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  • 1
    ISSN: 0947-3440
    Keywords: Oxepines ; Photoisomerization ; s-cis-Butadiene conformation ; Small ring systems ; Enantiomeric resolution ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Irradiation of the dihydrooxepine monoester (+)-4 gives the bridged bicyclobutane (+)-5 in 77% yield. On the other hand, the corresponding diester 1 yields the cyclobutene 2. Possible explanations for this differing behaviour are discussed.
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  • 2
    ISSN: 0947-3440
    Keywords: Barbaralanes ; Tricyclo[3.3.1.02,8]nona-3,6-dienes ; Bicyclo[3.3.1]nonane, derivatives of ; Vinyl sulphones ; Nitriles, α,β-unsaturated, from vinyl sulphones ; Esters, α,β-unsaturated, from nitriles ; 2-Oxaadamantanes, cleavage of ; 2-Oxatricyclo[3.1.1.15,7]decane, derivatives of ; Phenylcerium(III) chloride reagent ; Bromination with N-bromosuccinimide ; Cyclisation by debromination with the zinc-copper couple ; Reversed-phase liquid chromatography, preparative ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Short and efficient syntheses of the barbaralanes 2e-h are reported. Meerwein's diketone 9 is converted into the bis(vinyl sulphide) 10 by the action of 4-chlorothiophenol in the presence of titanium(IV) chloride and triethylamine. Oxidation of 10 with sodium perborate in acetic acid affords the bis(vinyl sulphone) 11 which is treated with potassium cyanide supported on aluminium oxide to yield the unsaturated dinitrile 8. Bromination of 8 with N-bromosuccinimide and subsequent reductive cyclisation of the resulting exo,exo dibromodinitrile 13 with the zinc-copper couple furnish 3,7-dicyanobarbaralane (2e) in 53% overall yield based on 9. - Treatment of dinitrile 8 with hydrochloric acid in boiling methanol followed by hydrolysis of the intermediate Pinner salt yields the dimethyl dicarboxylate 14 which is brominated to afford the exo,exo dibromo diester 15. Reductive cyclisation of 15 gives rise to the formation of dimethyl 3,7-barbaralanedicarboxylate (2f) in 49% overall yield based on 9. - Bicyclo[3.3.1]nonane-3,7-dione (7) is converted into a mixture of the bis(vinyl sulphones) C2- and Cs-17 (1:1) as described for 9 → 11. Bromination of this mixture yields a single exo,exo dibromo compound 18 which is cyclised by the zinccopper couple to afford 3,7-bis(phenylsulphonyl)barbaralane (2g) in 56% overall yield based on 7. - The known phenyl-2-oxaadamantanol 21a is cleaved by the action of boron trifluoride and acetic anhydride to yield the bicyclo[3.3.1]none-none 22. The difluoroboron complex 23 is obtained under more rigorous conditions. Addition of phenylcerium(III) chloride to 22 followed by dehydration of the endo alcohol 24 yields the diphenyldiene 25. Bromination of 25 and subsequent reductive cyclisation of the exo,exo dibromodiphenyldiene 26 furnish 3,7-diphenylbarbaralane (2h) which is isolated by preparative reversed-phase chromatography in 50% overall yield based on 7. - The configurations and conformations in the crystals are elucidated by X-ray diffraction analyses for the bicyclo[3.3.1]nonane derivatives 7, 13, 15, 18, 22-24, and 26 and the barbaralanes 2e-g. In the solid state, 2e and f exist as pairs of rapidly rearranging nonequivalent valence tautomers while the atomic distances observed for 2g indicate the presence of a single nonrearranging valence tautomer.
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  • 3
    ISSN: 0947-3440
    Keywords: Pyrrole-2,3-diones ; Nucleophilic addition ; Substituent effects ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions of 1H-pyrrole-2,3-diones 2a-i with N- and O-nucleophiles proceed by initial attack at C-2 or/and C-5. Consequently, earlier results suggesting structures of type A (attack at C-3) have to be revised (5, 13, 17, 18b). In general, electron-withdrawing substituents at C-4 favor attack at C-5 with formation of 2-pyrrolones 5, 6, 10, 13, 17, and 20. Alternatively, attack at C-2 leads to α-oxobutenoic acid derivatives 3, 7, 11, 12, 15, and 18. IR and 13C-NMR spectroscopic techniques were employed to differentiate between the possible isomeric addition products formed. The structure of methyl α-oxobutenoate 15 was established by an X-ray crystallo-graphic analysis.
