Johannes Leitich

The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than by electron transfer

Radiolytically generated azide radicals have been used for the formation of tyrosyl radical, TyrO˙ from tyrosine. The TyrO˙ radicals combine (2k= 4.5 × 108 dm3 mol–1 S–1, determined by pulse radiolysis) yielding bityrosine in a > 90% yield. Bityrosine formation is not suppressed in the presence of oxygen [k(TyrO˙+ O2) < 1 × 103 dm3 mol–1 S–1].When TyrO˙ and O2˙– radicals are generated side by side in a 1:1.2 ratio, bityrosine formation is strongly suppressed and (2S,3aR,7aS)- and (2S,3aS,7aR)-3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indole-2-carboxylic acids 10 become the major final products. Their hydroperoxidic precursor is only short-lived (t½= 4.2 h at room temperature and pH 8). Upon its decay H2O2 is released. Product 10 is believed to be formed by the addition of O2˙– to the ortho- and para-position of the phenoxyl radical, followed by protonation, ring closure and hydrolysis.Based on material balance considerations an electron transfer from O2˙– to TyrO˙, although thermodynamically feasible, must play a minor role (⩽10%). The rate constant k(O2˙–+ TyrO˙) has been determined by pulse radiolysis to be 1.5 × 109 dm3 mol–1 S–1.

The photolysis (λ = 254 nm) of tyrosine in aqueous solutions in the absence and presence of oxygen. The reaction of tyrosine with singlet oxygen☆

Abstract In the 254 nm photolysis of deoxygenated aqueous solutions of tyrosine (TYROH, 10 −3 mol dm −3 ) TYROH is consumed with a quantum yield of 9.1 × 10 −3 while the observed products (quantum yields in parentheses) are 2,2′-bityrosyl (BITYR, 3.1 × 10 −3 ) and 2-amino-4-ethenyl-hex-4-enic acid (AEHEA, 1.6 × 10 −3 ). In N 2 O-saturated solutions, N 2 (1.8 × 10 −2 ), in the presence of 2-propanol, H 2 (1 × 10 −3 ) are formed. In air-saturated solutions the tyrosine consumption is increased (9.7 × 10 −2 ); in addition to increased yields of BITYR ({if1.4 × 10 −2 }) but unaltered yields of AEHEA (1.6 × 10 −2 ), 3a-hydroxy-6-oxo-2,3,3a,6,7,7a-hexahydro-1H-indol-2-carboxylic acid (HOHICA,1.6 × 10 −2 ), 3,4-dihydroxyphenylalanine (DOPA, 3.8 × 10 −2 ) and 2,4-dihydroxyphenylalanine (2,4-DHPhe, 2.3 × 10 −3 ) are observed. The sum of the quantum yields of the O 2 -dependent products increases with increasing O 2 and TYROH concentrations, approaching a plateau value of approximately 7 × 10 −2 at high reactant concentrations. In D 2 O the quantum yields of BITYR, HOHICA and DOPA are increased considerably while those of AEHEA are decreased. Irradiation of Rose Bengal with visible light ( 1 O 2 formation) in the presence of TYROH yields HOHICA (90%) and BITYR (10%). It is concluded that from the excited singlet state ( 1 TYROH) photoionization and splitting of the phenolic OH bond occurs giving rise to tyrosine-derived phenoxyl radicals (TYRO) which are the precursors of BITYR. Also, from 1 TYROH a 1,3-H-shift of the phenolic OH and concomitant ring-opening occurs. Addition of water and loss of carbon dioxide in subsequent thermal reactions yields AEHEA as the final product. Quenching of the TYROH triplet state ( 3 TYROH) by O 2 yields 1 O 2 which reacts with TYROH by H abstraction. The resulting intermediates TYRO . and HO 2 . are the precursors of BITYR and HOHICA. In addition, a reaction of O 2 with 3 TYROH possibly yields 1,4- and 3,6-endoperoxides which are the precursors of further HOHICA and of DOPA and 2,4-DHPhe, respectively.

Über Evonymus-Alkaloide, 3. Mitt.: Konstitution und Konfiguration von Evonin und Evonolin (aus Evonymus europaea L.)

In continuation of our earlier studies1,2 the constitution and relative configuration of evonine (1) and an accompanying alkaloid, evonolin (2), were determined.ZusammenfassungIn Fortsetzung unserer Untersuchungen1,2 wurde die Konstitution und die relative Konfiguration des Evonins und eines Nebenalkaloides, des Evonolins, entsprechend den Formeln (1) bzw. (2) ermittelt.

