Pterinechemistry. Part 84. A new,regiospecific synthesis of L-biopterin

Pure L -biopterin was obtained in 42% yield by the condensation of 5-deoxy-L -arabinose-phenylhydrazone-triacetate with 4-hydroxy-2,5,6-triaminopyrimidine, followed by iodine oxidation of the formed tetrahydropterin derivative to 1′,2′-O-diacetyl-L -biopterin. Deacetylation was carried out with NH₄OH.     

Fibre-optic oxygen sensor with the fluorescence decay time as the information carrier

In constrast to existing fluoresensors for oxygen, a sensor is described that measures the decay time of the indicator rather than its emission than its emission intensity. A simple opto-electronic device is described for measuring lifetimes of a long-lived fluorophore with a frequency-modulated LED as light source. Measurement of lifetime provides as more linear Stern-Vomer plot and offers certain advantages with respect to the performance of the sensor, because it has an internal reference system that is an attractive to the frequently-used two-wavelenght referencing method. Long-term stability is distinctly better because the phase shift is independent of indicator bleaching and leaching a well as lamp intensity fluctuations. The detection limit is ca. 2 torr of oxygen.     

Bond dissociation energies and structures of CuNO(+) and Cu(NO)(2)(+)

The bond dissociation energies of CuNO(+), Cu(NO)(2)(+), and CuAr(+) are determined by means of guided ion beam mass spectrometry and quantum chemical calculations. From the experiment, the values D(0)(Cu(+)-NO) = 1.13 +/- 0.05, D(0)(ONCu(+)-NO) = 1.12 +/- 0.06, D(0)(Cu(+)-Ar) = 0.50 +/- 0.07, and D(0)(Cu(+)-Xe) = 1.02 +/- 0.06 eV are obtained. The computational approaches corroborate these results and provide additional structural data. The relative values of D(0)(Cu(+)-NO) and D(0)(Cu(+)-Xe) are consistent with the approximately thermoneutral formation of CuXe(+) upon interacting CuNO(+) with xenon. The sequential bond dissociation energies of Cu(NO)(2)(+) exhibit a trend similar to those of other Cu(I) complexes described in the literature. Although metathesis of nitric oxide to N(2) and O(2) is of considerable interest, no evidence for N-N- or O-O-bond formations in Cu(NO)(n)(+) ions (with n up to 3) is obtained within the energy range studied experimentally.     

Über Pterinchemie. 73. Mitteilung [1]. Zum Verlauf der katalytischen Reduktion von 7-Methylpterin

On the Pathway of the Catalytic Reduction of 7-Methylpterin The catalytic hydrogenation of 7-methylpterin (VII) in a neutral solution occurs first by the reduction of the 7,8-double bond (thermodynamically-controlled reaction) followed by the reduction of the 5,6-double bond. On the contrary, in an acidic medium like CF₃COOH, the 5,6-double bond is reduced first (kinetically-controlled reaction). The dihydro-intermediate then undergoes a [1,2]-H-rearrangement leading to the formation of the thermodynamically more stable 7-methyl-7,8-dihydropterin (XV) which on further reduction gives 7-methyl-5,6,7,8-tetrahydropterin (VIII). The catalytic reduction of 7-methyl-7,8-dihydropterin (XV) with deuterium gives stereoselectively a sole product with D at C(6) in the equatorial position.     

Transition probabilities of a string oscillator subject to impulsive collisions with a heavy mass point

Impulsive linear collisions between a string oscillator (a one-dimensional particle in a box) and a mass point are studied quantum mechanically. In the limit of a very heavy mass point (which corresponds classically to many collisions during a single encounter) the transition probabilities are determined exactly. The result permits a discussion of the mixed quantum-classical regime where the collider becomes almost classical while the oscillator remains quantum mechanical. While the average transition probabilities P(m-->n) are well reproduced by the Ehrenfest mean-field approximation, the prediction for the superimposed high-frequency resonance structure is qualitatively wrong for a genuine quantum oscillator. Only if the oscillator is also almost classical and if (m-n)2 square root(mu) < m, where mu is the mass ratio collider/oscillator, this structure is correctly predicted by the Ehrenfest approximation.     

New Cobalt-Catalyzed Cycloisomerization of epsilon-Acetylenic beta-Keto Esters. Application to a Powerful Cyclization Reactions Cascade

The full details of investigations into the cobalt(I)-catalyzed ene type reaction of epsilon-acetylenic beta-keto esters to form highly functionalized methylenecyclopentanes are described. The observed regio-, chemo-, and stereoselectivities support a process of cycloisomerization which controls the relative stereochemistry of two contiguous stereogenic centers. An efficient route to the basic skeleton of the phyllocladane family has been achieved via a one-pot sequence of cyclizations: ene type, [2 + 2 + 2], [4 + 2]. This new cascade created six carbon-carbon bonds and four rings in a totally stereoselective manner from an easily accessible acyclic polyunsaturated precursor.     

Non-exponential decay of the fluorescence from the 1Δg state in liquid oxygen at high excitation intensities

The ¹Δ_g state of liquid oxygen (natural O₂ and ¹⁸O₂) was excited by intense Nd: YAG laser pulses. The observed fluorescence decay is non-exponential and depends on pump laser intensity. Various decay channels of the ¹Δ_g state are discussed. The energy pooling reaction ¹Δ_g + ¹Δ_g → ¹Σ⁺_g + ³Σ^?_g is verified experimentally. Values f constant of liquid natural O₂ and ¹⁸O₂ are given.     

2,7,15,16-tetramethyl-trans-15,16-dihydropyrene

The synthesis of 2,7,15,16-tetramethyl-trans-15,16-dihydropyrene has been accomplished by two different routes. The compound is of interest in providing a better understanding of the ESR spectra of 15,16-dihydropyrenes.     

New synthesis routes to 2-halopyridines. III. 2-fluoropyridine

The solvent-free fluorination of 2-chloropyridine (III) with alkali metal fluorides and bifluorides was investigated. While complete degradation occurred with potassium fluoride at 315°, the use of potassium bifluoride under identical conditions provided a 74% yield of 2-fluoro-pyridine (II). Sodium bifluoride gave only a 3.5% yield of the desired product. These results are discussed in the light of: activation of the substrate, properties of the fluorinating agents, and the stability of 2-fluoropyridine.