Synthetic studies towards furosesquiterpenoids: total synthesis of (±) desmethylpallescensin-A, (±) isopallescensin-A and (±) isopallescensin-1

A new approach for a short and efficient synthesis of common cyclohexenone intermediate towards the total synthesis of some furosesquiterpenes and their analogues are described. Regioselective alkylation of Hagemann's ester with 2/3-furyl-2-ethyl bromide followed by hydrolysis cum in situ decarboxylation and 1,4-addition with Gilman's reagent produced the cyclohexanone derivatives which have been utilized for total synthesis of (±) isopallescensin-A, (±) 10-desmethylpallescensin-A, (±) 5-desmethyl-4,5-dehydromicrocionin-1 and (±) isopallescensin-1.     

Sugar amino acids and related molecules: Some recent developments

To meet the growing demands for the development of new molecular entities for discovering new drugs and materials, organic chemists have started working on many new concepts that can help to assimilate knowledge-based structural diversities more efficiently than ever before. Emulating the basic principles followed by Nature to build its vast repertoire of biomolecules, organic chemists are developing many novel multifunctional building blocks and using them to create ‘nature-like’ and yet unnatural organic molecules. Sugar amino acids constitute an important class of such polyfunctional scaffolds where the carboxyl, amino and hydroxyl termini provide an excellent opportunity to organic chemists to create structural diversities akin to Nature’s molecular arsenal. In recent years, sugar amino acids have been used extensively in the area of peptidomimetic studies. Advances made in the area of combinatorial chemistry can provide the necessary technological support for rapid compilations of sugar amino acidbased libraries exploiting the diversities of their carbohydrate frameworks and well-developed solid-phase peptide synthesis methods. This perspective article chronicles some of the recent applications of various sugar amino acids, furan amino acids, pyrrole amino acids etc. and many other related building blocks in wide-ranging peptidomimetic studies     

Steady-state temperature distribution in a finite spherical cone

A Naylor transform is employed to determine the axisymmetric steady-state temperature distributions in finite spherical cones where the boundary conditions include a variety of conditions at the spherical surface (insulation or heat loss in accordance with Newton's law) and an assigned temperature variation on the conical surface (in particular,f(r)=r ^m ,m0). Solutions are obtained in terms of inversion integrals and in Fourier-Legendre expansions. Computations based on the inversion integrals are carried out for cones of half-angles, =/6 and /2, and with several combinations of parameters relating to the boundary conditions.

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Résumé Une transformation de Naylor est employée afin de déterminer les distributions axisymétriques stationnaires de la température dans les cônes sphériques finis où les conditions aux limites comprennent une variété des conditions à la surface sphérique (isolation ou perte de chaleur conformément à la loi de Newton) et une variation de la température assignée à la surface conique (en particulierf(r)=r ^m ,m0). Les solutions sont obtenues sous forme d'intégrales d'inversion et de développements en série de Fourier-Legendre. Les calculs fondés sur les intégrales d'inversion sont accomplis pour les cônes de demi-angles, -/6 et /2, et pour différentes combinaisons de paramètres qui apparaissent dans les conditions aux limites.

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Acknowledgement. This research was supported in part by Grant AFOSR-69-1779 at North Carolina State University at Raleigh, Raleigh, North Carolina. 27607.     

Nickel(II)-induced excimer formation of a naphthalene-based fluorescent probe for living cell imaging

Ni(2+)-induced intramolecular excimer formation of a naphthalene-based novel fluorescent probe, 1-[(naphthalen-3-yl)methylthio]-2-[(naphthalen-6-yl)methylthio]ethane (L), has been investigated for the first time and nicely demonstrated by excitation spectra, a fluorescence lifetime experiment, and (1)H NMR titration. The addition of Ni(2+) to a solution of L (DMSO:water = 1:1, v/v; λ(em) = 345 nm, λ(ex) = 280 nm) quenched its monomer emission, with subsequent enhancement of the excimer intensity (at 430 nm) with an isoemissive point at 381 nm. The fluorescence lifetime of free L (0.3912 ns) is much lower than that of the nickel(2+) complex (1.1329 ns). L could detect Ni(2+) as low as 1 × 10(-6) M with a fairly strong binding constant, 2.0 × 10(4) M(-1). Ni(2+)-contaminated living cells of plant origin could be imaged using a fluorescence microscope.     

