Enantioselective syntheses of 2-alkyl- and 2,6-dialkylpiperidine alkaloids: preparations of the hydrochlorides of (-)-coniine, (-)-solenopsin A, and (-)-dihydropinidine

[reaction: see text] Sequences of lithiation-substitution, enantioselective hydrogenation, and diastereoselective lithiation-substitution provide efficient highly enantioselective syntheses of 2-substituted and cis and trans 2,6-disubstituted piperidines. The methodology is demonstrated by syntheses of (-)-coniine, (-)-solenopsin A, and (-)-dihydropinidine as their hydrochlorides.     

Highly enantioselective syntheses of anti homoaldol products by (-)-sparteine-mediated lithiation/transmetalation/substitution of N-Boc allylic amines

[reaction: see text](-)-Sparteine-mediated lithiation/transmetalation/substitution of N-Boc allylic amines provides anti-configured homoaldol precursors in yields of 38-85% and enantiomeric ratios of 83:17-99:1. Subsequent O-protection and hydrolysis allows access to O-protected homoaldol adducts in good yields. The absolute configurations of the homoaldol products have been assigned by calculation of optical rotations and by X-ray crystallography of derivatives. A stereochemical course of reaction for the lithiation/transmetalation/substitution sequence is proposed.     

Mechanism of oxygen transfer in the epoxidation of an olefin by molecular oxygen in the presence of an aldehyde

[reaction: see text] The reaction pathway for peroxide-initiated aldehyde-mediated oxidation of olefins to epoxides by molecular oxygen has been studied. The pathways of reaction via a peroxy acid or an acyl peroxy radical have been differentiated by investigation of the reaction of 4 with oxygen to provide 6 via 8.     

Quantitative analysis of fine nano-sized precipitates in low-carbon steels by small angle neutron scattering

The small angle neutron scattering (SANS) technique was used to determine the nano-sized precipitates in interstitial free (IF) and low-carbon (LC) steels with a hot-rolling temperature. The real-space direct model fitting method was applied to quantitatively analyze the SANS data. The magnetic and chemical properties of precipitates in the samples were also investigated by separation and comparison of nuclear and magnetic SANS scattering data. The size distribution of precipitates in the IF steel is in good agreement with the microstructure observation using transmission electron microscopy. The results revealed that the IF steel had two types of non-magnetic precipitates, Ti₄C₂S₂ and TiC, with the average size of about 30 nm in diameter and little difference in chemical composition. In case of commercial LC steel, the fine and large precipitates are identified as MnS and Fe₃C, respectively. The average size of spherical MnS precipitates was about 4.8 nm in radius and the distribution is isotropic. It is likely that the LC steels have almost the same composition and a similar size as precipitates such as MnS and Fe₃C with different finishing delivery temperatures. Interestingly, the average size and volume fraction of fine precipitates showed no significantly changes under the different finishing delivery temperatures.     

Improving the optical performance of InGaN light-emitting diodes by altering light reflection and refraction with triangular air prism arrays

The effect of triangular air prism (TAP) arrays with different distance-to-width (d/w) ratios on the enhancement of light extraction efficiency (LEE) of InGaN light-emitting diodes (LEDs) is investigated. The TAP arrays embedded at the sapphire/GaN interface act as light reflectors and refractors, and thereby improve the light output power due to the redirection of light into escape cones on both the front and back sides of the LED. Enhancement in radiometric power as high as 117% and far-field angle as low as 129° are realized with a compact arrangement of TAP arrays compared with that of a conventional LED made without TAP arrays under an injection current of 20 mA.     

Dynamic thermodynamic resolution: solvent effects, mechanism, and an asymmetric 3,4,5-substituted benzazepine synthesis

[reaction: see text] The resolution in the lithiation-substitution sequence from 1 to 4-11 in MTBE is shown to be under thermodynamic control in contrast to the previous report of kinetic control in diethyl ether. Diastereomeric equilibration of a soluble complex is shown to be controlling and an asymmetric synthesis of a 3,4,5-substituted benzazepine is reported.     

Air breakdown times of two-stage spark gaps and their applications

The air breakdown times of second spark formation, geometrically in series with trigatron switching, are studied and compared to that of a triggered spark gap. Despite a large breakdown jitter of ⩾1 μs in the trigatron, the latter is not observed to affect the electrical closure times of the series-connected second spark gap, which is triggered largely as a result of their direct electrical and optical coupling. While most of the multistage spark gaps are designed for fast and precision switching of high voltages, the air breakdown times of the second spark formation find a number of useful applications, particularly in achieving uniform glow discharge in pulsed gas lasers