Synthesis and reaction of a new type of 5-deazaflavin with axial and planar chirality

A new type of 5-deazaflavin derivative with axial and planar chirality was synthesized as a flavoenzyme model. A novel optical resolution gave an enantiomeric pair of the 5-deazaflavins 1a,b. Compounds1a,b were optically stable and effectively discriminated PNPH enantiomers in a model reaction of intercoenzyme hydrogen transfer.     

Magnetic field dip and the lifetime of the spherical tensor element of the density matrix in a He-Ne laser

The magnetic field dips (MFD) of the 0.633 and 1.15 μm lines of a He-Ne laser are investigated. It is found that the widths of the MFD signals are not determined by a single upper or lower laser state. Thus, there is some ambiguity in the interpretation of the MFD measurement for the determination of the lifetime, which is difficult to resolve theoretically or experimentally. The effect of pressure on MFD is also discussed, with emphasis on excitation-transfer collisions between the excited and ground states of neon atoms.     

The temperature dependence and the mechanism of the SO2 (3B1) quenching reactions

The Arrhenius parameters have been determined for the SO₂(³B₁) quenching reaction (9), SO₂(³B₁) + M → (SO₂ ? M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO₂ (³B₁) molecules was accomplished by two very different methods: (1) a 3829 Å laser pulse generated the triplet directly through absorption within the “forbidden” SO₂ (³B₁) → SO₂ (¹A₁) band; (2) a broadband Xe-flash system generated SO₂(³B₁) molecules and triplets were formed subsequently by intersystem crossing, SO₂(¹B₁) + M → SO₂(³B₁) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N₂, 2.9 ± 0.4; 0₂, 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO₂, 2.8 ± 0.4; CO, 2.7 ± 0.4; CH₄, 2.5 ± 0.6. This energy corresponds to the first region of the SO₂(³B₁) → SO₂(¹A₁) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO₂(³B₁) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n-butane, cyclohexane, and isobutane, which parallels closely the decrease in C? H bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge-transfer complex and subsequent chemical interactions between the SO₂(³B₁) molecule and the π-system of these compounds.     

Aldol reactions,conjugate additions,and alkylation reactionsof O-silylated ketene acetals under neutral conditions

Aldol reactions, conjugate additions, and alkylation reactions of O-silylated ketene acetals take place under neutral conditions by using a high pressure technique.     

Preparation of chiral 5-deazaflavin derivatives and their asymmetric reduction of ethyl benzoylformate

1,5-Dihydro-5-deazaflavin derivatives possessing a chiral substituent at N(3) position were synthesized, with which moderate asymmetric induction was observed in the reduction of ethyl benzoylformate.