Asymmetric synthesis of the C(1)---C(47) backbone of antitumour antibiotic A83586C

Protected pentapeptide 9 has been converted to hexapeptide 3 and this chemoselectively coupled to activated ester 2 to give 1 after glycal hydration during SiO₂ flash chromatography.     

Periodic solutions of singularly perturbed delay equations

We consider a general class of singularly perturbed delay differential systems depending on a singular parameter and another parameter . For =0, the equation defines a mapT which undergoes a generic period doubling at =0. If the bifurcation is supercritical (subcritical), these period two points define a stable period two square wave (unstable period two pulse wave). We give conditions on the vector field such that there is a sectorS in the , plane such that there is a unique periodic orbit if the parameters are inS, the orbit is stable (unstable) if the period doubling bifurcation is supercritical (subcritical) and approaches the square (pulse) wave as 0.Partially supported by NSF and DARPA.     

Mechanistic studies on the O-directed free-radical hydrostannation of disubstituted acetylenes with Ph3SnH and Et3B and on the iodination of allylically oxygenated alpha-triphenylstannylalkenes

[reaction: see text] The free-radical hydrostannation of 1 with Ph(3)SnH and catalytic Et(3)B in PhMe has been mechanistically probed. At high Ph(3)SnH concentrations, the O-directed hydrostannation pathway dominates, and 2 is formed with good selectivity (ca. 11.1:1). Substantially lower stannane/substrate concentrations increase the amount of tandem 5-exo-trig cyclization product 3 that is observed.     

Lower semicontinuity of attractors of gradient systems and applications

Summary For 0₀, let T(t), t0, be a family of semigroups on a Banach space X with local attractors A. Under the assumptions that T₀(t) is a gradient system with hyperbolic equilibria and T(t) converges to T₀(t) in an appropriate sense, it is shown that the attractors {A, 0₀} are lower-semicontinuous at zero. Applications are given to ordinary and functional differential equations, parabolic partial differential equations and their space and time discretizations. We also give an estimate of the Hausdorff distance between A and A₀, in some examples.Research supported by U.S. Army Research Office DAAL-03-86-K-0074 and the National Science Foundation DMS-8507056.     

Symbolic dynamics and nonlinear semiflows

Summary For a transverse homoclinic orbit of a mapping (not necessarily invertible) on a Banach space, it is shown that the mapping restricted to orbits near is equivalent to the shift automorphism on doubly infinite sequences on finitely many symbols. Implications of this result for the Poincaré map of semiflows are given.This work was supported by the Air Force Office of Scientific Research under Grant #81-0198, by the National Science Foundation under Grant #MCS-8205355 and by the Army Research Office under Grant ù DAAG-29-83-K-0029.     

Electron impact studies. CV—negative ion mass spectra of functional groups. The trimethylsilyl group. Singly charghed and doubly charged ions

The 70 eV negative ion spectra of m- and p-nitrophenyltrimethylsilyl-ethers and -esters show pronounced molecular anions and simple fragmentation patterns. The spectra of the corresponding ortho isomers show peaks produced by rearrangement processes; these reactions have been studied by ²H and ¹⁸O labelling. p-Nitrobenzoyl derivatives of amino trimethylsilyl esters are of limited use for characterization of amino acids. Certain of the compounds studied produce a variety of dianions.     

A one-pot synthesis of 3-substituted-5-carbonylmethyl-1,2,4-oxadiazoles from β-keto esters and amidoximes under solvent-free conditions

Herein we report a high yielding one-pot ‘green’ synthesis of 3-substituted-5-carbonylmethyl-1,2,4-oxadiazoles from readily available β-keto esters and amidoximes under simple and convenient solvent-free conditions. No additional base is needed. The reaction likely goes through an acyl ketene intermediate.     

Slug initiation and evolution in two-phase horizontal flow

A series of two-phase air–water experiments was carried out in order to study the initiation and the subsequent evolution of hydrodynamic slugs in a horizontal pipeline. Experiments were carried out at atmospheric pressure, 4.0 bar(a) and 9.0 bar(a), and the effects of superficial liquid and gas velocities were investigated. The test section used for these experiments is 37 m in length, with an internal diameter of 0.078 m. To study the interfacial development, measurements of interfacial structures were made at 14 axial locations along the test section, with data acquired at a sampling frequency of 500 Hz. A large number of slugs were initiated within the first 3 m of the test section, with the frequency subsequently reducing towards the fully developed value before the end of the pipe. This reduction in frequency was strongly influenced by the magnitude of the gas and liquid velocities. The frequency of slugging was not strongly affected when the system pressure was changed from 1 atmosphere, to 4.0 and 9.0 bar(a), closely similar values being obtained at the 10 downstream locations. However, higher pressure delayed the onset of slug initiation, with “slug precursors” being formed further downstream as the pressure was increased. The statistical distributions of slug lengths and of the time intervals between slug arrivals were examined in detail and compared to several standard distributions. This showed that slug initiation may be reasonably approximated as an uncorrelated Poisson process with an exponential distribution of arrival times. However, once slugs have developed, there is strong correlation and the arrival time intervals, as well as the lengths, are best represented by the log-normal distribution.