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  • 1
    Electronic Resource
    Electronic Resource
    Photoextrusion of Molecular Nitrogen from Annulated 5-Alkylidene-4,5-dihydro-1H-tetrazoles: Annulated Iminoaziridines and the First Triplet Diazatrimethylenemethane (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 317-327 
    Details
    ISSN: 1434-193X
    Keywords: Aziridines ; Cleavage reactions ; Diradicals ; EPR spectroscopy ; Photolysis ; Rearrangements ; Ring contractions ; Small ring systems ; Strained molecules ; Tetrazoles ; Valence isomerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Deprotonation of the annulated tetrazolium salts 4, 6, 8, 10, and 12 with sodium or potassium hydride yields the alkylidenedihydrotetrazoles 5, 7, 9, 11, and 13, respectively. While 5a and b are unstable, even in solution at low temperatures, 7, 9, 11, and 13 form yellow oils that are distilled under high vacuum. - Irradiation of solutions of 7, 9, and 11 in [D8]toluene at -60°C yields, besides molecular nitrogen, annulated iminoaziridines that have an exocyclic CN double bond, i.e. 14, 16, and 18, respectively. In addition, an equal amount of the isomer 19 with the endocyclic CN double bond is formed from 11. On thermolysis, 14, 16, and 18 undergo [2 + 1] cycloreversion into methyl isocyanide and the cyclic imines 15, 17, and 20, respectively. By contrast, 19 rearranges thermally to yield 18. While the doubly bridged alkylidenedihydrotetrazole 13a affords only unidentified decomposition products on photolysis, its methyl homologue 13b is converted into the hexahydronaphthyridine 22 which is also formed on thermolysis. - Irradiation of 13b in a 2-methyltetrahydrofuran or butyronitrile matrix at 77 K yields a triplet diradical showing a four-line EPR spectrum centred at 3362 G and a half-field transition (at 1669 G) with a hyperfine structure. The zero-field splitting parameters |D-hc| = 0.031 cm-1 and |E-hc| = 0.0014 cm-1 are obtained by simulation of the EPR spectrum. The signal-carrier is assigned the diazatrimethylenemethane structure 23 on the basis of the close similarity between its EPR spectrum and those of trimethylenemethane (28) and tris(N-methylimino)methane (29). - Structural features are discussed that are responsible for the observed differences between the photochemical pathways.
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  • 2
    Electronic Resource
    Electronic Resource
    The Effects of Substituents on the Degenerate Cope Rearrangement of Semibullvalenes and Barbaralanes (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 2209-2227 
    Details
    ISSN: 1434-193X
    Keywords: Automerization ; Carbocycles ; Fluxionality ; Hydrocarbons ; NMR spectroscopy ; Pericyclic reactions ; Polycycles ; Rearrangements ; Strained molecules ; Substituent effects ; Valence isomerisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The activation parameters [ΔG# (200 K), ΔH#, and ΔS#] for the degenerate Cope rearrangements of barbaralane (1a) and several semibullvalenes (3a, 7a, 8a), and for those of a number of their 3,7- and 2,6-disubstituted derivatives, have been determined by variable-temperature carbon-13 NMR spectroscopy at 126 and 151 MHz. the disubstituted compounds studied include the barbaralanes 1c, d, e, h, j, k, and 2e, h, j, and the semibullvalenes 3d, e, h, 4e, h, i, l, m, and 5h-8h. Careful attention has been given to the measurement of temperatures. The data for these compound, together with those for related compounds previously reported in the literature, show that conjugating subsituents (e. g. aryl, CH, CO2R) in the 2,6 positions lower the barriers of the degenerate Cope rearrangement, whereas substituents that are electron-withdrawing through the inductive effect (e. g. Cl, CF3, SO2Ar) retard the reaction. Substituents in the 3,7-positons have little effect or are rate-retarding. The presence of 1,5-tri- and tetramethylene bridges in semibullvalenes accelerates the rearrangements, the effect being larger in the case of the former.
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  • 3
    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.
