Structure and synthesis of a unique isonitrile lipid isolated from the marine mollusk Actinocyclus papillatus

The first chemical study of an Actinocyclidae nudibranch, Actinocyclus papillatus, resulted in the isolation of (-)-actisonitrile (1), a lipid based on a 1,3-propanediol ether skeleton. The structure was established by spectroscopic methods, whereas the absolute configuration of the chiral center was determined by comparing the optical properties of natural actisonitrile with those of (+)- and (-)-synthetic enantiomers, opportunely prepared. Both (-)- and (+)-actisonitrile were tested in preliminary in vitro cytotoxicity bioassays on tumor and nontumor mammalian cells.     

Iridium‐Catalyzed β‐Alkynylation of Aliphatic Oximes as Masked Carbonyl Compounds and Alcohols

An Ir‐catalyzed C(sp³)?H alkynylation of aliphatic ketones, aldehydes, and alcohols was achieved by using the corresponding oxime derivatives and a Ir^III catalyst. This general reaction is selective towards primary C(sp³)?H bonds and can be used for the late‐stage C?H alkynylation of complex molecules.     

Silica-Supported Cationic Tungsten Imido Alkylidene Stabilized by an N-Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α-Olefins

A well-defined silica-supported cationic W imido alkylidene was prepared through surface organometallic chemistry. This catalyst shows preferential activity towards α- over internal olefins, which is atypical for W-based catalysts, but consistent with the strong σ-donating ability of the NHC ancillary ligand. Complementing the studies on tungsten-based d⁰ metathesis catalysts, the silica-supported cationic W imido alkylidene displays the highest activity among W imido catalysts for α-olefins and shows improved selectivity for this class of olefins compared to Mo-based catalysts.     

Iridium-Catalyzed β-Alkynylation of Aliphatic Oximes as Masked Carbonyl Compounds and Alcohols

An Ir-catalyzed C(sp³)−H alkynylation of aliphatic ketones, aldehydes, and alcohols was achieved by using the corresponding oxime derivatives and a Ir^III catalyst. This general reaction is selective towards primary C(sp³)−H bonds and can be used for the late-stage C−H alkynylation of complex molecules.     

Singlet Heterofission in Tetracene–Pentacene Thin-Film Blends

Heterofission is a photophysical process of fundamental and applied interest whereby an excited singlet state is converted into two triplets on chemically distinct chromophores. The potential of this process lies in the tuning of both the optical band gap and the splitting between singlet and triplet energies. Herein, we report the time-domain observation of heterofission in mixed thin films of the prototypical singlet fission chromophores pentacene and tetracene using excitation wavelengths above and below the tetracene band gap. We found a time constant of 26 ps for endothermic heterofission of a singlet exciton on pentacene in blends with low pentacene fractions, which was outcompeted by pentacene homofission for increasing pentacene concentrations. Direct excitation of tetracene lead to fast energy transfer to pentacene and subsequent singlet fission, which prevented homo- or heterofission of a singlet exciton on tetracene.     

An Interplay Between Infrared Multiphoton Dissociation Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry and Density Functional Theory Computations in the Characterization of a Tripodal Quinolin-8-Olate Gd(III) Complex

A new hexadentate, tripodal 8-hydroxyquinoline based ligand (QH₃) and its gadolinium(III) tris-chelated (GdQ) complex with hemicage structure was investigated by using high resolution Fourier-transform ion cyclotron resonance mass spectrometry (FTICRMS). The protonated adduct of the free ligand and its hemicage tripodal Gd(III) complex, [GdQ + H]⁺, were first observed in experiments of electrospray ionization (ESI) with a linear ion trap (LTQ) mass spectrometer and further investigated by using high resolution FTICRMS. Gas-phase dissociation of the protonated Gd(III) complex, by infrared multiphoton dissociation (IRMPD) FTICR MS, demonstrated a fragmentation pattern with six main product cluster ions labeled as [F_n]⁺ (n?=?1 up to 6). These product ions suggest the elimination of 7-amino-alkyl or 7-alkyl chains of the hemicage moiety. High resolution MS conditions allowed the elucidation of the fragmentation pattern and product ion structures along with the determination, among the isotopic pattern of Gd, of the chemical compositions of closely related species, which differ in terms of hydrogen content. Among the Gd six naturally stable isotopes, ¹⁵⁸Gd is the most abundant, and its peak within each cluster was used as a reference for distinguishing each product ions. Computational DFT investigations were applied to give support to some hypothesis of fragmentation pathways, which could not have been easily justified on the basis of the experimental work. Furthermore, computational studies suggested the coordination geometry of the protonated parent complex and the five- and four-coordinated complexes, which derive from its fragmentation. Furthermore, experimental and computational evidences were collected about the octet spin state of the parent compound.      

Tuning the Fluorescence Through Reorientation of the Axle in Calix[6]arene-Based Pseudorotaxanes

This work describes a calix[6]arene-based wheel that binds, in non-polar media, a stilbazolium salt to yield a mixture of pseudorotaxane orientational isomers. The isomer's abundance ratio evolves with time and can be reversibly tuned by adjusting the temperature. The spectroscopic properties, and notably the emission spectrum, of the bound guest depend on its orientation inside the non-palindromic wheel, suggesting such a system as a switch with spectroscopic readout.     

Multiplicity of homoclinics for a class of time recurrent second order Hamiltonian systems

We prove the existence of infinitely many homoclinic solutions for a class of second order Hamiltonian systems in of the form , where we assume the existence of a sequence such that and as for any . Moreover, under a suitable non degeneracy condition, we prove that this class of systems admits multibump solutions. Received February 2, 1996 / In revised form July 5, 1996 / Accepted October 10, 1996     

New diastereomeric bis-sesquiterpenes from Hainan marine sponges Axinyssa variabilis and Lipastrotethya ana

Three unprecedented diastereoisomeric dimers, cis-dimer A (1), cis-dimer B (2) and trans-dimer C (3), exhibiting a bis-bisabolene skeleton, and a new sesquiterpene, dehydrotheonelline (4), their potential precursor, have been isolated from the South China Sea sponges Axinyssa variabilis and Lipastrotethya ana, along with known related sesquiterpenes. The structure of the novel molecules has been determined by extensive NMR spectral analysis.     

On a Sharp Sobolev‐Type Inequality on Two-Dimensional Compact Manifolds

Motivated by the asymptotic analysis of double vortex condensates in the Chern‐Simons‐Higgs theory, we construct a suitable minimizing sequence for a sharp Sobolev inequality “à la Moser” for two‐dimensional compact manifolds. As a consequence, we first obtain a direct proof of the sharp character of such an inequality. Secondly, and more interestingly, we use such minimizing sequence to show that for the flat torus the corresponding extremal problem attains its infimum. (Accepted April 6, 1998)