Structure and synthesis of leucettidine

The structure of leucettidine ($\text{1}$$\text{2}$) has been revised according to its UV-data to 6-(1-hydroxypropyl)-1-methyllumazine and has been proven by an unambiguous synthesis of this natural product.     

The weakly bound He-HCCCN complex: high-resolution microwave spectra and intermolecular potential-energy surface

Rotational spectra of the weakly bound He-HCCCN and He-DCCCN van der Waals complexes were observed using a pulsed-nozzle Fourier-transform microwave spectrometer in the 7-26-GHz frequency region. Nuclear quadrupole hyperfine structures due to the 14N and D nuclei (both with nuclear-spin quantum number I = 1) were resolved and assigned. Both strong a and weaker b-type transitions were observed and the assigned transitions were used to fit the parameters of a distortable asymmetric rotor model. The dimers are floppy, near T-shaped complexes. Three intermolecular potential-energy surfaces were calculated using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations. Bound-state rotational energy levels supported by these surfaces were determined. The quality of the potential-energy surfaces was assessed by comparing the experimental and calculated transition frequencies and also the corresponding spectroscopic parameters. Simple scaling of the surfaces improved both the transition frequencies and spectroscopic constants. Five other recently reported surfaces [O. Akin-Ojo, R. Bukowski, and K. Szalewicz, J. Chem. Phys. 119, 8379 (2003)], calculated using a variety of methods, and their agreement with spectroscopic properties of He-HCCCN are discussed.     

Serial potentiometric and conductometric detection in fast non-suppressed ion chromatography applied to the analysis of filter collected airborne particulates

Non-suppressed ion chromatography (NSIC) has been optimized to permit the determination of chloride, nitrate and sulfate in the low mg/l concentration range within 3 min. Using conductometric detection the extraneous (positive) injection peak was found to adversely affect the early eluting chloride signal when samples with high amounts of total cations are analyzed. The serial arrangement of a downstream potentiometric detector with a chloride ion selective electrode, which does not respond to other alterations of the composition of the eluent than the change in chloride concentrations, is shown to be suitable for interference-free, sensitive and reliable chloride evaluation. Application of the proposed system to the determination of chloride in extracts of filter collected airborne particulates revealed (for those samples that could be evaluated by both detection methods) high precision and no systematic deviations between conductometric and potentiometric detection.Dedicated to Professor Dr. Peter Brätter on the occasion of his 60th birthday     

Catalytic asymmetric synthesis of Secondary (E)-allyl alcohols from acetylenes and aldehydes via (1-alkenyl)zinc intermediates. Preliminary Communication

Hydroboration of aliphatic 1-alkynes with freshly prepared dicyclohexylborane (1 mol-equiv., hexane), treatment of the resulting [(E)-1-alkenyl]boranes 5 with Et₂Zn or Me₂Zn (1.05 mol-equiv.) followed by addition of (?)-3-exo-(dimethylamino)isoborneol (DAIB, 8 ; 0.01 mol-equiv.), subsequent addition of a solution of an aromatic or aliphatic aldehyde (1 mol-equiv., hexane), and quenching with aq. NH₄Cl provided (E)-allyl alcohols 6 usually in 70–95% yield with 79–98% enantiomeric excess (Scheme 3 and Table).     

Thermal and photochemical studies with stilbene oxides intramolecular trapping of carbonyl ylide intermediates

The annelated tetrahydrofuran derivatives 14 and 15, resp., are formed in moderate yield by intramolecular trapping reaction of the carbonyl ylide intermediates 9, which are generated either by thermal ring opening of the trans-stilbene oxides 7t/8t or by photolysis of the cis-isomers 7c/8c.     

Second-order nonlinear optical properties of the Ag(111)/electrolyte interface in the presence of self-assembled monolayers containing conjugated pi systems. I. Alpha-functionalized terthiophene films on Ag(111)

The nonlinear optical properties of self-assembled monolayers obtained from bonding two different alpha-functionalized terthiophenes (alpha-T3) to (111) silver electrode surfaces have been investigated using second harmonic generation (SHG). The two (alpha-T3) compounds used were functionalized with alkane chains of different lengths (C8 and C4), and each was terminated with a thiol anchoring group. A nitrile group was attached to the terminal thiophene ring of the (alpha-T3) compound with the C4 chain. The orientation of the polarization of the incident beam was changed systematically and gradually between "p" and "s" orientation and the SH signal (isotropic and anisotropic contributions) analyzed in both directions ("P" and "S"). The symmetry of the system was reduced by the presence of the adlayers from C3v to C3. The dependence on the applied potential and the incident wavelength has also been studied. The relative magnitudes and phases of the various second-order tensor elements have been estimated and compared with the values for a bare surface. A resonance process in the (alpha-T3) pi moiety has been investigated, and from this, the effective "band-gap" energies of the organic semiconductor SAMs (i.e., the energy difference between the pi-pi bands) have been estimated.     

Synthesis of 2,3-dihydropyrazolo[5,1-b]thiazoles via a tandem condensation sulfur-extrusion reaction

2,3-Dihydropyrazolo[5,1-b]thiazoles 3 are the first representatives of a heterocyclic system, which are conveniently prepared by heating 3-aminorhodanines 1 with ethyl 2-bromo-3,3-diethoxypropionate ( 2 ), via a tandem condensation-sulfur extrusion reaction.     

Copper—A “Modern” Bioelement

Copper is a bioessential element in biology with truly unique chemical characteristics in its two relevant oxidation states +I and +II. Significant progress has been made in recent years in the elucidation of the frequently surprising biochemistry of this trace element. Those advances were especially furthered through mutual stimulation involving results from biochemistry, molecular biology, and medicine on one hand and the synthesis as well as the structural and spectroscopic characterization of low molecular weight model complexes on the other. The most notable features of protein-bound active copper are its almost exclusive function in the metabolism of O₂ or N/O compounds (NO, N₂O) and its frequent association with oxidizing organic and inorganic radicals such as tyrosyl, semiquinones, superoxide, or nitrosyl. This unique biological role of copper can be rationalized given its chemical and assumed evolutionary background.     

A Convergent Synthesis of (±)-α- and β-Himachalenes

(±)-α and β-Himachalene, 5 and 6 , have been synthesized in a convergent manner from 3,3-dimethylacrolein ( 9 ), the ester enolate 10 and the silyloxypentadienyllithium 7 . The key steps are the regioselective γ-addition of the dienal 13 to 7 and the intramolecular Diels-Alder addition 15 → 16 . Hydrogenolysis of the diethylphosphate group and functionalization at C (5) completed the synthesis of 5 and 6 .