Absolute Configuration of 2-Substituted 2-Azabicyclo[2.2.1]hept-5-enes

The diastereoisomeric 2-substituted 2-azabicyclo[2.2.1]hept-5-enes 2 – 4 were prepared by aza-Diels-Alder reaction of cyclopentadiene with the corresponding methaniminium ions. Their relative configurations were deduced using ¹H, ¹H-ROESY experiments, and their absolute configurations were assigned from the crystal structure of the aziridinium derivative (?)- 5 . The absolute configuration of (+)- 1 , i.e. (1R), was assigned by CD spectroscopy.     

Pseurotin, a new metabolite of Pseudeurotium ovalis Stolk having an unusual hetero-spirocyclic system.

The structure and absolute configuration of pseurotin ( 1 ), a new metabolite, isolated from culture filtrates of Pseudeurotium ovalis STOLK (Ascomycetes), has been shown to be 2-[1′(S), 2′ (S)-dihydroxhex-3′-ene-yl]-3-methyl-8(S)-methoxy-8-benzoyl-9(R)-hydroxy-(5S)-1-oxa-7-aza-spiro[4.4]non-2-ene-4, 6-dione ( 1 ), by spectral data and chemical transformations, and by X-ray analysis of its dibromo derivative 2 [1].     

D-Gluconhydroximo-1,5-lactam and Related N-Arylcarbamates Theoretical Calculations,Structure, Synthesis,and Inhibitory Effect on β-Glucosidases

The known D -gluconhydroximo-1,5-lactam (= D -glucono-1,5-lactam oxime) 7a , its nitrogen isotopomers 7b and 7c , and the N-arylcarbamates 26–29 were synthesized from 2,3,4,6-tetra-O-benzyl-D -glucono-1,5-lactam ( 11a ) and its nitrogen isotopomer 11b to establish the controversial structure of 7a and to study the inhibition of β-glucosidases by the N-arylcarbamates 26–29 . Conversion of 11a with Lawesson's reagent yielded a mixture of the thionolactam 15a and its manno-configurated isomer 16a , which was transformed into a mixture of the benzylated hydroximo-lactam 13a and the manno-isomer 17a . Debenzylation (Na/NH₃) and acetylation of this mixture led to the gluco-configurated pentaacetate 14a and the manno-isomer 18a . Treatment of 11a with Et₃O·BF₄ and then with H₂NOH gave exclusively the benzylated D -gluconhydroximo-1,5-lactam (benzylated D-nojirilactam oxime) 13a , which was transformed into 14a . Deacetylation of 14a yielded the hydroximo-lactam 7a . The isotopomers 7b and 7c were obtained by analogous reaction sequences, using either ¹⁵NH₃ or ¹⁵NH₂OHHCl. To prepare the acetylated N-arylcarbamates 20–25 , 13a was debenzylated and acetylated (→ 14a ), followed by selective deacetylation to the tetraacetate 19a and treatment with the appropriate isocyanates. The structure of the 2-chlorophenyl carbamate 21 was established by X-ray analysis. Deacetylation of 20–23 led to the N-arylcarbamates 26–29 . The ¹⁵N-NMR spectra of 7b , 7c , and of their precursors 13b , 13c , 14b , and 14c , show that the C?N bond in all these lactam oximes is exocyclic as predicted from semiempirical and ab initio SCF-MO calculations on the structure of acetamide oxime and 5-pentanelactam oxime. According to these calculations, 5-pentanelactam oxime is a (Z)-configurated, flattened chair. X-ray analysis established the structure of D -glucono-1,5-lactam oxime ( 7a ) in the solid state, where it adopts a conformation between ⁴C₁ and ⁴H₃. In H₂O, 7a is a flattened ⁴C₁. The calculations also predict that protonation at the exocyclic N-atom strengthens the conjugation between the endocyclic N-atom and the hydroxyimino group, and leads to a half-chair conformation. This is evidenced by the chemical shift differences in the ¹⁵N-NMR spectra observed upon protonation of 7b and 7c . The hydroximolactam 7a and the N-arylcarbamates 26–29 are competitive inhibitors of the β-glucosidases from sweet almond (emulsin) and from Agrobacterium faecalis (= Abg), with K_I values between 8 and 21·10^?6M against emulsin (at pH 6.8) and between 0.15 and 1.2·10^?6M against Abg (at pH 7.0).     

