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  • General Chemistry  (51)
  • Rearrangements  (3)
  • Ring expansion  (3)
  • [3 + 2] Cycloaddition  (3)
  • Asymmetric synthesis  (2)
  • 1
    Electronic Resource
    Electronic Resource
    Ring Expansion of 2-Alkylidenedihydroquinazolines to Iminodihydro-1,4-benzodiazepines by Methanesulfonyl and Trifluoromethanesulfonyl Azide (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1577-1587 
    Details
    ISSN: 1434-193X
    Keywords: Azides ; Cleavage reactions ; Cycloadditions ; Nitrogen heterocycles ; Polycycles ; Ring expansion ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---2-Alkyl-1-methylquinazolinium hexafluorophosphates 9 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinazolines 10, which were investigated by NMR spectroscopy. Trapping with methanesulfonyl azide (5a) of 10 in situ or subsequent treatment with trifluoromethanesulfonyl azide (5b) gives mixtures of colourless (15) and intensely yellow N-sulfonylimino-1,4-benzodiazepines 16 along with products due to cleavage of the exocyclic double bond of 10, viz. 11 and 13. The ethylidene compound 10b yields the bicyclic products 18 and 19, apparently by complex sequences of reactions that are triggered by removal of the acidic proton at C-2 of 16b and 16f. The structures of the products are based on spectroscopic evidence and X-ray diffraction analyses performed on 15b, 16d, 16e, and 19.
    Additional Material: 6 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Synthesis of Axially Chiral Biaryls by Atropo-Diastereoselective Cleavage of Configurationally Unstable Biaryl Lactones with Menthol-Derived O-Nucleophiles (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3047-3055 
    Details
    ISSN: 1434-193X
    Keywords: Axial chirality ; Biaryls ; Atropisomerism ; Dynamic kinetic resolution ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Configurationally unstable lactone-bridged biaryls 4 are cleaved atropo-diastereoselectively using chiral menthol-derived alkali metal alkoxides, to give axially chiral biaryl esters of type 5 in high yields and excellent diastereomeric ratios of up to 〉 99:1. The method permits the optional preparation of each of the two atropisomers from the same lactone precursor (“atropo-divergence”), simply according to the choice of the appropiate mentholate or its enantiomer as the O-nucleophile - or by the use of the mentholate in solution or in suspension. Undesired minor atropisomers of 5 possibly formed (if at all) can be recycled (“axially chiral economy”) by cyclization back to the lactone 4. For the preparation of larger amounts of enantiopure biaryl alcohols 9, an efficient reaction sequence was developed: alcoholysis of 4 → in situ reduction → crystallization. The synthetic value of these alcoholysis reactions for asymmetric biaryl synthesis is illustrated by the transformation of 5 into a broad series of enantiopure biaryls of type 6 with various functional groups ortho to the axis.
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  • 3
    Electronic Resource
    Electronic Resource
    Ring Expansion of Heterocyclic Ketene N,X-Acetals and 2-Alkylidenedihydroindoles with Methanesulphonyl Azide by [3 + 2] Cycloaddition and Subsequent Extrusion of Molecular Nitrogen (1996)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1541-1549 
    Details
    ISSN: 0947-3440
    Keywords: [3 + 2] Cycloaddition ; Ketene N,X-acetals, cyclic ; Azides, electrophilic ; Amidines, cyclic N-sulphonyl- ; Ring expansion of heterocycles ; 1,2-Shift of carbon, nitrogen, sulphur, or selenium ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methanesulphonyl azide (2) reacts readily with isolated cyclic ketene N,X-acetals of type 1, viz. 10a, d, 15b, d, and 22a-c, or those that are generated in situ by deprotonation of the corresponding 2-alkylbenzazolium tetrafluoroborates, i.e. 14a → 15a, 14b → 15b, and 18 → 19. Ring-expanded products are formed by extrusion of molecular nitrogen from intermediate labile [3 + 2] cycloadducts 3 with concomitant 1,2-shift of N (route A1 → 12, 24) or X (route A2 → 16, 20, 21). In addition, 3 may undergo [3 + 2] cycloreversion into N-sulphonylimine 5 and diazoalkane 6 (route B → 13, 17, 25). The configurations of the cyclic N-sulphonylamidines 16b and 21b, the N-sulphonylimine 24 and the N-sulphonylamine 27 are elucidated by means of X-ray diffraction analyses. The ratio of the (useful) ring-expansion reactions vs. the unwanted formation of 5 + 6 is hardly influenced by the solvent employed and temperature of the experiment but strongly by the nature of the potential migrating atom and the substituents at the α-carbon atom.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199619961008
