Fluorimetric determination of acidity constants of naphthoic and anthroic acids

A fluorimetric method of determining the acidity constants (pK_a values) of some naphthoic and anthroic acids is presented, based on the changes in fluorescence of solutions of these acids in water on changing the pH. The method is useful in the absence as well as in the presence of excited-state proton transfer. In the latter case, when the excited-state protonation occurs in the same pH region as the ground-state dissociation, resolution of the two processes can be accomplished by addition of a suitable quencher such as iodide. The method permits the accurate determination of the pK_a value of fluorescent compounds, even when they are poorly soluble in water, because of the high sensitivity of the spectrofluorimetry.     

Synthèse des (R)- et (S)-O-isopropylidène-1,2-glycérols. Détermination de la pureté optique

Synthesis of the (R)- and (S)-Glycerol Acetonides. Determination of the Optical Purity The optical purity of (R)-glycerol acetonide ( 1 ) and (S)-glycerol acetonide ( 1 ′) has been determined with great accuracy by gas chromatography after derivatization. The optical purity of (S)-glycerol actonide prepared from D -mannitol was > 99.4% while for (R)-glycerol acetonide obtained from L -serine it was 95%.     

Physikalische und spektroskopische daten von trifluormethylselenol und derivaten

The characterization of the compounds CF₃SeX (X = H, Cl, Br, CN, CF₃, SeCF₃) is completed by the report of melting points, boiling points, enthalpies of vaporization and entropies of vaporization. Ultraviolet and mass spectra are presented and discussed. An improved synthesis for CF₃SeH is reported.     

Zum Mechanismus der Photochemie des Benzfurazans

Mechanistic studies on the photochemical reactions of benzfurazan . From other works it is known that irradiation of benzfurazan ( 1 ) in methanol gives the carbaminacid-ester 4 , whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5–8 (Scheme 2) have been proposed as intermediates. In our investigations we detected and characterized by means of UV.- and IR.-spectroscopy the two species 5 and 8 . Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H₂O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1 . The rate constant for this reaction at room temperature is 2 · 10^–5s^–1. The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene ( 6 ) or nitrene ( 7 ). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).     

Enhanced fluorescence from o-phthalaldehyde and fluorescamine fluorophores using Triton and β-cyclodextrin

Mexiletine— and lysine hydrochloride—o-phthalaldehyde and mexiletine hydrochloride—, cysteine—, cysteamine—, homocysteine— and lysine hydrochloride—fluorescamine derivatives were subjected to Triton and β-cyclodextrin enhancement treatments. Of several classical fluorescence-enhancing reagents tested (Triton, β-cyclodextrin, sodium dodecyl sulphate, Brij), Triton provided the best results, followed by β-cyclodextrin. Increases in fluorescence emission by a factor of up to about 10 (mexiletine—fluorescamine—Triton X-100) were observed, with a generally negligible influence of the enhancing reagents on the excitation and emission maxima. Fluorescence enhancement by the addition of a suitable reagent solution to the final analyte solution may, in specific instances, enhance the detectability of native or chemically induced fluorophores.     

New stereoselective syntheses of cis- and trans-2-methyl-4-arylpiperidines

A stereoselective approach has been developed for the synthesis of cis- and trans-2-methyl-4-arylpiperidines from a common intermediate. The Ni-catalyzed hydrogenolysis of N-Boc-2-methyl-4-aryl-4-piperidinols, obtained by addition of organometallic reagents on N-Boc-2-methyl-4-piperidone, afforded the trans derivatives with up to 95% selectivity whereas the corresponding cis isomers were obtained in the presence of palladium catalysts.     

Xyloketal G,a Novel Metabolite from the Mangrove Fungus <Emphasis Type="Italic">Xylaria sp.</Emphasis> 2508

A novel metabolite 1, named xyloketal G, was isolated from cultures of marine derived mangrove fungus Xylaria sp. 2508. Its structure was elucidated by analysis of spectroscopic data.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 22–23, January–February, 2005.     

Triplet-singlet radiationless energy transfer between benzophenone and perylene in vitreous solution

We have measured at 77 K the “critical radius” R₀ of the resonant intermolecular energy transfer from the triplet state of benzophenone to the singlet state of perylene, these two comporents being in vitreous solution.