    Additional Material: 4 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Thermal Isomerisation of Substituted Semibullvalenes and Cyclooctatetraenes - A Kinetic Study (1999)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 813-822 
    Details
    ISSN: 1434-193X
    Keywords: Cyclizations ; Hydrocarbons ; Isomerizations ; Kinetics ; Polycycles ; Rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Whereas 1,5-dimethylsemibullvalene (1b) equilibrates with 1,5-dimethylcyclooctatetraene (2b), the semibullvalene-2,6-dicarbonitriles 13 rearrange irreversibly to afford mixtures of the isomeric cyclooctatetraene-1,5-dicarbonitriles 14 and 15. Arrhenius and Eyring parameters of these thermal isomerisation reactions have been determined for the gas phase (1b → 2b) and [D6]benzene solutions (1b → 2b, 13 → 14 + 15). Furthermore, the activation parameters of the known rearrangement of octamethylcyclooctatetraene (3) to octamethylsemibullvalene (4) have been determined. - The data for these compounds, together with those for related compounds previously reported in the literature, show that substituents not only influence the relative stabilities of the semibullvalene and cyclooctatetraene systems but also the height of the energy barrier for their interconversion. Those substituents that lower the barrier toward the degenerate Cope rearrangement of semibullvalenes simultaneously accelerate their rearrangement to cyclooctatetraenes thus limiting the thermal stability of the former.
    Additional Material: 2 Ill.
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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    Synthese und Photolyse von 1,4,4-Trialkyl -4,5-dihydro-5-imino -1H-1,2,3-triazolen.  -  Regio- und diastereoselektive Bildung von (Z)-Aziridiniminen (1990)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 123 (1990), S. 2195-2202 
    Details
    ISSN: 0009-2940
    Keywords: [3 + 2] Cycloaddition ; [3 + 2] Cycloreversion ; 5H-Tetrazoles, 5-alkylidene-1,4-dihydro- ; Aziridinimines ; 1,2,3-Triazoles, 1,4,4-trialkyl-4,5-dihydro-5-imino- ; 1,2,3,4,6,7,8-Heptaazaspiro[4.4]nona-2,7-dienes ; Photolysis ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis and Photolysis of 1,4,4-Trialkyl-4,5-dihydro-5-imino-1H-1,2,3-triazoles.  -  Regio- and Diastereoselective Formation of (Z)-AziridiniminesThe novel 5-iminodihydro-1,2,3-triazoles 14 are synthesized from the 5-isopropylidenedihydrotetrazole 11 through a ring transformation involving cycloaddition and cycloreversion of alkyl azides 12. Thus, [3 + 21 cycloaddition of 11 and 12 pro- duces high yields of the spiro compounds 13. On thermolysis at 120- 130°C, the latter undergo clean [3 + 2] cycloreversion into methyl a i d e (12a) and the iminodihydro-1,2,3-triazoles 14, which exist as mixtures of diastereomers, e.g. (E)-/(Z)-14a = 955, or as a single stereoisomer (E)-14c. When toluene solutions of 14 are irradiated (≥ 1 280 mm) at - 60°C, the configuration of the products is under kinetic control. Irradiation of 14a affords a quantitative yield of the aziridinimines (E)-and (Z)-15a (11:89) besides molecular nitrogen. In order to allow the distinction between the least-motion and the non-least-motion path of product formation, the photolysis of iminodihydro-1,2,3-triazoles is studied which are labelled at N-1 by a deuterated methyl group (14b) or a neopentyl group (14c). Neither photolysis (λ ≥280 nm) nor heating to 120 to 130°C induces any scrambling of the alkyl groups attached to N-1 and the exocyclic nitrogen atom of 14b and c. The major product from (E)-/(Z)-14b (84%) is formed by the non-least-motion path leading diastereoselectively to (Z)-15 b, while the least-motion products (E)- and (Z)-17b (16%) exhibit a diastereo-meric ratio of 3: 1. Very similar results are obtained on photolysis of (E)-14c, except that the least-motion product (15%) arises only in a single configuration [(E)-l7c]. In the non-least-motion products (Z)-15b, c, the geometry of the Me—N=-C—N???—R group, fixed by the ring in 14b, c, is completely retained. Therefore, the (Z) diastereoselectivity is interpreted in terms of a one-bond cleavage leading to diazenyl-1,3-diazaallyl diradicals 22 having lifetimes too short for changes of their configuration which is determined by the structure of the precursor.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19901231118
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  • 7
    Electronic Resource
    Electronic Resource