Verknüpfung von 1,3-dienen mit acrylsäuremethylester durch photochemische umsetzung mit pentacarbonyleisen ☆

Abstract Pentacarbonyliron and methyl acrylate/1,3-diene (2,3-dimethylbutadiene, isoprene, butadiene) mixtures react photochemically via diene—Fe(CO)3 and methyl acrylate—Fe(CO)4 to give products in which a methyl acrylate—diene adduct is 1,4,5,6-η-coordinated to the Fe(CO)3 moiety. (η2-diene)(η2-methyl acrylate)Fe(CO)3 is proposed to be an intermediate.

Conversion of d-fructose into 6-deoxy-d-threo-2,5-hexudiulose by γ-irridation: a chain reaction in the crytalline state☆

Abstract γ-lrradiation of crystalline β- d -fructose yields 6-deoxy- d - threo -2,5-hexodiulose ( 2 ) via a chain reaction. The initial G -value at 25° is 38. With decreasing temperature, G( 2 ) decreases strongly; however, no change in G( 2 ) is observed on going to higher temperatures. G( 2 ) is independent of dose rate, but decreases with increasing dose. A mechanism for the formation of 2 is proposed. Although G( 2 ) decreases with increasing dose, γ-irridiation of d -fructose is a convenient method for obtaining 2 on a preparative scale. At a dose of 10 21 eV.g −1 , d -fructose is converted into ∼6% of 2 .

The photo-Nazarov cyclization of 1-cyclohexenyl phenyl ketone revisited. Observation of intermediates

Abstract The photochemical transformation of 1-cyclohexenyl phenyl ketone ( 1 ) to the hexahydrofluorenone 2 (“photo-Nazarov cyclization”) was reinvestigated on the level of conventional preparative chemistry backed up by nuclear magnetic resonance (NMR) spectroscopy and deuteration experiments. Hitherto unreported intermediates were detected. These include the enol 8 and, in non-protic media, the crystalline dimers 3 and 4 . Dimer 3 , which was characterized by single-crystal X-ray analysis, is a kinetically stable non-conjugated enol which readily isomerizes to 4 on acid-base catalysis. Dimer 4 is photolysed to give ultimately 2 . The enol 8 is sufficiently long-lived to be seen in the 1 H NMR spectra of freshly irradiated solutions of 1 in CD 3 OD and of 4 in CD 3 OD or CD 3 CN before it isomerizes to 2 . Enol 3 resuls from trapping of the oxyallyl 7 , the presumed precursor of the enol 8 , by 8 in a novel type of reaction. Irradiation of 1 in the presence of 1,3-cyclopentadiene (CPD) gives adducts 10–16 . Adducts 13–16 appear to be derived from 7 , whereas 10 , 11 and 12 possess the structures of Diels-Alder adducts of CPD to the trans -cyclohexene isomer of 1 ( 6 ). The intermediacy of 6 as the primary photoproduct is also suggested by futher circumstantial evidence.

Thermal [2 + 2] cyclodimerisation of strained olefins. Cis, trans-1,5-cyclo-octadiene and trans-cyclo-octene

Abstract The title reaction in the absence of catalysts has been investigated both with the racemic and with the optically active cycloolefins. Both title olefins form [2 + 2] dimers of the trans double bonds with complete retention of configuration (trans-syn-trans 2 by R + S and trans-anti-trans 3 by R + R combination) and with mono-inversion (cis,trans 4, both by R + S and R + R combinations), but almost none with di-inversion (cis-syn-cis 5 and cis-anti-cis 6), together with trans-cis isomerised starting olefins.

Zur Photodimerisierung von 3-Methyl-2-cyclopentenon

ZusammenfassungBei der Belichtung von 3-Methyl-2-cyclopentenon in Lösung entstehen die Isomeren Cyclobutane I, II, III und IV sowie zwei weitere Dimere, V und VI, die ein Oxocyclopentylcyclopentenon-Gerüst besitzen. Das Verhältnis der gebildeten Dimeren ist abhängig vom Lösungsmittel und von der Konzentration der belichteten Lösung. Die Logarithmen der Dimerenverhältnisse korrelieren linear mit demKirkwood-Onsager-Parameter des Lösungsmittels; der Konzentrationseinfluß läßt sich durch den gemessenenKirkwood-Onsager-Parameter der Lösung erfassen.AbstractIrradiation of 3-methyl-2-cyclopentenone in solution yields the isomeric cyclobutanes I, II, III and IV, and two other dimers with oxocyclopentyl-cyclopentenone structure (V and VI, resp.). The ratio of the dimers is dependent on the solvent and on the concentration of the irradiated solution. The logarithms of the dimer ratios are related linearly to theKirkwood-Onsager parameter of the solvent; the concentration effect can be correlated with the measuredKirkwood-Onsager parameter of the solution.

