Oxabicyclo[3.2.1]octane derivatives as highly reactive dienophiles: synthesis of bicyclo[5.n.0] systems

[reaction: see text] We have developed highly versatile, homochiral oxabicyclo[3.2.1]octadiene building blocks for the synthesis of natural products. We have found that these bridged alkenes undergo exceptionally facile Diels-Alder reactions and react faster than several well studied bicyclo[2.2.1]heptene dienophiles. The reaction proceeds with high levels of stereochemical control and in very good to excellent yields, providing access to bicyclo[5.4.0]undecane and bicyclo[5.3.0]decane systems. This reactivity is attributed to strain and homoconjugation effects.     

Control of ultracold collisions with frequency-chirped light

We report on ultracold atomic collision experiments utilizing frequency-chirped laser light. A rapid chirp below the atomic resonance results in adiabatic excitation to an attractive molecular potential over a wide range of internuclear separation. This leads to a transient inelastic collision rate which is large compared to that obtained with fixed-frequency excitation. The combination of high efficiency and temporal control demonstrates the benefit of applying the techniques of coherent control to the ultracold domain.     

A perceptual learning investigation of the pitch elicited by amplitude-modulated noise

Noise that is amplitude modulated at rates ranging from 40 to 850 Hz can elicit a sensation of pitch. Here, the processing of this temporally based pitch was investigated using a perceptual-learning paradigm. Nine listeners were trained (1 hour per day for 6-8 days) to discriminate a standard rate of sinusoidal amplitude modulation (SAM) from a faster rate in a single condition (150 Hz SAM rate, 5 kHz low-pass carrier). All trained listeners improved significantly on that condition. These trained listeners subsequently showed no more improvement than nine untrained controls on pure-tone and rippled-noise discrimination with the same pitch, and on SAM-rate discrimination with a 30 Hz rate, although they did show some improvement with a 300 Hz rate. In addition, most trained, but not control, listeners were worse at detecting SAM at 150 Hz after, compared to before training. These results indicate that listeners can learn to improve their ability to discriminate SAM rate with multiple-hour training and that the mechanism that is modified by learning encodes (1) the pitch of SAM noise but not that of pure tones and rippled noise, (2) different SAM rates separately, and (3) differences in SAM rate more effectively than cues for SAM detection.     

Cooperative metal-coordination and ion pairing in tripeptide recognition

[structure: see text] The recognition of tripeptides by a metal-centered receptor consisting of a rigid backbone region and variable tripeptide arms is described. The studies showed that the receptor was selective for l-Xxx-l-Lys-l-Lys, with Xxx = His, Cys, and Met, giving association constants near 1.0 x 10(6) M(-)(1). Binding enhancement through cooperative interactions of the peptidic arms is demonstrated.     

Coherent shear phonon generation and detection with ultrashort optical pulses

Using an optical technique we generate and detect picosecond shear and quasishear coherent acoustic phonon pulses in the time domain. Thermoelastic and piezoelectric generation are directly achieved by breaking the sample lateral symmetry using crystalline anisotropy. We demonstrate efficient detection in isotropic and anisotropic media with various optical incidence geometries.     

Spectral encoding of the polarization state of light in multiplexed polarization-sensitive lasing structures

We report the experimental demonstration of spectral encoding of the polarization state of light by use of multiplexed distributed-feedback lasers in a dye-doped organic thin film by exploiting the pump polarization selectivity of the superimposed resonators. Measurement of the Stokes parameters of the pump light without a priori knowledge of the lasing properties of the structure is discussed.     

Catalytic activity of dodecacarbonyltetracobalt in aqueous media: a "greening" of the Pauson-Khand reaction

The unprecedented reactivity of Co(4)(CO)(12) with enynes under aqueous conditions, representing the development of a mild and simple aqueous-phase cobalt-catalyzed PK reaction protocol, is described herein.     

Measurement of internal magnetic field fluctuations in a reversed-field pinch by Faraday rotation

Magnetic field fluctuations (and the associated current perturbation) have been measured in the core of a high-temperature reversed-field pinch using a newly developed fast-polarimetry system. Radial magnetic field fluctuation levels of approximately 1% are measured in standard-reversed-field pinch discharges which increase to approximately 4% during the sawtooth crash (enhanced dynamo). The fluctuation level is reduced fourfold for high-confinement plasmas where the core-resonant tearing modes are suppressed.