Synthese von terpenartigen bicyclischen Systemen über die Cycloaddition von Dimethylketen an Methylcyclopentadien

The structure of the cyclo-addition products of ketenes ( 2 ) and methylcyclopentadiene could not be predicted, since this ketenophile exists as a rapidly equilibrating 45:54:1 mixture of the 1- ( 4 ), 2- ( 5 ) and 5-methyl ( 6 ) isomers. We studied this reaction using dimethylketene ( 2a ) with a view to monoterpenoid synthesis. Only two ( A and B ) of the many possible cyclo-adducts were formed in good yield. Using three methods to generate dimethylketene, the ratios of A:B were 90:6, 77:16 and 55:31 respectively. The structures of A and B were proved without making use of the known ketene cyclo-addition rules: Of the many possibilities, all but 7, 8, 9 and 10 were excluded by the C?O and C?C? CH₃ IR.-bands and NMR. signals. Structure 7 (filifolone) was rejected by the difference of its NMR. spectrum from those of both A and B , leaving only 8 , 9 and 10 . For A , structures 9 and 10 were eliminated by two chemical approaches: (1) Peracetic acid oxidation of a mixture rich in A resulted in a combination of Baeyer-Villiger reaction, epoxidation, and epoxide-ketone rearrangement affording a C₁₀-keto-lactone 14 , which was cleaved by alkali to acetone and a C₇-keto-acid 18 . (2) Another peracetic acid treatment produced a keto-epoxide 11, which was converted to a diol 22 and then to a diketo-aldehyde 23 with an NMR. doublet for the aldehyde proton. Of the two structures ( 9 and 10 ) left for B , the latter was excluded by the NMR. spectrum of keto-epoxide 15 isolated from the peracid treatment of a mixture rich in B : A decoupling experiment with 15 showed that the -CH₂- was placed next to the α-carbon of the carbonyl function. Thus the major cyclo-adduct of dimethylketene and methylcyclopentadiene is 8 , and the minor product is 9 . This result demonstrates further ketene cyclo-addition specificities on top of the ones expressed by the three known rules. The different ratios of 8 : 9 are considered to be due to varyingly effective competition between the preliminary interconversion of the methylcyclopentadienes and their cyclo-addition of dimethylkctene. A number of other oxidation and reduction products of 8 are described. One of them proved that C7 had migrated in the Baeyer-Villiger reaction. A partial separation of the enantiomers of the major cycloadduct 8 was achieved. By the use of 0.5 equivalent of three chiral primary amines (31, 32, and 33) to form a Schiff base, the excess ketone 8 was in each case distilled off in a chirally enriched form with positive optical rotation. The derived 2.3-dihydroketone 36 was shown to have a positive Cotton effect and, by an application of the octant rule, to have the (1R,5S) chirality orientation.     

Tunable room-temperature cw laser action in Cr3+: GdScGa-Garnet

We report for the first time on tunable room temperature cw laser action of a transition metal laser material. In Cr³⁺:GdScGa-garnet crystals almost the total fluorescence is channelled into the broad band⁴ T ₂→⁴ A ₂ transition. The fluorescence lifetime is τ=120 μs. Laser pumping in the blue-green and yellow-red spectral range yields pump thresholds around 250 mW and slope efficiencies up to 11%. The wavelength of the free running Cr³⁺ laser is centered at 777 nm.     

Electron capture by slow multiply charged Ar and Ne ions from atomic hydrogen

At collision energies below 15 keV total charge transfer cross sections have been measured for multiply charged Ar^ζ+(2≦ζ≦6) and Ne^ζ+(2≦ζ≦4) ions colliding with atomic hydrogen. A Wood discharge was used to provide a hydrogen target with a sufficiently high degree of dissociation. Results are compared with measurements performed at higher energies and with theoretical calculations. For ζ=2 cross sections in atomic hydrogen are much smaller than in the molecular case, for ζ>2 the ratio of these cross sections varies between 0.7 and 1.6.     

Trapping of excitation in the averageT-matrix approximation

We investigate the trapping of excitation by a random array of acceptor ions embedded in an assembly of donors. The problem is formulated in terms of coupled rate equations. Exact results for the fraction of excited donors are obtained in the zero and rapid donor-donor transfer limits for arbitrary ratios of the donor to trap concentration. The averageT-matrix approximation (ATA) is introduced to interpolate between these limits in situations where the concentration of traps is much less than the concentration of donor ions. In three-dimensional systems the ATA reproduces the results of earlier calculations in appropriate limits. The extension of the theory to higher trap concentration is discussed, as are problems connected with the application to one- and two-dimensional arrays.     

EID study on the system platinum-hydrogen

Electron induced desorption of hydrogen on polycrystalline platinum has been studied in continuation of former work. The results remain consistent with a supposed adsorption model implying that electron induced H⁺ emission is restricted to hydrogen adsorbed only at particular sites occupying not more than 1 per cent coverage. These sites are ascribed to characteristic and stable surface impurities which are not influenced by the improved target treatment applied. Depletion of sites by sputter cleaning is cancelled by recovery during annealing. The highest observed H⁺ yield (all available sites filled with hydrogen) is 2 · 10⁻⁵ ions per electron. The total cross section for desorption by electron impact is 2 · 10⁻¹⁷ cm². Complex interaction phenomena between the electrons and the hydrogen covered platinum surface have been observed.     

Formation of branched calixarene aggregates-a time-resolved static light scattering study

Mixtures of a calix[4]arene and a naphthyridine derivative dissolved in 1,2-dichlorobenzene form thermoreversible aggregates. The aggregation process was followed by means of time-resolved multiangle light scattering at two different mixing ratios, 1:3 and 1:4, yielding a detailed record of the relative mass, the radius of gyration, and the particle scattering function of the growing aggregates. On the basis of these data, a conclusive model of the structure is presented for the developing aggregates: monomers aggregate to wormlike filaments which form branching points. Formation of branching points proceeds in a frequency and distribution which is similar to the polycondensation of ABC monomers toward non-randomly branched macromolecules (Burchard, W. Macromolecules 1977, 10, 919-927). Thus, aggregation results in hyperbranched-like particles with striking analogies to the polymerization of glucose to amylopectin.     

Acidic peptides acting as growth modifiers of calcite crystals

Small acidic peptides comprising a repeating Phe-Asp sequence motif exert control, in vitro, on the morphology of calcite crystals similar to natural proteins from calcified tissues.