experimental findings. Depending on the substitution pattern, the [1,3]-sigmatropic rearrangement of vinylcyclopropanecarbaldehydes may compete with the [3,3]-sigmatropic rearrangement. For the other investigated structures, the [1,3] rearrangement is strongly disfavored.Supporting information for this article is available on the WWW under -http://www.wileY-Vch.de/contents/jc_2046/1999/98456_s.pdf or from the author.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    ISSN: 0170-2041
    Keywords: Amino acids ; Peptides ; Cyclopropanes, silylamino- ; Condensation, fluoride-induced ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The N-silylated methyl 2-aminocyclopropanecarboxylate 1 can be incorporated into a dipeptide via a CsF-mediated condensation reaction with N-tosylated phenylalanine chloride 5. Due to its instability the corresponding free β-amino acid 4 could not be isolated up to now, and peptides containing this structural element have previously not been accessible. After desilylation, the aminomethyl-substituted cyclopropane or cyclopropene derivatives trans- 10, 12, and 13 also provided N-protected dipeptides in good yields.
    Additional Material: 2 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: 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.
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  • 6
    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.
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  • 7
    ISSN: 0947-3440
    Keywords: Pericyclic reactions ; Hetero-Cope-type rearrangement ; Cyclopropanes ; Heterocycles ; Substituent effects ; Transition structures ; Density functional calculations ; Ab initio calculations ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The prototypical 1,2-cis-vinylcyclopropanecarbaldehyde-to-2,5-dihydrooxepin hetero-Cope-type rearrangement was studied by “exact” first-principle methods. The reaction pathway was examined. The reaction, as well activation energies, was calculated for the unimolecular transformation of vinylcyclopropanecarbaldehyde and various derivatives. The derivatives differ from vinylcyclopropanecarbaldehyde by replacement of the formyl (CH=O) by the thioformyl (CH=S) or formiminyl (CH=NH) group and, in part, by replacement of hydrogen atoms at the substituted carbon atoms of the cyclopropane ring by hydroxyl or formyl substituents. The experimental reaction parameters of vinylcyclopropanecarbaldehyde are surprisingly well reproduced by B3LYP/6-31G* density functional and MP2/6-31G* ab initio quantum theoretical calculations. Reactant and product are nearly isoenergetic, while the activation energy amounts to about 25 kcal/mol. In the case of the nitrogen and sulfur containing compounds the isomeric seven-membered ring structures are considerably favoured over the cyclopropanes. Due to a low calculated activation energy a rapid formation of the 2,5-dihydrothiepin is expected. Substitution of the hydrogen at the substituted ring carbon atoms in vinylcyclopropanecarbaldehyde by OH and CH=O also lowers the barrier and increases, in general, the exothermicity of the reaction. As shown by the reaction energies of isodesmic reactions both reactants and products are stabilized by substitution. However, the seven-memberd ring compounds are more strongly stabilized than the cyclopropanes. The OH group exerts a different effect depending on whether the linkage is geminal or vicinal to the C=X group. The difference is caused by hydrogen bond formation in geminal arrangements. The substituent effects in the cyclopropane series parallel those for the prototypical Cope-type and Claisen-type series. The particular feature of the cyclopropane series is the lower stability of the cyclopropanes relative to the corresponding open chain congeners. This is obviously due to the ring strain which over-compensates for attractive interactions between cyclopropane and the substituents. In consequence, the formation of seven-membered ring compounds proceeds more easily than the formation of the corresponding compounds in related Cope-type rearrangements.
    Additional Material: 7 Ill.
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  • 8
    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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  • 9
    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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  • 10
    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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