HPLC analysis of methylxanthines in human breast milk

A sensitive and specific high-performance liquid chromatographic (HPLC) procedure is developed for simultaneously quantitating the levels of caffeine, theophylline, theobromine and paraxanthine in breast milk. The method involved the precipitation of proteins present in the milk samples with a 6% v/v perchloric acid solution containing the internal standard, proxyphylline, followed by centrifugation at 12,800 Xg for 10 minutes. The clear supernatant was then chromatographed on a C18 reversed-phase analytical column at ambient temperature using a wavelength of 272 nm. Samples were eluted from the column at a constant flow rate of 1.5 mL/min using a gradient program in which the concentration of methanol in the mobile phase varied from 0 to 16%. The mean recoveries of the methylxanthines averaged over all the concentrations examined were generally excellent and ranged from 96.3 +/- 5.4% for caffeine to 102.3 +/- 8.9% for paraxanthine. The assay precision was very good and the peaks of interest were extremely well resolved. The method is recommended for assessing the total caffeine and dimethylxanthine load to which the nursing infant is exposed in mothers ingesting typical amounts of caffeine.     

MOVPE growth of ternary and quaternary tensile strained MQW structures for polarization insensitive devices

Devices for future all-optical network systems will have a strong requirement for polarization independent operation, which can be achieved by introducing biaxially tensile strain into MQW active regions. We report on InP based low pressure MOVPE growth of MQW structures for 1.55 μm wavelength with tensile strained InGaAs and InGaAsP wells and lattice matched InGaAsP barrier layers. Taking into account measurement results from X-ray and PL together with theoretical evaluations we were able to grow polarization insensitive device structures. For all-optical wavelength converters we used MQW stacks with 5 tensile strained ternary wells and achieved λ conversion with a polarization dependence of less than 1 dB. Using quaternary wells a polarization insensitive electroabsorption modulator was realized. The residual polarization dependence is < 0.4 dB for 1550 nm and < 1 dB in the wavelength range from 1540 to 1560 nm (extinction ratio 10 dB).     

Darstellung und Struktur von Sn(NH2)2F2

Synthesis and Structure of Sn(NH₂)₂F₂ Diamido-difluoro-tin Sn(NH₂)₂F₂ can be produced by ammonolysis from (NH₄)₂SnF₆ at 613 K. The compound is a product from Sn(NH₃)₂F₄ formed during the ammonolysis reaction. Sn(NH₂)₂F₂ crystallizes in space group C2/m (No. 12) with lattice constants a = 1070.18(7), b = 325.38(3) pm, c = 505.02(3) pm and β = 105.728(3)° (V = 169.271(6) · 10⁶ pm³) containing two formula units per unit cell. Data refinement by the Rietveld method yields a Bragg R-value of R_Bragg = 0.0514 (Profile R-value R_wp = 0.177). Tin is octahedrally coordinated by two fluorine atoms and for amido groups. The octahedra are connected to one-dimensional strings by edge sharing. The NH₂ groups are in the bridging position whilst the fluorine atoms are terminal.     

Niob- und Tantal-Komplexe des N-metallierten Hexamethylimidophosphorsäuretriamids und des Tris-tert-butyliminophosphorans

Niobium and Tantalum Complexes of N-Metalated Hexamethyl Imidophosphorus Acid Trisamide and Tris-tert-butyl Iminophosphorane Sterically most demanding phosphorane iminato ligands [N=PR₃]^– (R = ^tBu, NMe₂) with pronounced π-basic character are used for the stabilization of π-acidic d⁰ metal centers. In this respect the reaction of niobium and tantalum oxo chlorides MOCl₃ (M = Nb, Ta) with Me₃SiN=P(NMe₂)₃ leads to selective formation of (Me₃Si)₂O and [M{NP(NMe₂)₃}₂Cl₃] (M = Nb 1 , Ta 2 ). By reaction of the metal pentachlorides with the N-lithiated ligands the hitherto unknown tris-substituted derivatives [M(NPR₃)₃Cl₂] (M = Nb: R = ^tBu 3 , NMe₂ 4 ; M = Ta: R = ^tBu 5 , NMe₂ 6 ) are obtained. [Ta(NP^tBu₃)₃Cl₂] ( 5 ) was characterized by an X-ray crystal structure analysis. The tantalum center is trigonal-bipyramidally coordinated by three equatorial phosphorane iminato and two axial chloro ligands. The high temperature ³¹P-NMR spectrum of [Nb{NP(NMe₂)₃}₃Cl₂] ( 4 ) allows the observation of a decett, characterized by ²J(Nb–P) = 279 Hz.