Regioselective formation of alkylidenecyclopropanes from 2-substituted cyclobutylidenes generated from geminal dibromocyclobutanes and methyllithium

Four different 2-substituted geminal dibromocyclobutanes were reacted with methyllithium at -78 degrees C. In contrast to previous studies using diazocyclobutanes as carbene precursors at temperatures above 200 degrees C via reaction of the corresponding tosylhydrazone sodium salts, the organometallic route in each case produces only an alkylidenecyclopropane that could be isolated in good yields. B3LYP calculations were employed to rationalize the observed regioselective ring contraction of the generated cyclobutyliden(oid)s. [reaction: see text]     

Kennzeichnungen von Sphären

Ohne Zusammenfassung     

Cervimycin A-D: a polyketide glycoside complex from a cave bacterium can defeat vancomycin resistance

Cervimycins A-D are novel polyketide glycosides with significant activity against multi-drug-resistant staphylococci and vancomycin-resistant enterococci. They are produced by a strain of Streptomyces tendae, isolated from an ancient cave. The structures of the cervimycins were determined by performing extensive NMR and chemical degradation studies. All cervimycins have a common tetracyclic polyketide core that is substituted with unusual di- and tetrasaccharide chains, composed exclusively of trideoxysugars; however, they differ in the acetyl and carbamoyl ring substituent and in the highly unusual terminal methylmalonyl and dimethylmalonyl residues.     

Refinement of Convergence Rates for Tail Probabilities

Let X₁, X₂,... be, i.i.d. random variables, and put . We find necessary and sufficient moment conditions for , where δ≥ 0 and q>0, and with a_n>0 and b_n is either or The series f(x) we deal with are classical series studied by Hsu and Robbins, Erdős, Spitzer, Baum and Katz, Davis, Lai, Gut, etc     

Nearly quantum-noise-limited timing jitter from miniature Er:Yb:glass lasers

We report on nearly quantum-limited timing-jitter performance of two passively mode-locked Er:Yb:glass lasers with a repetition rate of 10 GHz. The relative timing jitter of both lasers was measured to be 190 fs (100 Hz-1.56 MHz) root mean square. The remaining cavity-length fluctuations are below 7.5 pm in the 6 Hz-8 kHz frequency range, indicating the stability of a rugged miniature cavity setup. By actively controlling the cavity length we reduced the timing jitter to 26 fs (6 Hz-1.56 MHz). We also discuss the influence of cavity length on the practically achievable timing jitter.     

Semisynthetic preparation of leucomycin derivatives: Introduction of aromatic side chains by reductive amination

A small library of leucomycin A7 derivatives was prepared by NaCNBH₃/ZnCl₂-mediated reductive amination of the C18 aldehyde moiety with a variety of lipophilic benzylamines and tested for antibiotic activity.     

Large Kerr effect in bulk Se-based chalcogenide glasses

High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ~1-pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.     

16.2-W average power from a diode-pumped femtosecond Yb:YAG thin disk laser

We demonstrate a power-scalable concept for high-power all-solid-state femtosecond lasers, based on passive mode locking of Yb:YAG thin disk lasers with semiconductor saturable-absorber mirrors. We obtained 16.2 W of average output power in pulses with 730-fs duration, 0.47-muJ pulse energy, and 560-kW peak power. This is to our knowledge the highest average power reported for a laser oscillator in the subpicosecond regime. Single-pass frequency doubling through a 5-mm-long lithium triborate crystal (LBO) yields 8-W average output power of 515-nm radiation.