Total synthesis of the immunosuppressant FR901483 via an amidoacrolein cycloaddition

[reaction: see text]. The total synthesis of the potent immunosuppressant FR901483 is described. In a key step, the intermolecular Diels-Alder cycloaddition of an amidoacrolein with 2-(triisopropylsilyloxy)-1,3-butadiene produced the desired 3-cyclohexene-1-carboxaldehyde. This compound was subjected to basic followed by acidic conditions which effected two sequential aldol cyclizations to deliver the tricyclic ring system of the natural product, suitably functionalized for completion of the total synthesis.     

Quantitative HPTLC determination of prostaglandin E1

Direct in situ determination of PGE₁ in dioxane/water (1:1) at concentrations between 50 μg/ml and 1 mg/ml is described. HPTLC of PGE₁ was carried out on HPTLC Kieselgel 60 o.F. plates; application (200 nl) was with a Nano-Appllcator and ethyl acetate/formic acid (400:5, v/v) served as mobile phase. The spots were located by dipping the plates into a 3% cupric acetate solution in 15% aqueous phosphoric acid, followed by heating at 120°C for 15 min. The plates were then evaluated directly at λ = 345 nm by chromatogram spectrometry. The coefficient of variation is 3.6%.     

A Frequency-Selective pH-Responsive paraCEST Agent

Paramagnetic chemical exchange saturation transfer (paraCEST) agents are well-suited for imaging tissue pH because the basis of CEST, chemical exchange, is inherently sensitive to pH. Several previous pH-sensitive paraCEST agents were based on an exchanging Ln³⁺-bound water molecule as the CEST antenna but this design often added additional line-broadening to the bulk water signal due to T₂ exchange. We report herein a pH-sensitive paraCEST agent that lacks an inner-sphere water molecule but contains one Ln-bound −OH group for CEST activation. The Yb³⁺ complex, Yb( 1 ), displayed a single, highly shifted CEST peak originating from the exchangeable Yb-OH proton, the frequency of which changed over the biologically relevant pH range. CEST images of phantoms ranging in pH from 6 to 8 demonstrate the potential of this agent for imaging pH. Initial rodent imaging studies showed that Gd( 1 ) remains in the vascular system much longer than anticipated but is cleared slowly via renal filtration.     

Measurement of moisture in grains at extreme temperatures

Summary This research has demonstrated that the dielectric method can be used successfully for grain moisture measurements for diverse grain types over wide ranges of temperatures if the grain moisture content is below certain (grain-specific) threshold values. These high-moisture limits were estimated. Optimum temperature correction coefficients for 149 MHz moisture measurements were determined for grain samples at different moisture levels. This information should be very helpful for grain moisture meter manufacturers and for moisture meter users who need to determine grain moisture contents at temperature extremes to be able to market grain more efficiently.     

Two-dimensional synthetic aperture imaging in the optical domain

In scan-mode synthetic aperture imaging radar, spatial resolution in a range is given by a frequency-swept waveform, whereas resolution in the orthogonal direction is derived from the record of phase as the beam footprint executes linear motion over the object. We demonstrate here what is to our knowledge the first two-dimensional imaging that uses exactly this process in the optical domain for a 1 cm x 1 cm object with 90 mumx170 mum resolution.     

Hypocholesterolemic agent compactin. an alternative diels-alder strategy for the synthesis of the hexahydronaphthalene portion

The synthesis of the hexahydronaphthalene segment (4) of compactin (1) has been accomplished in a short sequence (8 steps from 9) featuring the intramolecular Diels-Alder cycloaddition of 7b.     

A study of the induced magnetism in the Au spacer layer of Co/Au/CoO exchange-bias trilayers and related systems

We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at a magnetic/non-magnetic interface, applying low temperature nuclear orientation (LTNO) to Co/Au(x)/CoO trilayer systems. This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au γ-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurements on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.     

Nearest-neighbor configuration in (GaIn)(NAs) probed by x-ray absorption spectroscopy

Ga(1-x)In(x)N(y)As(1-y) is a promising material system for the fabrication of inexpensive "last-mile" optoelectronic components. However, details of its atomic arrangement and the relationship to observed optical properties is not fully known. Particularly, a blueshift of emission wavelength is observed after annealing. In this work, we use x-ray absorption fine structure to study the chemical environment around N atoms in the material before and after annealing. We find that as-grown molecular beam epitaxy material consists of a nearly random distribution of atoms, while postannealed material shows segregation of In toward N. Ab initio simulations show that this short-range ordering creates a more thermodynamically stable alloy and is responsible for blueshifting the emission.     

Laser induced decomposition of a designed and a commercial polymer studied by ns-interferometry and shadowgraphy

Commercially available polymers often exhibit quite poor laser ablation properties for irradiation wavelengths around 248 nm. At these wavelengths, the absorption is due to photostable aromatic groups. Photolabile triazene polymers were developed to compare the influence of a photolabile group on the laser ablation process. The photochemically active triazene group has a strong absorption band at 332 nm, whereas the second absorption maximum at 220 nm is due to the photostable aromatic group. By irradiating at 308 nm and 193 nm, the influence of the photochemically active group on the ablation process can be studied. The etching of the triazene polymer starts and ends with the laser pulse. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than with 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer compared with polyimide. Received: 28 August 2002 / Accepted: 20 September 2002 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/310-4412, E-mail: thomas.lippert@psi.ch