Studies directed towards the total synthesis of lycoperdinosides: stereoselective construction of the C1-C9 and C10-C21 segments of the molecules

The three chiral centres of the C1-C9 moiety of the six-membered lactone glycosides, lycoperdinosides A and B, have been derived from a common starting material containing a single chiral centre. In contrast, the C10-C21 segment of these molecules has been synthesized using, as key steps, a highly stereoselective aldol reaction, a Ti(III)-mediated opening of a trisubstituted epoxy alcohol and an efficient directed hydrostannylation of a suitably substituted internal alkyne.     

Stereoselective synthesis of conformationally constrained omega-amino acid analogues from pyroglutamic acid

Bicyclic lactams derived from pyroglutamic acid provide a useful scaffold for synthesis of conformationally restricted analogues of lysine, ornithine and glutamine, as well as an Ala-Ala dipeptide analogue. Amino alcohol and carboxylic acid derivatives are accessible from a common intermediate. In this strategy, the bicyclic lactam system not only controls, but also facilitates the determination of the stereochemistry of the synthetic intermediates.     

Synthesis and biological activity of bridgehead nitrogen heterocycles

The reaction of 3-(2,4-dichlorophenyl)-5-mercapto 1,2,4–1H-triazole with α-haloketones and with 1,2-dibromoethane leading to the formation of fused heterocycles were carried out and the orientation of cyclization was studied. The reaction of 3-(2,4-dichlorophenyl)-5-mercapto-4-amino-1,2,4-s-triazole with α-haloketones     

A proposed k0 based methodology for neutron activation analysis of samples of non-standard geometry

An internal mono-standard method has been proposed for multi element analysis. This method gives the relative concentration of the elements in a sample of non-standard shape and size. It utilizes an in-situ relative efficiency calibration and hence, does not need the cumbersome procedures, otherwise required to correct for -attenuation in the sample. To validate this method, the relative concentration of elements in IAEA RM's SL-3 and Soil-7 were analyzed with sample amounts ranging from a few milligrams to grams. The samples were counted in different non-specific geometries. The results are in good agreement with the recommended values, suggesting that this methodology could be applied for the analysis of samples of non-standard size and shape, and in principle, for the analysis of large samples.     

Synthesis of (3R,4S,5S,9S)-3,5,9-trihydroxy-4-methylundecanoic acid δ-lactone

Radical-mediated opening of a chiral trisubstituted epoxy alcohol using cp₂TiCl furnished the ‘2-methyl-1,3-diol’ moiety with the desired stereochemistry, which led to a total synthesis of (3R,4S,5S,9S)-3,5,9-trihydroxy-4-methylundecanoic acid δ-lactone 1.     

Kinetic and mechanistic studies on the interaction between glycylglycine and <Emphasis Type="Italic">cis</Emphasis>-diaqua-<Emphasis Type="Italic">bis</Emphasis>-(bipyridine)-ruthenium(II) complex in aqueous medium

The kinetics of interaction between glycylglycine and cis-[Ru(bipy)₂(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Ru(bipy)₂(H₂O) ₂ ²⁺ ], [diglycine] and temperature at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and diglycine as the zwitter ion. The reaction has been found to proceed via two consecutive steps. The first step involves the ligand-assisted anation, while the second step involves chelation when the second aqua ligand is displaced. Rate constants have been evaluated and activation parameters calculated. The low enthalpy of activation and large negative values of entropy of activation indicate an associative mode of activation for both steps.     

Computing the maximum clique in the visibility graph of a simple polygon

In this paper, we present an algorithm for computing the maximum clique in the visibility graph G of a simple polygon P in O(n²e) time, where n and e are number of vertices and edges of G respectively. We also present an O(ne) time algorithm for computing the maximum hidden vertex set in the visibility graph G of a convex fan P. We assume in both algorithms that the Hamiltonian cycle in G that corresponds to the boundary of P is given as an input along with G.     

Temperature dependent positron annihilation studies in Nafion-117 polymer

We have measured the S-parameter and life-time as a function of temperature from 15 to 300 K in acid Nafion-117. In addition, high temperature measurements from 300 to 550 K have been carried out in acid as well as Cs neutralised Nafion membrane. The results are discussed.     

Partially observed semi-Markov zero-sum games with average payoff

We study a zero-sum partially observed semi-Markov game with average payoff on a countable state space. Under certain ergodicity conditions we show that a saddle point equilibrium exists. We achieve this by solving the corresponding average cost optimality equation using a span contraction method. The average value is shown to be the unique zero of a Lipschitz continuous function. A value iteration scheme is developed to compute the value.     

Effect of sodium hydroxide pre-treatment on the optical and structural properties of lyocell

Lyocell is a type of regenerated cellulose. Fibres spun from cellulose solution in N-methylmorpholine-N-oxide hydrate consist of crystalline cellulose II and amorphous cellulose. Lyocell fabrics were treated with aqueous sodium hydroxide solution (NaOH) to study the influence of alkali on optical and structural properties. It was observed that sodium hydroxide treatment causes the density, orientation and crystallinity of lyocell fibre to decrease with increasing sodium hydroxide concentration, a corresponding decrease in tensile strength is also observed. The greatest change in fibre properties occurs between 3.0 and 5.0 mol dm⁻³ NaOH. This is attributed to the onset of formation of Na-cellulose II at 3.0 mol dm⁻³ NaOH; a fully formed Na-cellulose II structure is expected above 6.8 mol dm⁻³ NaOH. Formation of Na-cellulose II causes plasticization of the lyocell fibres as both inter- and intra-molecular hydrogen bonds are broken by these higher sodium hydroxide concentrations.