    Retention of Configuration in Two Photochemical Reactions: Formation of Cyclopropanimines by Extrusion of Molecular Nitrogen from Tetraalkyl-4-imino-1-pyrazolines and [2 + 1] Cycloreversion of Cyclopropanimines to Isocyanides and Alkenes (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 2545-2554 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,3,5,5-tetraalkyl-4-imino-3,5-dihydro ; Cyclopropanes, 2,2,3,3-tetraalkyl-1-(alkylimino) ; Alkanamines, 2,2,3,3-tetraalkylcyclopropylidene ; Photolysis ; Extrusion of molecular nitrogen ; Photoreduction ; Configuration, retention of ; Azatrimethylenemethanes ; [2 + 1] Cycloreversion, stereospecific, photochemical ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1-pyrazolin-4-ones 7 and 9 and the pyrazolidin-4-one 13 are condensed with alkanamines 8 to produce the imines 3,10 and 12 in high yields. Direct irradiation of 3 with 350-nm light at 90°C in deuterated hydrocarbon solvents affords the cyclo-propanimines 4 in almost quantitative yields besides molecular nitrogen and small amounts of the imines 12 as a result of photoreduction. In [D6]acetone, the cyclopropanimine 4b isomerizes in part of the α,β-unsaturated imine 14. Direct irradiation of 3 with the unfiltered light of the high-pressure mercury lamp results in quantitative [2 + 1] cycloreversion of the primary photoproducts 4 into the alkene 6 and an isocyanide 5. At low temperature (10°C), photolysis of 3 occurs much more slowly giving rise to photoextrusion of nitrogen (→4) and photoreduction (→12) to about the same extent. - Photolysis of the stereochemically labelled iminopyrazolines cis- and trans-10 (d.e. 99%) at 90°C produces the cyclopropanimines cis- and trans-16 (d.e. 94%) with high stereospecificity. The configuration of cis- and trans-16 is established by a comparison with the corresponding methylenecyclopropanes cis- and trans-19 and the quantitative and completely stereospecific [2 + 1] cycloreversion into methyl isocyanide (5a) and the 3,4- dimethyl-3-hexenes (Z)- and (E)-17 on irradiation with the unfiltered light of the mercury arc. The necessity of thermal activation for efficient nitrogen extrusion from the 1(n,π*) state of 3 and 10 is indicative of a considerable energy barrier towards the transition into a dissociative state. At low temperature, hydrogen abstraction from the solvent or other molecules becomes important for the deactivation of the 1(n,π*) state, in addition to decay and fluorescence. The stereospecific formation of cis- and trans-16 is interpreted in terms of diastereomeric bis-orthogonal azatrimethylenemethane diradicals as intermediates which retain the configuration on cyclization. The minor non-stereospecific path may involve mono-orthogonal azatrimethylenemethane diradicals. Thus, mechanisms that involve the same types of diradical intermediates can rationalize the photolysis of the iminopyrazolines 3, 10 and of the methylenetriazoline 1 as well. The [2 + 1] cycloreversion of cis- and trans-16 into the alkenes (Z)- and (E)-17 and methyl isocyanide (5a) demonstrates for the first time that such photoreactions can be entirely stereospecific.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911241125
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  • 8
    Electronic Resource
    Electronic Resource
    Photolysis of Tetraalkyl-1-pyrazoline-4-thiones. - Diastereoselective Formation of (E)/(Z)-Alkylidenethiiranes from cis- and trans-Tetraalkyl-1-pyrazoline-4-thiones (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1747-1755 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazole-4-thiones, 3,3,5,5-tetraalkyl-3,5-dihydro ; Thiiranes, 3,3-dialkyl-2-alkylidene ; 1,3-Pentadiene-3-thiol, 2,4-dimethyl ; Photolysis ; Extrusion of molecular nitrogen ; (E)/(Z) Stereoselectivity ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tetraalkyl-1-pyrazolin-4-ones 5 react with hydrazine to afford the hydrazones 6 which are transformed into the thiones 7 in high yields by treatment with disulphur dichloride in the presence of triethylamine. Selective excitation of the azo chromophor of 7a with 350-nm light gives rise to the isopropylidenethiirane 8, besides molecular nitrogen, in a very clean reaction; at almost quantitative conversions, less than 2% of byproducts are observed, and about 10% of 8 have isomerized to the pentadienethiol 9. Photolysis of the stereoisomers cis- and trans-7b yields mixtures of the alkylidenethiiranes (E)-and (Z)-10 with (E)/(Z) ratios of 35:65 and 49:51. The results are interpreted in terms of diastereomeric bis-orthogonal (cis- and trans-21) and mono-orthogonal thioxyallyl diradicals (E)- and (Z)-23 which cyclize to furnish (E)- and (Z)-10. There seems to be a qualitative resemblance between the photochemical and thermal stabilities in the series of 4-substituted tetramethyl-1-pyrazolines, viz. 7a 〈 12 〈 5a, 13, similar to that suggested by Engel for 2,3-diazabicyclo[2.2.2]oct-2-ene derivatives.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240813