Zur Kenntnis organischerLewis-Säuren, 36.Cis—trans-Gleichgewichte und Konformationen von 2,(x)-Dialkyl-1-dicyanomethylencyclohexanen. Wahlweise einfache Darstellung dercis- odertrans-Formen

Knoevenagel condensation of malononitrile withcis- andtrans-2,5-dimethylcyclohexanone (1c and1t, respectively) leads to2c and2t, respectively, and withtrans-1-decalone to4t. The equilibria2c⇌2t and4c⇌4t have been determined as well as, by means of 270-MHz-1H-NMR, the conformations of these four compounds. The dicyanomethylene group is found to induce axial positions of neighbouring alkyl residues on the cyclohexane ring (or when this is impossible, as in the case of4t, the twist form of the cyclohexane ring). This results in a strong predominance of thecis isomers in the equilibria2c⇌2t and4c⇌4t whereas thetrans isomers strongly predominate in the equilibria among the starting ketones. This situation allows the optional preparation of2c or2t, and of4c or4t, from the more stable starting ketone (or ketone mixture). The free conformational energy of the distorted twist form of4t amounts to 17 kJ/mol.

Photochemistry of 1,1-dicyano-1-alkenes: The olefin-to-cyclopropane rearrangement

Abstract 1,1-Dicyano-1-alkenes (DCNA) that lack further unsaturation undergo formation of 1,1-dicyano-cyclopropanes via 1,2-migration of either hydrogen or methyl/alkyl from C-3 to C-2 in their lowest exited singlet state. Quantum yields for this “olefin-to-cyclopropane photorearrangement” (OCPR) were found to span a wide range (≤0.1) and to depend characteristically on alkyl substituents on C-3 and C-2. OCPR occurs preferentially via such 1,2-migration that leaves behind the more alkylated C-3 atom. 1,2-Migration was found to occur suprafacially (i.e. to follow maximum orbital overlap), but to be rather tolerant towards unfavorable orientation of the migrant in the starting DCNA. The ring-closure that completed OCPR was found to be devoid of any intrinsic stereoselectivity; thus, in cyclohexane, each of the two epimeric 2-[(2-methyl-cyclohexyl)-methylene]-malononitriles ( 3 and 4 ) yielded the same approximately 1:1 mixture of the two epimeric 4-methyl-spiro[2.5]octane-1,1-dicarbonitriles ( 28 and 29 ). OCPR proceeded via an intermediate that also led to isomeric DCNA and to 1,1-dicyano-3-alkenes as minor by-products. Some deprotonation at C-3 of the photoexcited DCNA was noticeable in methanol, but not in hexane. Supplementary experiments included preparative and kinetic investigations of thermolyses of 1,1-dicyano-cyclopropanes. The combined evidence allowed the deduction of the following reaction path for OCPR. In their lowest excited singlet state, a ππ ∗ state, DCNA exhibit cationic reactivity of their C-2 atoms (presumably in the perpendicular conformation of C-2 relative to C-1, according to Salem’s seminal concept). This reactivity triggers the 1,2 (Wagner–Meerwein type) migration to yield, still on the excited hypersurface, a 1,3 dipole. This 1,3-dipole achieves an almost complete conformational equilibration in cyclohexane (though less so in more polar solvents) before it decays to the electronic ground-state thereby becoming a 1,3-diradical. This 1,3-diradical undergoes three competing terminating reactions: ring closure to cyclopropane (the major path); 1,2-back migration of hydrogen to form starting or isomeric DCNA; 1,4-hydrogen shift to produce 1,1-dicyano-3-alkenes. The 1,3-dipole is too short-lived to undergo a potentially favorable Wagner–Meerwein rearrangement. Like the reactive excited DCNA singlet state, the 1,3-dipole is not trapped to any significant extent by nucleophilic addition of the solvents tert -butanol or methanol to its cationic center. The reason for this failure appears to be the excited-state nature of this species, which bars the formation of a ground-state adduct in an adiabatic reaction.