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  • Inorganic Chemistry  (22)
  • Regioselectivity  (3)
  • Transition structures  (3)
  • 6H-1,2-Oxazines  (2)
  • Amino acids  (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.
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  • 2
    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.
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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: 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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  • 5
    ISSN: 0170-2041
    Keywords: 6H-1,2-Oxazines ; γ-Lactams ; Lewis acids ; Ring contraction ; Phosphite, trimethyl ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Unexpected Formation of γ-Lactams from 6H-1,2-Oxazines Promoted by Lewis AcidsThe reaction of 6H-1,2-oxazine 1 with trimethylsilyl cyanide in the presence of titanium tetrachloride provides α,β-unsaturated γ-lactam 3. Similarly, Lewis acid-promoted addition of trimethyl phosphite towards 1 affords a mixture of γ-lactams 8 and 9. The structure of 9 was established by X-ray analysis. A plausible mechanism for the formation of these γ-lactams by Lewis acid-induced ring contraction is proposed.
    Additional Material: 1 Ill.
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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: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Preparation of Vinylsulfonates from Trimethylsilyl Enol Ethers  -  Synthetic Consequences of a Remarkable Cation EffectThe C/O-selectivity observed in the sulfonylation of the enolate ion 5 with benzenesulfonyl fluoride depends strongly on the nature of the gegenion. Li⊕ yields the β-oxosulfone 3 by C-sulfonylation exclusively. The fraction of O-Sulfonylation is increased with the size of the cation, yielding the vinylsulfonates 6b exclusively in the presence of Cs⊕ or quaternary ammonium ions. From this behaviour a regio- and stereoselective synthesis of vinylsulfonates is developed starting from the corresponding trimethylsilyl enol ethers.
    Notes: Die C/O-Selektivität der Sulfonylierung des Enolat-Ions 5 mit Benzolsulfonylfluorid zeigt einen starken Einfluß des Gegenions. Während mit Li⊕ ausschließlich C-Sulfonylierung zum β-Oxosulfon 3 erfolgt, wächst mit der Größe des Kations der Anteil an O-Sulfonylierung, bis mit Cs⊕ oder quartären Ammonium-Ionen ausschließlich Enolsulfonsäureester 6b entstehen. Dieses Verhalten wird für eine regio- und stereoselektive Synthese von Enolsulfonsäureestern aus den entsprechenden Trimethylsilyl-enolethern genutzt.
    Additional Material: 2 Tab.
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  • 9
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Several phenyl-substituted cyclopropane derivatives were investigated regarding rate and regiochemistry of their hydrogenolysis. The following reactivity sequence has been established: 1〈2〈4〈7〈9. Substitutent effect are discussed and compared to those reported for monoactivated cyclopropanes.
    Notes: Einige phenylsubstituierte Cyclopropane wurden hinsichtlich der Geschwindigkeit und Regiochemie ihrer Hydrogenolyse untersucht. Dabei wurde die folgende Reaktivitätsfolge ermittelt: 1〈2〈4〈7〈9. Die Substituenteneffekte werden diskutiert und mit denen bei monoaktivierten Cyclopropanen verglichen.
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  • 10
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Alkyl-substituted methyl 2-(trimethylsiloxy)cyclopropanecarboxylates 8a - 8f are opened by hydrogen in the presence of palladium on carbon affording 4-oxoalkanoci esters 10a - 10e by desilylation. 2-Phenyl-substituted cyclopropanes 8g - 8j, however, provide 4-phenylbutanoic esters 13g - 13j under these conditions. Formation of 13j is not stereoselective. Here the primary cyclopropane cleavage is followed by hydrogenolysis of the benzylic C - O bond. This subsequent reaction can be suppressed in the case of 8g by poisioning the catalyst with triethylamine; otherwise the tert-butyldimethylsiloxy compounds 11g - 11i have to be used. Butanoic esters 14h and 14i, respectively, are formed as mixtures of diastereomeres, which is another indication for nonstereoselective hydrogenolysis of cyclopropanes. Whereas slow desilylation by hydrogen was observed with the 3-phenyl-substitued cyclopropane derivative 8k, the vinylcyclopropane 8m in part suffers cleavage of the 1 - 3 bond of the three-membered ring. Regio- and stereoselectivity of the hydrogenolysis of the donor-acceptor-substituted cyclopropanes 8 and 11 are discussed.
    Notes: Die alkylsubstituierten 2-(Trimethylsiloxy)cyclopropancarbonsäureester 8a - 8f werden von Wasserstoff in Gegenwart von Palladium auf Kohlenstoff durch Desilylierung zu den γ-Oxocarbonsäureestern 10a - 10e geöffnet. Dagegen ergeben die 2-phenylsubstituierten Cyclopropane 8g - 8j unter diesen Bedingungen die 4-Phenylbutansäureester 13g - 13j. Die Bildung von 13j erfolgt stereounselektiv. Der primären Cyclopropanspaltung schließt sich hier eine Hydrogenolyse der benzylischen C - O-Bindung an. Diese Folgereaktion kann bei 8g durch Vergiftung des Katalysators mit Triethylamin, in anderen Fällen durch Einsatz der tert-Butyldimethylsiloxy-Verbindungen 11g - 11i verhindert werden. Die Butansäureester 14h bzw. 14i fallen als Diastereomerengemische an, was wiederum als Beleg für stereounselektive Cyclopropanhydrogenolyse gewertet wird. Während das 3-phenylsubstituierte Cyclopropanderivat 8k lediglich unter langsamer Desilylierung mit katalytisch erregtem Wasserstoff reagiert, findet man bei dem Vinylcyclopropan 8m auch teilweise eine Spaltung der 1 - 3-Bindung des Dreiringes. Regio- und Stereo-selektivität der Hydrogenolyse der Donor-Acceptor-substituierten Cyclopropane 8 und 11 werden diskutiert.
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