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  • 9
    Electronic Resource
    Electronic Resource
    Photoreduction on Irradiation of 3,3,5,5-Tetramethyl-4-methylene-1-pyrazoline.  -  An Overlooked Example of a Reaction Competing with the Extrusion of Molecular Nitrogen from Reluctant Azoalkanes (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1613-1618 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,5-dihydro-3,3,5,5-tetramethyl- ; 1-Pyrazoline, 4-methylene- ; 1H-Pyrazole, 2,3,4,5-tetrahydro-3,3,5,5-tetramethyl-4-methylene- ; Photolysis ; Photoreduction ; Extrusion of molecular nitrogen ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photolysis of the 4-methylene-1-pyrazoline 1 is reinvestigated under a variety of experimental conditions. Direct irradiation (350-nm light) affords the alkylidenecyclopropanes 4 and 5 as predominant products, besides molecular nitrogen, and in a constant ratio of (55 ± 1): (45 ∓ 1), which is not altered in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) or piperylene nor in [D6]acetone solution. When oxygen is excluded, an additional compound is formed whose proportion depends on the capability of the solvent to act as hydrogen donor. In the presence of three equivalents of DABCO, it eventually becomes the major product. The 4-methylenepyrazolidine structure 13 of the novel photoproduct is established by NMR spectroscopy, a comparison with an authentic sample, obtained in quantitative yield by zinc reduction of 1, and reoxidation to 1 as well. An extensive investigation to detect any traces of the 4-isopropylidene-2-pyrazoline 6 in the irradiated solutions has failed. This result is at variance with recent disclosures reporting evidence for a turnaround photoisomerization of 1 via diazenylallyl diradicals. The contribution of photoreduction to the decay of the first excited singlet state of reluctant azo compounds is emphasized.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240723
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  • 10
    Electronic Resource
    Electronic Resource
    Retention of Configuration and E/Z Stereoselectivity in the Photolysis of cis- and trans-Tetraalkyl-4-methylene-1-pyrazolines (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1619-1626 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,3,5,5-tetraalkyl-3,5-dihydro-4-methylene- ; Cyclopropanes, 2,2,3,3-tetraalkyl-1-methylene- and 2,2-dialkyl-1-alkylidene- ; Photolysis ; Extrusion of molecular nitrogen ; Configuration, retention of ; E/Z Stereoselectivity ; Trimethylenemethanes, bis-orthogonal and mono-orthogonal ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The isomeric 4-methylene-1-pyrazolines cis- and trans-2 are synthesized by a Wittig reaction from the pyrazolin-4-ones cis- and trans-1, respectively. Direct irradiation with 350-nm light of cis-2 affords the alkylidenecyclopropanes cis-8, (E)- and (Z)-9 (64: 17: 19) besides molecular nitrogen and small amounts of the 4-methylenepyrazolidine cis-7 as a result of photoreduction. Under the same conditions, trans-2 is converted into trans-8 as well as (E)- and (Z)-9 (64: 14: 22). The configurations of cis- and trans-8 have been established by means of a 1,3-dipolar cycloaddition with mesityl nitrile oxide which furnishes two cycloadducts from cis-8, e.g. cis-14A and B, but only one cycloadduct from trans-8, viz. trans-14. The configurations of (E)- and (Z)-9 have been assigned on the basis of homoallylic 1H,1H coupling constants and nuclear Overhauser experiments. The results demonstrate that, on the least-motion paths leading to cis- and trans-8, the configuration of the ring substituents in cis- and trans-2 is completely retained. Of the non-least-motion products (E)- and (Z)-9 the latter is favoured regardless of the configuration of the precursors cis- and trans-2 though the extent of (Z) selectivity depends on that configuration. The results are interpreted in terms of bis-orthogonal trimethylenemethane diradicals as primary intermediates from which irreversible paths bifurcate. Cyclization yields cis-8 (from cis-2) or trans-8 (from trans-2) and 90°-bond rotations afford two diastereomeric mono-orthogonal trimethylenemethane diradicals [(E)- and (Z)-18] which eventually cyclize to the corresponding non-least-motion products (E)-9 and (Z)-9.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240724
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