Rearrangement of 4,5-epoxy-9-trimethylsilyldecalines. Application to the synthesis of the natural eremophilane (-)-aristolochene

Several 4,5-epoxy-9-trimethylsilyl-eudesmanes and 15-nor-eudesmanes, having different relative stereochemistry and substitution at the oxirane ring, have been prepared starting from (-)-carvone and subjected to acid-promoted rearrangement. The presence of the silicon at C9 favors two different main reaction pathways involving C14-methyl or C1-methylene migration through the stabilization of a C10 carbocation intermediate. Selective 1,2-migration of the bridgehead methyl group takes place with trisubstituted beta-epoxide and tetrasubstituted alpha-epoxide, yielding 4-hydroxy-eremophilane and 15-nor-eremophilane compounds, while the trisubstituted alpha-epoxide suffers successive rearrangements to give a 1-hydroxy-15-nor-eremophilane through a pathway involving the initial migration of the C1 methylene. The synthetic utility of these rearrangements is shown by the synthesis of natural (-)-aristolochene.     

Large yet Flexible N‐Heterocyclic Carbene Ligands for Palladium Catalysis

A straightforward and scalable eight‐step synthesis of new N‐heterocyclic carbenes (NHCs) has been developed from inexpensive and readily available 2‐nitro‐m‐xylene. This process allows for the preparation of a novel class of NHCs coined ITent (“Tent” for “tentacular”) of which the well‐known IMes (N,N′‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene), IPr (N,N′‐bis(2,6‐di(2‐propyl)phenyl)imidazol‐2‐ylidene) and IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene) NHCs are the simplest and already known congeners. The synthetic route was successfully used for the preparation of three members of the ITent family: IPent (N,N′‐bis(2,6‐di(3‐pentyl)phenyl)imidazol‐2‐ylidene), IHept (N,N′‐bis(2,6‐di(4‐heptyl)phenyl)imidazol‐2‐ylidene) and INon (N,N′‐bis(2,6‐di(5‐nonyl)phenyl)imidazol‐2‐ylidene). The electronic and steric properties of each NHC were studied through the preparation of both nickel and palladium complexes. Finally the effect of these new ITent ligands in Pd‐catalyzed Suzuki–Miyaura and Buchwald–Hartwig cross‐couplings was investigated.     

Solvothermal synthesis of nitrides

Abstract

III-V nitrides are large band-gap semiconductors. They are very promising materials for opto-electronic and microelectronic applications.

The present paper deals with a new process for preparing nitrides. Two nitrides, gallium nitride (GaN) and aluminium nitride (AlN), have been synthesiaed through a solvothermal route. This process uses ammonia in supercritical conditions as solvent and metals as source of gallium and aluminium. Moreover, an additive is used to improve the nitridation of the metals.

The resulting microcrystallites of nitrides have been charactensed by X-ray diffraction and the morphology has been observed by scanning Electron Microscopy.     

A novel discriminant approximation of periodic differential equations

A new approximation of the discriminant of a second order periodic differential equation is presented as a recursive summation of the evaluation of its excitation function at different values of time. The new approximation is obtained, at first, by means of Walsh functions and then, by using some algebraic properties the dependence on the Walsh functions is eliminated. This new approximation is then used to calculate the boundaries of stability. We prove that by letting the summation elements number approach to infinite, the discriminant approximation can be rewritten as a summation of definite integrals. Finally we prove that the definite integrals summation is equivalent to the discriminant approximation made by A. Lyapunov which consists in an alternating series of coefficients defined by multiple definite integrals, that is, a series of the form $A=A_0?A_1+\dots +{(?1)}^n{A}_n$, where each coefficient $A_n$ is defined as an n-multiple definite integral.     

Nickel-Catalysed Cross-Electrophile Coupling of Benzyl Bromides and Sulfonium Salts towards the Synthesis of Dihydrostilbenes

A nickel-catalysed reductive cross-coupling reaction between benzyl sulfonium salts and benzyl bromides is reported. Simple, stable and readily available sulfonium salts have shown their ability as leaving groups in cross-electrophile coupling, allowing the formation of challenging sp³–sp³ carbon-carbon bonds, towards the synthesis of interesting dihydrostilbene derivatives. In addition, benzyl tosyl derivatives have been demonstrated to be suitable substrates for reductive cross-coupling by in-situ formation of the corresponding sulfonium salt.     

Enantioselective, chemoenzymatic synthesis, and absolute configuration of the antioxidant (−)-gloeosporiol

The natural antioxidant (−)-gloeosporiol, isolated as a peracetylated derivative from a culture of the fungus Colletotrichum gloeosporioides, has been enantioselectively prepared from 3,4-dihydroxybenzaldehyde by means of a chemoenzymatic synthesis. The key intermediate was obtained by resolution with a lipase from Pseudomonas cepacia. Its stereochemistry, initially assigned as R, according to the Kazlaukas empirical rule for secondary alcohols, was independently confirmed by NMR and chirooptic methods. This, in turn, allowed the assignment of compound (−)-1 as (−)-(2S,3R,4R)-2-(3′,4′dihydroxyphenyl)tetrahydrofuran-3,4-diol.     

Metabolites from Eutypa species that are pathogens on grapes

Structural and synthetic studies of the metabolites isolated from Eutypa lata are reviewed. This fungus is the causative agent of Eutypa dieback disease, also known as eutyposis or dying-arm disease, a perennial canker that affects grapevines and many other woody fruit plants. The review, which encompasses all the literature in this field up to the present and in which 76 references are cited, also includes a detailed study of the biological activity of the metabolites, especially the role of toxins in the development of the plant disease. Some aspects of the synthesis and biosynthesis of these metabolites and related compounds are discussed.     

Kinetic study of the oxidation of indigocarmine by hydrogen peroxide with ferrous and ferric ions in acid medium

The use of hydrogen peroxide in addition to ferrous and ferric salts has been shown an effective method in the decoloration of indigocarmine (IDS), Indigo-5,5′-disulfonic acid disodium salt. The kinetic experiments control have been carried out by means of a conventional spectrophotometric technique. In acid medium and the initial absence of Fe⁺², the decoloration process obeys the following rate law: Effects of reagents concentrations, temperature, and acidity on the rate experimental constant have been discussed. A feasible reaction mechanism is finally proposed. The rate equation deducted from the mechanism is according to the achieved experimental results. © 1997 John Wiley & Sons, Inc.