Ionic liquid mediated 1,3-dipolar cycloaddition of azomethine ylides: a facile and green synthesis of novel dispiro heterocycles

A greener, facile and efficient one-pot, three-component procedure for the synthesis of novel dispiropyrrolidine-bisoxindole derivatives by cycloaddition trapping of azomethine ylides generated in situ, via decarboxylative condensation of isatin with sarcosine (α-amino acid), has been reported in [bmim]PF₆, an ionic liquid as a recyclable solvent in excellent yield without using any catalyst. This protocol provides mild reaction conditions, high yields of product in short reaction time, high regio- and stereoselectivity, operational simplicity and environmentally benign procedure.     

Do Carbenes Have a “Hidden” Carbon(0) Character? Revisiting the Electronic Structure of 2,2′‐Bipyridyl Carbene

Density functional calculations predict that 2,2′‐bipyridyl carbenes have some degree of “hidden” carbon(0) character. This is supported by very high second proton affinity values and bond dissociation energies (BDEs) for the dissociation of one AuCl and one Ni(CO)₂ molecule from [(AuCl)₂(2,2′‐bipyridyl carbene)] and [{Ni(CO)₂}₂(2,2′‐bipyridyl carbene)]. An Arduengo type carbene also showed significant BDE values for gem‐dimetallation, thus indicating the possible existence of carbon(0) character. All of the dimetallated derivatives showed metallophilic interactions.     

Effect of sol–gel-based silica buffer layer on the formation of NaA zeolite membrane

The support substrates were modified with sol?Cgel-based silica coating as intermediate buffer layer followed by NaA zeolite seed (prepared hydrothermally at 85?°C for 3?h) coating with 2?wt% aqueous dispersion. The secondary crystallization of NaA zeolite in the silica-seed-coated supports was carried out hydrothermally through crystal growth process at 65?°C for 4 and 8?h (single-stage each), (6?+?4) and (6?+?6)?h (double-stage each), and (6?+?4?+?2) and (6?+?4?+?4)?h (triple-stage each). The crystallization behaviours of the prepared NaA membranes were studied by X-ray diffraction, and the microstructures of the same films were observed by field emission scanning electron microscope. The efficiency of the membranes was tested with gas permeation study. The double-stage secondary crystallization at 65?°C for (6?+?4)?h showed highly interlocked NaA grains in the membrane and it rendered the permeance values of 3.7?×?10^?8?mol?m^?2?s^?1?Pa^?1 and 15.9?×?10^?8 for single gas, nitrogen (N₂) and hydrogen (H₂) respectively with the ideal selectivity of 4.3.     

Stabilization of cyclic and acyclic carbon(0) compounds by differential coordination of heterocyclic carbenes: a theoretical assessment

Recently, donor stabilized divalent carbon(0) compounds have undergone intense experimental and theoretical investigation due to their strong electron rich character. In this Article, some new cyclic and acyclic carbon(0) compounds stabilized by differential coordination modes (such as abnormal, remote and a mixture of both) of N-heterocyclic carbenes are studied theoretically. The cyclic carbon(0) compounds proposed in this study are unusual in the sense that they contain a five membered ring consisting of only carbon atoms with a central carbon atom in the formal oxidation state of zero. All these compounds are found to be very strong nucleophiles which might have wide implications in catalysis. Calculation of first proton affinities of these molecules reveal that they are better σ donors than the carbon(0) compound supported by normal N-heterocyclic carbenes. Quantum chemical calculations indicate that these molecules possess very high donor-acceptor L → C bond strengths and are thermodynamically stable. Calculation of the bond dissociation energies for the complexation of one and two molecules of AuCl indicates the possible isolation of their gem dimetalated derivatives.     

The Euler class group of a polynomial algebra with coefficients in a line bundle

Let $A$ be a commutative Noetherian ring and $P$ be a projective $A$ -module of rank $=(\text {dim}(A)-1)$ . An intriguing open question is to find the precise obstruction for $P$ to split as: $P\simeq Q\oplus A$ for some $A$ -module $Q$ . In this paper we settle this question when $A=R[T]$ for some ring $R$ containing the field of rationals and $P$ is a projective $A$ -module of rank $=\text {dim}(R)$ .