Total Synthesis of Crenulatan Diterpenes: Strategy and stereocontrolled construction of a bicyclic keto-lactone building block

The bicyclic keto lactone 26 was synthesized for the purpose of developing a viable route to marine diterpenes of the crenulatan type. Following the efficient conversion of (S)-citronellol ( 5 ) to the allylated alcohol 9a (Scheme 2), the αβ-unsaturated lactone 12 was efficiently accessed in preparation for stereocontrolled conjugate addition. The hydroxymethyl equivalent most suited to this task was (i-PrO)Me₂SiCH₂MgCl, which gave 13 predominantly in the presence of CuI and Me₃SiCl. Once the OH group was deprotected (→ 14 ), it proved an easy matter to implement acid-catalyzed isomerization to lactone 15 , oxidation of which gave the pivotal aldehyde 16 . Condensation of 16 with PhSeCH₂Li led via 21 to 22 (Scheme 3). Once the OH group was protected (→ 22b ), it proved possible to effect aldolization with crotonaldehyde (→ 23 ). Exposure of 23 to acid gave the sub-target compound 25 . Its subsequent oxidation and thermal activation resulted in sequential selenoxide elimination with Claisen rearrangement (→ 26 ). The structural features of 26 require that a chair-like transition state be adopted during the [3.3]sigmatropic event. With the clarification of these issues, a highly serviceable and more advanced assault on the crenulatans should prove capable of being mounted.     

A new method for frequency calibration and control of a laser

A new method is introduced for high-precision measurement of the wavenumber of monochromatic radiation emitted by a single-mode tunable laser. The main task of the apparatus (called sigmameter) is the direct measurement of the wave-number. Furthermore it serves as a reference for the stabilization and piloting of the laser frequency.     

Quenching of the cathodoluminescence of some alkali halides as a function of concentration of colour centres at 90°K and 300°K

The study of the cathodoluminescence kinetics at 300°K and 90°K of some alkali halides (KCl, KBr, Kl) shows that, in its decreasing part, the emission intensity is described by an exponential law as a function of concentration of interstitial centres. This quenching process has its origin in the non-radiative recombination of electron-hole pairs on these defects created by the electronic irradiation.     

Detection and study of Rb I Rydberg states

We describe the details of an experiment using an atomic beam of rubidium which allowed us to detect by field ionization techniques the np Rydberg states from n = 28 up to n = 78, to detect also ns and nd states using a Stark mixing, and for all of these detected states to check the classical law E_c = [16n^*4]^?1 concerning the critical ionizing electric field E_c. It turns out that for n as high as 65 this law is quite well verified.     

A short and efficient synthesis of the NMDA glycine site antagonist: (3R,4R)-3-amino-1-hydroxy-4-methyl pyrrolidin-2-one (L-687,414)

A short and efficient synthesis of the NMDA glycine site antagonist: (3R,4R)-3-amino-1-hydroxy-4-methyl pyrrolidin-2-one (L-687,414) is described.     

Metal-Assisted and Microwave-Accelerated Evaporative Crystallization

We describe a platform technology, called metal-assisted and microwave-assisted evaporative crystallization (MA-MAEC), based on the combined use of silver nanoparticles and microwave heating for selective and rapid crystallization of small molecules. In this regard, the crystallization of a model small molecule (glycine) was achieved in several seconds. Glycine crystals grown on silver nanostructures with and without microwave heating were found to be larger than those grown on blank glass slides. The MA-MAEC technique has the potential to selectively grow the desired polymorphs of small molecules "on-demand" in a fraction of the time as compared to the conventional evaporative crystallization.     

Collective atomic spin squeezing and control

Using a quantum theory for an ensemble of two- or three-level atoms driven by electromagnetic fields in an optical cavity, we show that the various spins associated with the atomic ensemble can be squeezed. Two kinds of squeezing are obtained: on the one hand self-spin squeezing when the input fields are coherent ones and the atomic ensemble exhibits a large non-linearity; on the other hand squeezing transfer when one of the incoming fields is squeezed. Received 14 August 2001 and Received in final form 7 November 2001     

Doppler-free spectroscopy of neon atoms dressed by optical photons in nearly-degenerate four-wave mixing

We have investigated nearly degenerate four-wave mixing in neon on the 607 nm line. We have considered the case where one pump beam saturates the medium while the two other incident beams have a much weaker intensity. The frequency of the most intense pump beam is equal to the frequency of the probe beam. When the frequency of the second pump beam is scanned, narrow resonances are observed. These resonances correspond to transitions between the energy levels of the atom dressed by the photons of the intense pump beam.     

Nuclear moments and charge radius changes in platinum isotopes

Isotope shift (IS) and hyperfine structure (HFS) measurements have been performed for^{185, 187, 189, 191, 195}Pt using the PILIS (Post ISOCELE Laser Isobar Separator) apparatus installed at the ISOCELE facility. Magnetic and quadrupole moments have been deduced from the HFS results. The charge radius changes determined for these odd nuclei from the IS results, added to the σ<r²> values of the even-A nuclei are compared to the results of lattice Hartree-Fock+BCS calculations for asymmetric solutions:¹⁸⁵Pt is confirmed to be prolate shaped whereas^{187, 189, 191}Pt are likely triaxial in their ground states.