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  • 4
    ISSN: 0947-3440
    Keywords: Acivicin ; Photochemistry ; 1,3-Dipolar cycloadditions ; Nitrile oxides ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: α-Amino-3-chloro-4,5-dihydro-5-methyl-5-isoxazoleacetic acid (8), a ring-methylated analogue of the potent antitumor agent acivicin (AT-125), is synthesized in a 6-step procedure in 63% overall yield from (S)-valine. Key step is the 1,3-dipolar addition of bromonitrile oxide to the N,C-protected (S)-isodehydrovaline (6) available from (S)-valine in four steps involving the photoisomerization of N-phthaloylvaline methyl ester (1). The stereochemical course of the 1,3-dipolar cycloaddition is proven by means of a X-ray structure analysis of the major diastereoisomer 7a formed in the chloronitrile oxide cycloaddition. The absolute configuration of the major (u) diastereomer 7a and the bromo derivative 7b is (αS,5R).
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  • 5
    ISSN: 0947-3440
    Keywords: Autoxidation ; Copolymerisations ; Cyclizations ; Diradicals ; Epoxidations ; Liquid Chromatography ; Oxygen ; Peroxides ; Polycycles ; Radical Reactions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Addition to the diphenyldiketone 11 of the reagent prepared from rigorously dried cerium(III) chloride and phenyllithium yields the tetraphenyldiol endo,endo-13 after extended periods of time. The configuration of this diol, which was previously assigned the (incorrect) configuration exo,exo-13, is established by an X-ray diffraction analysis. Dehydration of endo,endo-13 with sulphuric acid in acetic acid affords a mixture of the oxatwistane 16 and the tetraphenyldiene 17 (3:1). Only the latter is obtained from endo,endo-13 by the action of boron trifluoride-diethyl ether in dichloromethane solution. While attempts at allylic bromination of 17 with N-bromosuccinimide in conventional solvents inevitably lead to complete decomposition, use of cyclohexane as solvent allows to obtain solutions of allylic dibromides that can be cyclised with the zinc-copper couple to afford tetraphenylbarbaralane 9 in 58% yield based on 17. Thus, 9 is now available from the diphenyldiketone 11 in only three steps with an overall yield of 42%. - The autoxidation of 9, studied in various solvents, yields mixtures of products of which the epoxyendoperoxides 20 and 21, the unsaturated ketone 22, and a labile, probably oligomeric or polymeric product C are isolated in pure form. The structures 21 and 22 are elucidated by X-ray diffraction analysis and independent synthesis, respectively. On warming, 20 rearranges into 21. Traces of acid convert the unknown autoxidation product A instantaneously into the unsaturated ketone 22. - Rate studies show that the autoxidation of 9 starts after a short initiation period. 2,6-Di-tert-butyl-4-methylphenol exhibits powerful inhibitory effects. These results demonstrate the free radical nature of the autoxidation of 9. - The results are interpreted in terms of a mechanistic scheme involving initiation of the radical chain by addition of triplet oxygen to 9 to generate the diradical 28 followed by a radical chain 1:1 copolymerisation of 9 and oxygen to produce diradical 29. Eventually, endo addition of oxygen to 29 gives rise to the formation of the epoxyendoperoxides 20 and 21. Intramolecular hydrogen abstraction from the methylene group (C9) is accompanied by cleavage of a bridgehead bond (C1-C2) and the adjacent O-O bond to afford the extremely labile product A (perhaps 32 or 33) which undergoes acid-catalysed elimination to furnish ketone 22.