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  • 4
    Electronic Resource
    Electronic Resource
    An Attempted Synthesis of 2,6-Bis(phenylazo)barbaralane (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1733-1738 
    Details
    ISSN: 0947-3440
    Keywords: Azo compounds ; Cyclizations ; Hydrazones ; Michael additions ; Nitrogen heterocycles ; Oxidations ; Polycycles ; Rearrangements ; Strained molecules ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Treatment of the unsaturated ketone 6 with phenylhydrazine fails to afford the unsaturated phenylhydrazone 3 but gives the tricyclic phenylhydrazoketone exo-8, which is oxidised with iron(III) chloride to yield the phenylazoketone exo-9. The configuration of exo-9 is elucidated with the help of an X-ray diffraction analysis. The unsaturated phenylazo compound 14 is obtained from Meerwein's diketone 10 in two ways. Iodination of the phenylhydrazone 11 in the presence of pyridine followed by elimination of pyridinium iodide from the bispyridinium salt 13 with aqueous sodium hydroxide in dimethyl sulphoxide furnishes 14 in low yield. A somewhat better yield is more conveniently achieved when the dibromodiketones exo,exo- and exo,endo-12 are allowed to react with phenylhydrazine in the presence of pyridine. Bromination of 14 with N-bromosuccinimide in cyclohexane as solvent affords the labile dibromide 15. Both reagents employed, viz. the zinc/copper couple in refluxing tetrahydrofuran and butyllithium at low temperature, fail to convert 15 into the hoped-for bis(phenylazo)barbaralane 1 but instead give rise to the formation of the isomer 17, either by cyclisation of the intermediate anion 16 or by rearrangement of 1.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199719970815
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  • 5
    Electronic Resource
    Electronic Resource
    Extension of Saunders' Isotopic Perturbation Method as Probe for the Structures in Solution of 2,4,6,8-Substituted Barbaralanes - NMR-Spectroscopic Evidence for the Coexistence of Localised and Delocalised States (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 691-693 
    Details
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---No Abstract.
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  • 6
    Electronic Resource
    Electronic Resource
    Tetraacylethenes as Dienophiles and Hetero Dienes in Two-Step Diels-Alder Reactions (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 3343-3351 
    Details
    ISSN: 1434-193X
    Keywords: C-C coupling ; Cycloadditions ; Dehydrogenations ; Electron deficiency ; Isomerisations ; Oxidations ; Zwitterionic intermediates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation with barium manganate of the enol of tetraacetylethane (10) affords tetraacetylethylene (7a) in good yield. Treatment of the 1,3-diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling yielding 10 and diacetyldibenzoylethane 17, respectively, as has been reported by Moriarty et al. Instead the known difluoroborylenolates 16 are formed. Oxidative coupling of the sodium enolate of 15b with iodine affords the meso-tetraketone meso-17 and diacetyldibenzoylethylene (E)-7b besides small amounts of rac-17 and the bisenol 19. In the presence of 1,4-diazabicyclo[2.2.2]octane, meso-17 and 19 tautomerise yielding identical equilibrium mixtures of meso-17, rac-17, and 19 (26:29:45). Dehydrogenation with 5,6-dichloro-2,3-dicyanobenzoquinone of meso-17 yields (E)-7b and an isomer (77:23) which was assigned structure (Z)-7b on the basis of spectroscopic evidence. - Tetraacetylethylene (7a) reacts with 1,2-dimethylenecyclohexane to afford two different [4+2]cycloadducts, viz. the spirocyclic dihydropyran 25 (30 %) and the tetraacetyloctalin 27. Whereas the latter is stable, the former isomerises to the latter on heating. Formation of both [4+2]cycloadducts and the isomerisation 25 → 27 are interpreted by invoking the zwitterion 26 as common intermediate. Monoolefines that cannot generate stable carbenium ions don't give [4+2]cycloadducts with 7a. In contrast, α-methylstyrene yields the oxa Diels-Alder product 30. The structures of (E)-7b, 19, 25, and 27 are established by X-ray diffraction analyses.
