Ruthenium-catalyzed isomerization of oxa/azabicyclic alkenes: an expedient route for the synthesis of 1,2-naphthalene oxides and imines

1,2-Naphthalene oxides and imines can be rapidly accessed through a ruthenium-catalyzed isomerization of readily available 7-oxa/azabenzonorbornadienes. These mild reaction conditions were found to be tolerant to various functional groups and the isomerization is highly regioselective.     

Particle counting and numerical models: Effect of instrumental size resolution and particle shapes on optical cross-sections

The effect of instrumental size resolution measurements on numerical calculations of optical cross-sections is investigated. The particle counting instruments considered are a FSSP-300, a large scattering angle probe instrument similar to a ASASP-X, and, an aerodynamical system ELPI instrument. The scattering and hemispheric backscattering cross-sections, C_sca and C_bk, are calculated on the basis that the full width of the instrumental size bin should be considered in modeling. An average process is applied on these quantities over the full size bin of the instrument; they are then compared to their usual estimation on the single mean diameter D_p per channel. The effect of particle shape is investigated with ellipsoids and spheres. Results show sensitivity of the optical cross-sections to the shape of the particles as well as position of the mean geometrical diameter D_p of the channels within the interferences modes of the scattering efficiencies. The value of the width of the size bins, d log D, of each channel is crucial in the results. This comparison gives a useful estimation of error important in optical particle counting instruments based on inversion process of optical quantities. In addition, accuracy of size distribution measurements is found not to be representative of accuracy in the calculations of optical cross-sections.     

Planned and unplanned halogenations in route to selected oroidin alkaloids

Highly diastereoselective, substrate-controlled, halogenation/ring contraction sequences delivered the naturally occurring chlorinated and unnatural brominated and iodinated axinellamine core structure. An unexpected azide displacement of the chlorinated cyclopentane, which proceeded with retention of stereochemistry, suggested a modification of the Scheuer/Kinnel proposal that may account for the related natural product massadine. Two unsuccessful routes to access the stereochemistry proposed for palau'amine were S_N2 displacement of the bromo- and iodocyclopentane with excess chloride anion and an intramolecular directed chlorination pathway. Finally, an unexpected chlorination during our phakellstatin synthesis proceeded with retention of stereochemistry during tosylation possibly resulting from neighboring group participation.     

Surface Chemistry on Small Ruthenium Nanoparticles: Evidence for Site Selective Reactions and Influence of Ligands

The reactivity of two classes of ruthenium nanoparticles (Ru NPs) of small size, either sterically stabilized by a polymer (polyvinylpyrrolidone, PVP) or electronically stabilized by a ligand (bisdiphenylphosphinobutane, dppb) was tested towards standard reactions, namely CO oxidation, CO₂ reduction and styrene hydrogenation. The aim of the work was to identify the sites of reactivity on the nanoparticles and to study how the presence of ancillary ligands can influence the course of these catalytic reactions by using NMR and IR spectroscopies. It was found that CO oxidation proceeds at room temperature (RT) on Ru NPs but that the system deactivates rapidly in the absence of ligands because of the formation of RuO₂. In the presence of ligands, the reaction involves exclusively the bridging CO groups and no bulk oxidation is observed at RT under catalytic conditions. The reverse reaction, CO₂ reduction, is achieved at 120 °C in the presence of H₂ and leads to CO, which coordinates exclusively in a bridging mode, hence evidencing the competition between hydrides and CO for coordination on Ru NPs. The effect of ligands localized on the surface is also evidenced in catalytic reactions. Thus, styrene is slowly hydrogenated at RT by the two systems Ru/PVP and Ru/dppb, first into ethylbenzene and then into ethylcyclohexane. Selectively poisoning the nanoparticles with bridging CO groups leads to catalysts that are only able to reduce the vinyl group of styrene whereas a full poisoning with both terminal and bridging CO groups leads to inactive catalysts. These results are interpreted in terms of location of the ligands on the particles surface, and evidence site selectivity for both CO oxidation and arene hydrogenation.     

Spontaneous Biomimetic Formation of (±)‐Dictazole B under Irradiation with Artificial Sunlight

Guided by biosynthetic considerations, the total synthesis of dictazole B is reported for the first time. Experimental evidence for an easy access to challenging cyclobutane alkaloids of marine origin, which are often postulated to be biosynthetic precursors of more complex structures, is provided.     

A Strategy Utilizing a Recyclable Macrocycle Transporter for the Efficient Synthesis of a Triazolium‐Based [2]Rotaxane

A general synthesis of triazolium‐containing [2]rotaxanes, which could not be accessed by other methods, is reported. It is based on a sequential strategy starting from a well‐designed macrocycle transporter which contains a template for dibenzo‐24‐crown‐8 and a N‐hydroxysuccinimide (NHS) moiety. The sequence is: 1) synthesis by slippage of a [2]rotaxane building‐block; 2) its elongation at its NHS end; 3) the delivery of the macrocycle to the elongated part of the axle by an induced translational motion; 4) the contraction process to yield the targeted [2]rotaxane and recycle the initial transporter.     

Synthesis of Novel 2‐Amino‐(2‐thioxo‐3‐alkyl‐4‐methyl‐3H‐thiazol‐5‐yl)‐[1,3,4]thiadiazole Derivatives and Their Growth Stimulant Activity

On the base of synthesized 2‐amino and 2‐ethylamino‐(2‐thioxo‐3‐alkyl‐4‐methyl‐3H‐thiazol‐5‐yl)‐[1,3,4]thiadiazoles, their alkyl, acetyl, and alkylacetylamino derivatives are obtained. The alkylation of 2‐ethylamino derivatives can occur at both exo and endo nitrogen atoms of amidine group, and the acetylation takes place exclusively at the exocyclic nitrogen atom. At acetylation of 2‐amino‐[1,3,4]thiadiazoles, only exo substitution is observed. At the further alkylation of these products, a mixture of exo‐ and endo‐substituted forms is obtained. At preliminary screening, the synthesized compounds have shown expressed growth stimulant properties. The activity of the most active derivatives was in the range of 65–100%, compared with that of heteroauxin.