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Detection of O2(1Δg) phosphorescence emission, λmax = 1270 nm, following laser excitation and steady-state methods was employed to determine the total rate constant, kT, and the chemical reaction rate

– Cyclic 1,4-diazadiene (2,3-dihydro-5-methyl-6-phenylpyrazine) and diketone (1-phenyl-1,2-propanedione) yielded two products via dehydration–condensation of the diamine and diketone moieties when a mixture of H2O and MeOH was used as the solvent. The str

The first asymmetric hydrogenation of unfunctionalized 2-substituted and 2,3-disubstituted 5,6-dihydropyrazines catalyzed by chiral cationic Ru-diamine complex (R,R)-1a was developed, affording chiral piperazine derivatives with good enantioselectivities (up to 89% ee).

Quinoxaline and dihydropyrazine derivatives were obtained in high yields by simple addition of 1,2-diamines and 1,2-dicarbonyl compounds in water. In some cases, the products spontaneously precipitated from the reaction mixture, making it possible to recover and reuse the mother liquor for further condensations. The very mild reaction conditions, the high yields of the products, and the absence of any catalyst make this methodology an efficient and green route to quinoxalines and dihydropyrazines. Georg Thieme Verlag Stuttgart New York.

Tautomerization of methyl-substituted dihydropyrazine (DHP) derivatives to their latent enamine form was investigated theoretically and empirically. Among two types of hydrogen transfer model simulated by means of density functional theory calculation, a

Monophenyl-substituted dihydropyrazines (Ph-DHP-1 to 4) of 2,3-dihydro-5,6-dimethylpyrazine (Me-DHP-1), which have the inductive effects of apoptosis and mutagenesis, were synthesized and their biological effect was investigated in terms of DNA strand-breakage. Differences between the phenyl- and methyl-substituted dihydropyrazines were examined.

The reaction of singlet molecular oxygen with a series of cyclic and acyclic α-diimines was studied. Time-resolved methods were used to measure total reaction rate constants and steady-state methods were used to determine chemical reaction rate constants. GC-MS was used to tentatively assign the reaction products. 5,6-Disubstituted cyclic α-diimines are singlet oxygen quenchers, but become more effective in polar solvents. A reaction mechanism involving a perepoxide intermediate or transition state leading to a hydroperoxide seems to be a key reaction path for product formation. A replacement of the phenyl substituent for a methyl substituent opens up an additional reaction involving a perepoxide-like exciplex, which increases singlet oxygen quenching of the cyclic α-diimines. The reactivity of 5,6-disubstituted cyclic α-diimines towards singlet oxygen is highly dependent on steric interactions arising from vicinal phenyl rings and from electronic effects. 1,4-Disubstituted acyclic α-diimines are, by comparison, moderate or poor singlet oxygen quenchers. Total rate constants are scarcely dependent on solvent properties, but instead correlate with the Hildebrand parameter. These results are explained in terms of a mechanism involving a dioxetane-like exciplex that gives rise to a charged intermediate leading to products.

Dye-sensitized photooxygenations of cyclic and acyclic 1,4-diaza-1,3-butadienes (α-Diimines), carrying a methyl or an ethyl group in the 2-position, yield isonitriles and amides, as well as formaldehyde or acetaldehyde.Thus, cyclic α-diimines such as 5,6-dimethyl- (1a), 5-methyl-6-phenyl- (1b), 5-methyl-6-(p-tolyl)- (1c), and 5,6-diethyl-2,3-dihydropyrazine (1d) yield the respective 1-isocyano-2-(acylamino)ethanes 2a-d, whereas 5-isopropyl-6-phenyl- (1e) and 5,6-diphenyl-2,3-dihydropyrazine (1f) are inert toward photooxygenation.Acyclic α-diimines such as 1,8-bis(acetylamino)-3,6-diaza-4,5-dimethyl-3,5-octadiene (4a), 2,5-diaza-3,4-dimethyl-2,4-hexadiene (4b), 1,4-diaza-2,3-dimethyl-1,4-diphenyl-1,3-butadiene (4c), and N,N'-bis-2,3-butanediimine (4d) afford the respective isonitriles 5a-d and acetamides 6a-d.With retention of configuration, chiral α-diimines such as (-)-N,N'-bis-2,3-butanediimine and its R,R enantiomer , and (3S,8S)-(-)-3,8-dicarbomethoxy-4,7-diaza-2,5,6,9-tetramethyl-4,6-decadiene yield the corresponding enantiomerically pure isonitriles (S)-5e, (R)-5e, and (S)-5f and acetamides (S)-6e, (R)-6e, and (S)-6f.Evidence for a singlet oxygen reaction is presented.A working hypothesis is proposed which includes the formation of a transient hydroperoxide 7 that fragments into an isonitrile molecule, an aldehyde, and an N-alkyl- or N-arylnitrilium cation and a hydroxide ion; the latter combine to yield the amide groups in 2a-d and the amide compounds 6a-f.

Rose bengal and tetraphenylporphin sensitized photooxygenations of 5,6-dimethyl- (1a) and 5-methyl-6-phenyl-2,3-dihydropyrazine (1b) yield 1-isocyano-2-(N-acetylamino)-ethane (2a) and 1-isocyano-2-(N-benzoylamino)-ethane (2b), respectively. 5,6-Diphenyl-2

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