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  • 6
    ISSN: 0947-6539
    Keywords: chiral phenols ; facial selectivity ; photooxidations ; quinols ; singlet oxygen ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three of the four possible isomeric para-substituted hydroxyethyl-methylphenols were prepared from readily available starting materials and submitted to photooxygenation. In the case of 3-hydroxy-α,6-dimethylbenzenemethanol (1 a) and 2-hydroxy-α,5-dimethylbenzene-methanol (1 b) the corresponding 4-hydroperoxy-3,5-cyclohexadienones were obtained as oxy-functionalized products (≥90%, d.r. 85:15 and 75:15). The stereochemistry of the products was assigned after reduction to the corresponding p-quinols. The ready cleavage of the initial oxygenated product, which can be observed at low temperature as a single diastereomer, prevented rigorous assignment of the π-facial selectivity of the singlet oxygen attack for the derivative 4-hydroxy-α,2-dimethylbenzenemethanol (1 c). The present stereochemical results indicate that the hydroxyl group directing effect, that is, coordination of the incoming singlet oxygen dienophile with the hydroxyl group, is also operating in the photooxygenation of the chiral phenols 1 a-c. However, in the case of the derivative 1 b, hydrogen bonding in the starting material leads to an opposite stereochemical outcome. This conclusion is further substantiated by solvent effects and the fact that the methyl ether 1 d of alcohol 1 c displays a significant lower diastereoselectivity.
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  • 7
    ISSN: 0947-6539
    Keywords: amino acids ; C-H activation ; electron transfer ; photochemistry ; state selectivity ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photochemistry of the methyl esters of N-phthaloylcysteine derivatives 1b-5b was studied. The results are remarkable, because they prove a pronounced, multiplicity-controlled regioselectivity of the initial CH activation step. From substrates 1b-4b the benzazepine-1,5-dione 6 was produced exclusively by the singlet path. The formation of compound 6 is initiated by a γ-H abstraction; this was demonstrated by deuterium labeling experiments. The penicillamine derivative 5b was unreactive in the singlet manifold. From substrates 1b-3b and 5b the thiazinoisoindoles 7-9 and 11 were produced exclusively by the triplet path. The sterically hindered S-isopropylcysteine derivative 4b also furnished this product type (10) as a proportion of the products in the singlet manifold. These annulation products result from a primary photoinduced electron-transfer (PET) step followed by heterolytic ∊-H activation. The mechanistic scenario was elucidated by quenching and sensitization experiments. An interplay of CH activation steps and electron back-transfer is probably responsible for this type of spin selectivity.
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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: Crystal and Molecular Structure of Bromo(3,8-diisopropyl-2,9-dimethyl-4,7-diaza-4,6-decadienyl-N, N′) nickel(II), a Square Planar Complex with a Tridentate N, N, C-ChelateThe title compound crystallizes monoclinically in the space group P21/a with a = 14.654 Å, b = 11.333 Å, c = 11.664 Å, β = 92.51° and 4 formula units. THe structure of the complex is characterized by a square planar coordination for the nickel atom with the ligand atoms Br, N, N,C. The bond distances are Ni—Br = 2.302 Å, Ni—N = 1.995 and 1.820 Å, and Ni—C = 1.907 Å. The details of the structure are discussed.
    Notes: Die Titelverbindung kristallisiert monoklin in der Raumgruppe P21/a mit a=14.654 Å, b = 11.333 Å, c =11.664 Å, β = 92.51° und 4 Formeleinheiten. Die Struktur des Komplexes ist durch eine planar-quadratische Nickel-Koordination mit den Ligandenatomen Br,N,N,C ausgezeichnet. Die Bindungsabstände betragen Ni—Br = 2.302 Å, Ni—N = 1.995 bzw. 1.820 Å und Ni—C = 1.907 Å. Die Einzelheiten der Strukture werden diskutiert.
