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Nicotinamide N-Oxide A Biological Oxidant.

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Oxidation Reactions of the Thioamide Group
By W. Walter [ *I
On oxidation with Hz02, thioamides afford thioamide Soxides ( I ) , which can be clearly differentiated from the
isomeric thiohydroxamic acids. The vibrations of the C = S
group of thioamides couple so strongly with other vibrations
that they cannot be used for identification, but the S=O
vibration of the >C=S=O system offers a key band for this
group. The equilibrium between a thioamide and a n imido
thiol corresponds to the equilibrium between ( I ) and the
iminomethanesulfenic acid (2) in the oxidation products.
representative of those of heterocyclic N-oxides in general,
then these compounds can be classified as biological oxidizing
agents and should be investigated as potential electrophilic
hydroxylating agents.
Lecture at Freiburg (Germany), November l l t h , 1966
[VB 49 IE]
German version: Angew. Chem. 79, 476 (1967)
[*I Prof. Dr.
S. Chaykin
Chemisches Laboratorium der Universitat
Albertstr. 21
78 Freiburg (Germany)
[l] K . N. Murray and S. Chaykin, J. biol. Chemistry 241, 3468
R I - C - N H R ~e R ' - C = N R ~
s=o;'( I )
Reactions of thioamide S-oxides have been found in which
the form (2) takes part; in the thiourethane series compounds of structure (2) can be isolated, e.g. when R1 =
and R2 = - C ~ H ~ ( C H ~ ) ~ - O , The
hydrogen bond shown in ( I ) plays a part in the stability of
compounds ( I ) and (2).
Thioamides show cis-trans-isomerism a t their partial C = N
double bonds. One isomer of a n N-benzyl-N-methylthioformamide has been isolated and the other has been enriched to the extent of 75 %; the activation energy for their
0.66 kcal/
interconversion has been determined as 25.16
mole. Their respective structures were assigned on the basis
of N M R and X-ray diffraction data.
N-Isobutylthioformamide is a n equilibrium mixture of 82 %
of the trans- and 18 % of cis-form, but after oxidation to the
S-oxide only the cis isomer remains; thus the intramolecular
hydrogen bond is so strongly stabilizing that the barrier to
rotation cannot be overcome at room temperature.
Lecture at Krefeld (Germany) on November 24th, 1966
[VB 45 IE]
German version: Angew. Chem. 79, 426 (1967)
[*] Prof. W. Walter
Chemisches Staatsinstitut Hamburg,
Institut fur Organische Chemie
2 Hamburg 1 3 (Germany)
Papendamm 6
Photoreduction of Ketones and Ketimines
By M. Fischer [*I
On irradiation by UV light, benzophenone ( I ) is reduced by
hydrogen donors such as alcohols and hydrocarbons to
benzopinacol (2)W The quantum yield for the photoreduction reaches a n optimal 2.0 in 2-propanol since hydrogen
is abstracted relatively easily from this solvent, and it is
vanishingly small in benzene whose hydrogen atoms are
more firmly b o u n d W Studies with quenchers [31 show that
the triplet state is the excited state responsibIe for the
light-induced reaction.
2-Methylbenzophenone (3) cannot be reduced to the pina~01141 because the excitation by light causes an intramolecular migration of hydrogen to give the dienol (4), which
spontaneously reverts to the ketone. Formation of the
dienol (4) was demonstrated by Diels-Alder addition to dimethyl acetylenedicarboxylate.
Nicotinamide N-Oxide : A Biological Oxidant
By S. Chaykin[*l
Hog liver xanthine oxidase can catalyse the reduction of nicotinamide N-oxide [I]. Either reduced nicotinamide adenine
dinucleotide (NADH), hypoxanthine, or xanthine can be
oxidized by the N-oxide. Since the enzyme also catalyses
reactions with oxidizing agents that can act only as electron
acceptors, it has been generally assumed that oxidizing
agents always function solely as electron acceptors in
xanthine oxidation. Water would serve as an oxygen donor
when required. Although the N-oxide certainly is an electron
acceptor in the oxidation of NADH, the possibility existed
that it might act as an oxygenating agent in the oxidation of
hypoxanthine or xanthine. The latter reactions were therefore
investigated using 18O-labeled oxidants.
When xanthine is oxidized in an atmosphere of 1 8 0 2 no
oxygen-18 is found in the uric acid which is formed. If the
reaction is carried out in Ha80 and a n atmosphere of 1602
oxygen-18 is found in the uric acid. However, when 180labeled nicotinamide N-oxide is used, direct transfer of
oxygen to xanthine occurs and 0.67 g-atom of 1 8 0 per mole
of uric acid are found. The N-oxide also functions as a n
electron acceptor, but to a much lesser extent; on oxidation
with unlabeled N-oxide in the presence of H i 8 0 0.18 gatoms of 1 8 0 were incorporated per mole of uric acid.
Similar results were obtained with milk and liver xanthine
oxidases. If the properties of nicotinamide N-oxide are
Photoreduction is also not observed with those benzophenones[Zl that have substituents with electron-donor
properties at the 0- or p-position, since on irradiation
electron shifts lead to a x,x* triplet instead of the reactive
n,x* triplet.
Ketimines derived from benzophenone, e.g. benzophenone
methylimine (5), with R = CH3, are reduced by 2-propanol
under the influence of UV light to the corresponding amines
(6) [51. Quantum yield: 0.01 o n irradation with light of wavelength 265 nm. Reaction conditions: Hanau immersion lamp
T Q 81; Solidex filter; yield: 90%.
The aminodiphenylmethyl radical (7) is formed in the
primary photochemical process, as could be shown by means
of radical trapping agents and by isotope experiments.
Angew. Chem. internat. Edit. 1 Yol. 6 (1967) 1 No. 5
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oxide, nicotinamide, biological, oxidant
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