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  • 7
    Electronic Resource
    Electronic Resource
    Ring Expansion of 2-Alkylidenedihydroquinolines to 2-Iminodihydro-1-benzazepines by Phenyl, Methanesulphonyl, and Trifluoromethanesulphonyl Azide[1] (2000)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 507-520 
    Details
    ISSN: 1434-193X
    Keywords: Azides ; Cleavage reactions ; Cycloadditions ; Nitrogen heterocycles ; Photolysis ; Ring expansion ; Spiro compounds ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---2-Alkyl-1-methylquinolinium hexafluorophosphates 1 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinolines 2, which are investigated by NMR spectroscopy. 1,3-Dipolar cycloaddition of phenyl azide to 2 yields the spirocyclic products 10. While, at 80-110 °C, the [3 + 2] cycloaddition that afforded (u)-10f is reversible and accompanied by epimerisation to give (l)-10f, thermolysis of the dimethyl compounds 10b and d affords the ring-expanded products 14b and d, respectively, in good yields along with molecular nitrogen. Irradiation of 10d with light of λ 〉 320 nm results in the formation of similar amounts of 14d and [3 + 2] cycloreversion products, viz. 2-diazopropane (5b) and the N-phenylimine 15d. - Trapping of 2 by methanesulphonyl azide (18a) gives mixtures of the products of ring expansion (21b, d-f, 10-50 %) and [3 + 2] cycloreversion (22a, d, 10-80 %) of the apparently very labile intermediate spirocyclic cycloadducts 19. The ratio of 21 vs. 22 is significantly improved when 18a is replaced by trifluoromethanesulphonyl azide (18b), which affords the iminodihydrobenzazepines 21i-k in 50-75 % yield. The structures of the products are based on NMR evidence and X-ray diffraction analyses performed with 21b, d, and (ax,E)-21e.
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  • 8
    Electronic Resource
    Electronic Resource
    Selective Reactions, Resolution and Absolute Configuration of Bridged Hydroazulenes (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2651-2657 
    Details
    ISSN: 1434-193X
    Keywords: Functionalized hydroazulenes ; Wittig reaction ; Selective additions ; Resolution ; X-ray analyses ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Treatment of the bridged oxepine (±)-1 with a large excess of powdered sodium methoxide in Et2O or with triphenylphosphonium methylide leads to the functionalized hydroazulenes (±)-2 (69%) and (±)-3 (81%). Regioselective and stereoselective cyclopropanation of (±)-2 leads to the dibromocyclopropane derivative (±)-4. Epoxidation gives the epoxides (±)-5 and (±)-6. The resolution of the hydro-azulenone (±)-2 is achieved by fractional crystallization of the quinine salt (-)-8 of the corresponding mono acid (±)-7. The absolute configuration of (+)-7 is established by X-ray analysis of the quinine salt (-)-8.
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  • 9
    Electronic Resource
    Electronic Resource
    Photocycloadditions of Chloranil to Homobenzvalene, Norbornadiene, and Quadricyclane (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2093-2101 
    Details
    ISSN: 1434-193X
    Keywords: Photochemistry ; Cycloadditions ; Small ring systems ; Rearrangements ; Electron transfer ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Solutions of chloranil (CA) in toluene or benzene have been irradiated in the presence of the C7H8 valence isomers homobenzvalene (HB), norbornadiene (N), and quadricyclane (Q). In the case of HB, the adducts 3-6 were obtained, all of them having a rearranged structure. By a separate irradiation it was shown that 4 is a consecutive product of 3. The structure of 4 was established by X-ray diffraction analysis. These reactions are most probably initiated by an electron transfer (ET) from HB to 3CA, continue by bond formation between the radical ions to give the zwitterion 14, which undergoes Wagner-Meerwein rearrangements, and are completed by ring-closure between the charged centres of the resulting zwitterions. The oxidation potential of N is not larger than that of HB and the products 8-10 may well be formed through an ET to 3CA. However, the pathway leading to product 3 most probably involves diradical intermediates. This should also be the case for the adducts 11 and 12, which were produced upon irradiation of methyl phenylglyoxylate (PG) in the presence of N. In the case of Q, an ET to 3CA is again highly likely, followed by a combination of the radical ions to generate the zwitterion 29, which eventually collapses to produce the oxetane 8 as the sole identifiable product. The photoreactions of CA with N and Q proceeded much less efficiently in acetonitrile solution.
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  • 10
    Electronic Resource
    Electronic Resource
    Cycloallenes, 11.Part 10: Ref. [1] 1-Phenyl-1,2-cyclononadiene: Preparation and Dimerisation (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1791-1796 
    Details
    ISSN: 0947-3440
    Keywords: Allenes ; Carbenoids ; Cycloadditions ; Dimerisations ; Rearrangements ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1-Phenyl-1,2-cyclononadiene (10) was prepared by addition of dibromocarbene to 1-phenylcyclooctene and treatment of the resulting 9,9-dibromobicyclo[6.1.0]nonane 7 with methyllithium. In addition to 10, the 9-bromo-9-methylbicyclononanes cis- and trans-8 as well as the bicyclopropylidene 9 were formed. The dimerisation of 10 proceeded slowly already at room temperature and gave rise to the 1,2-bismethylenecyclobutane derivatives cis- and trans-11. From the complex mixture that resulted on thermolysis at 200°C of cis- and trans-11 only their isomer 12 could be isolated. Heating of a benzene solution of tetracyanoethylene and cis- and trans-11 afforded the Diels-Alder adduct 13 of cis-11, which rearranged to the benzodicyclononene derivative 14 on thermolysis.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199719970825
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