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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: Cycloaddition von Singulettsauerstoff und 4-Phenyl-4H-1,2,4-triazol-3,5-dion mit 7-substituierten 1,3,5-CyclooctatrienenDie Cycloadditionen von 7-substituierten (X; Y) 1,3,5-Cyclooctatrienen 3 (N3; H), 4 (OAc; H), 5 (OEt; OEt), 6 (Me; H) und 7 (iPr; H) mit Singulettsauerstoff (1O2) und 4-Phenyl-4H-1,2,4-triazol-3,5-dion (PTAD) wurden untersucht. Singulettsauerstoff reagierte mit diesen Cyclooctatrienen aus dem bicyclischen Valenzisomeren (Bicyclo[4.2.0]octa-2,4-dien) heraus zu den entsprechenden tricyclischen Endoperoxiden. Bicyclische Endoperoxide, ausgehend von monocyclischen Valenzisomeren (1,3,5-Cyclooctatrien), wurden lediglich bei den alkylsubstituierten Derivaten 6 (Me; H) und 7 (iPr; H) erhalten. Die Cyclcoaddition mit PTAD führte zu den bicyclischen und tricyclischen Urazolen, mit Ausnahme des Acetals 5 (OEt; OEt), welches nur das tricyclische Produkt gab. Das Bromderivat 2 reagierte weder mit 1O2 noch mit PTAD. Kontrollexperimente mit der Stammverbindung zeigten, daß die Cycloadditionsprodukte entsprechend den mono- und bicyclischen Valenzisomerenanteilen des Eduktgemisches erhalten wurden.
    Notes: The cycloaddition of the 7-substituted (X; Y) 1,3,5-cyclooctatrienes 3 (N3; H), 4 (OAc; H), 5 (OEt; OEt), 6 (Me; H), and 7 (iPr; H) with singlet oxygen (1O2) and 4-phenyl-4H-1,2,4-triazole-3,5-dione (PTAD) was investigated. Singlet oxygen gave with all cyclooctatrienes the corresponding tricyclic endoperoxides derived from the bicyclic valence isomers (bicyclo[4.2.0]octa-2,4-dienes). The corresponding bicyclic endoperoxides derived from the monocyclic valence isomers (1,3,5-cyclooctatriene) were only obtained for the alkyl-substituted derivatives 6 (Me; H) and 7 (iPr; H). PTAD cycloaddition led to both the bicyclic and tricyclic urazoles, except with the acetal 5 which gave only tricyclic product. The bromo derivative 2 did not react with 1O2 nor PTAD. Control experiments with the parent cyclooctatriene showed that the cycloaddition products were derived from the “static” amounts of the monocyclic and bicyclic valence isomers in the substrate mixture.
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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: Thermolabile Hydrocarbons, XVIII, 1-Substituted Neopentyl Radicals and their DimersFive 3,4-diaryl-2,2,5,5-tetramethylhexanes 1a-e were prepared as pure meso- and DL-isomers. According to the NMR spectra, x-ray analyses for meso- and DL-1e (with an (FB)2E conformation10) as energy minimum for DL-1e), and force field calculations the diastereomers have distinctly different minimum energy conformations, rotational potentials, and strain enthalpies. Also the activation parameters for the thermal dissociation into 1-arylneopentyl radicals 2 are typically differing. From an entropy discussion it is concluded that sandwich-like diastereomeric radical complexes are formed in these reactions as first intermediates. Their tightness influences ΔS≠. The recombinations of the radicals 2 likewise take place stereoselectively. The substituent effects on the selectivity can also be understood by primary formation of diastereomeric complexes of radical pairs.
    Notes: Die 3,4-Diaryl-2,2,5,5-tetramethylhexane 1a-e wurden als reine meso- und DL-Isomere dargestellt. Nach NMR-Analyse, Röntgenstrukturbestimmungen für meso- und DL-1e (mit (FB)2E-Konformation10) als Energieminimum für DL-1e) und Kraftfeldrechnungen unterscheiden sich die Diastereomeren charakteristisch in ihren Vorzugskonformationen, Rotationspotentialen und Spannungsenthalpien sowie in den Aktivierungsparametern ihres thermischen Zerfalls in 1-Arylneopentyl-Radikale 2. Aus einer Entropiediskussion wird gefolgert, daß die thermische Dissoziation dabei über primär gebildete diastereomere sandwichartige Radikalkomplexe verläuft, deren Festigkeit ΔS≠ beeinflußt. Die Rekombination der Radikale 2 erfolgt ebenfalls stereoselektiv. Die Substituenteneffekte auf die Selektivität werden durch primäre Bildung der diastereomeren Radikalpaar-Komplexe verständlich.
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