Forty years of hydride reductions

Forty years ago, it was observed that diborane, a hydride, reduces aldehydes and ketones with exceptional ease even at 0°. Since then various hydride reagents have evolved for the convenient reduction of typical organic functional groups. Electrophilic reducing agents such as borane and alane possess markedly different reducing characteristics than those of nucleophilic reducing agents such as sodium borohydride and lithium aluminum hydride. Explorations have revealed means of both enhancing and diminishing the electrophilic character of borane and alane on the one hand and both enhancing and diminishing the nucleophilic character of borohydrides and aluminohydrides on the other. Such derivatives reveal unique characteristics making possible valuable selective reductions, not practical previously. These developments have completely altered the procedures normally used in organic chemistry for the reduction of functional groups.     

Finite segmentp-adic number systems with applications to exact computation

Proceedings - Mathematical Sciences - A fractional weighted number system, based on Hensel’sp-adic number system, is proposed for constructing a unique code (called Hensel’s code) for...     

Theory of insulator metal transition and colossal magnetoresistance in doped manganites

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.     

Compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) in multiplicity-edited HSQC

Use of adiabatic pulses in broadband inversion and decoupling is well known. Replacement of the rectangular pi pulses in the INEPT and rev-INEPT parts of the HSQC and gHSQC experiments with adiabatic pulses substantially improves the sensitivity of these experiments. However, modulation of cross peak intensity in multiplicity-edited HSQC or gHSQC experiments can be quite severe. These modulations arise during the multiplicity-editing periods due to the inefficient refocusing of the spin-echo caused by the mismatch of the echo delay with the one-bond coupling constant. These modulations (which we call echo modulations) are field strength (and hence spectral width) independent. Use of adiabatic pulses with the inversion sweep synchronized to the 1H-13C coupling constant range typically observed in a 13C spectrum will provide substantial improvement in sensitivity. The inversion profile problems associated with rectangular pi pulses can be moderately compensated by composite pulse schemes and these schemes could prove to be reasonable alternatives to adiabatic pulses. However, the adiabatic sweep provides a unique method to compensate the echo modulations for multiplicity-edited experiments. The origin and the compensation of refocusing inefficiency with synchronized inversion sweep (CRISIS) method to minimize these modulations is described.     

2J,(3)J-HMBC: A new long-range heteronuclear shift correlation technique capable of differentiating (2)J(CH) from (3)J(CH) correlations to protonated carbons

The development of a series of new, accordion-optimized long-range heteronuclear shift correlation techniques has been reported. A further derivative of the constant time variable delay introduced in the IMPEACH-MBC experiment, a STAR (Selectively Tailored Accordion F(1) Refocusing) operator is described in the present report. Incorporation of the STAR operator with the capability of user-selected homonuclear modulation scaling as in the CIGAR-HMBC experiment, into a long-range heteronuclear shift correlation pulse sequence, (2)J,(3)J-HMBC, affords for the first time in a proton-detected experiment the means of unequivocally differentiating two-bond ((2)J(CH)) from three-bond ((3)J(CH)) long-range correlations to protonated carbons. Copyright 2000 Academic Press.     

Development and validation of a HPLC method for quantification of rivastigmine in rat urine and identification of a novel metabolite in urine by LC‐MS/MS

A sensitive, specific and accurate HPLC method for the quantification of rivastigmine (RSM) in rat urine was developed and validated. The method involves the simple liquid–liquid extraction of RSM and pyridostigmine as an internal standard (IS) from rat urine with tertiary methyl butyl ether. The chromatographic separation of RSM and IS was achieved with 20 mm ammonium acetate buffer (pH 6.5) and acetonitrile (65:35, v/v) delivered at flow‐rate of 1 mL/min on a Kromasil KR‐100. The method was in linear range from 50 to 5000 ng/mL. The validation was done as per FDA guidelines and the results met the acceptance criteria. The method was successfully applied for the quantification of RSM in rat urine. Besides method validation, we have identified two metabolites of RSM in urine. Both the metabolites were characterized by HPLC‐PDA and LC‐MS/MS and it was found that one metabolite is novel. Copyright © 2010 John Wiley & Sons, Ltd.     

Self-quenched dynamics

We introduce a model for the slow relaxation of an energy landscape caused by its local interaction with a random walker whose motion is dictated by the landscape itself. By choosing relevant measures of time and potential this self-quenched dynamics can be mapped on to the “True” Self-Avoiding Walk model. This correspondence reveals that the average distance of the walker at time t from its starting point is , where for one dimension and 1/2 for all higher dimensions. Furthermore, the evolution of the landscape is similar to that in growth models with extremal dynamics. Received 8 August 2000