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  • 1
    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.
    Additional Material: 4 Tab.
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  • 2
    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.
    Additional Material: 1 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Synthesis, Crystal Structure, and Circular Dichroism Spectra of (1S)-4,8-Diphenylbarbaralane-2,6-dicarbonitrile - Chiroptical Properties of the Transition State of a Degenerate Cope Rearrangement (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 1811-1823 
    Details
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Automerisation ; Circular dichroism ; Conformation analysis ; Enantiomeric resolution ; Polycycles ; Solid-state structures ; Solvent effects ; Thermochromism ; Transition states ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Diphenylbicyclo[3.3.1]nonane-2,6-dione rac-3 is resolved in 57 % overall yield by chromatographic separation of the diastereomeric (R)-N-(1-phenylethyl)carbamates 9 which are obtained from (R)-(1-phenylethyl) isocyanate (8) and the 6-hydroxydiphenylbicyclo[3.3.1]nonan-2-ones endo- and exo-4. The enantiomers (1R)- (e.r. = 98:2) and (1S)-3 (e.r. = 97:3) are regenerated from 9 by reduction with lithium aluminium hydride followed by Swern oxidation of the resulting diols 5. The title compound (1S)-1 is synthesised in three steps from (1S)-3 in improved yield on the route that had led to rac-1. The absolute configurations are established by X-ray diffraction analyses of the carbamates endo-(1R)-9 and exo-(1S)-9. X-ray diffraction analyses were also performed of the camphanoate (1R)-7, the intermediates rac-endo-4 and (1S)-3, and the title compound (1S)-1. Hydroxy ketone rac-endo-4 adopts similar conformations in the solid state and in solution as shown by a comparison of vicinal 1H,1H coupling constants from proton spectra with those calculated from torsional angles in the crystal. The molecular structures of (1S)-1 and (1S)-3 closely resemble those of the corresponding racemates investigated previously. These results show (i) that intermolecular interactions in the solid state are of minor importance and (ii) that the unusually long C2-C8 distance of (1S)-1 and rac-1 (168 pm) is a molecular but not an averaged property due to a non-degenerate Cope rearrangement in the crystal. CD spectra are reported for (1R)- and (1S)-3, the unsaturated dicarbonitrile (1S)-13, and (1S)-1. The CD spectrum of (1S)-1 exhibits a weak positive band at 459 nm where rac-1 shows a temperature-dependent absorption which has been assigned to the higher, vibronic state represented by rac-1*. The intensity of the weak CD band depends on the temperature and the solvents in the same way as the UV/Vis absorption of rac-1. This supports the conclusion that both bands originate from the same source, viz. the transition state 1* of the degenerate Cope rearrangement 1 ⇄ 1′.
    Additional Material: 11 Ill.
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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
    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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