Synthesis and reactivity of cyclam-based enediynes

Cyclam-based enediynes 1-3 have been synthesized for the first time either by direct bis- or tetra-alkylation of the cyclam or via double alkylation of the 1,8-bis-sulfonyl derivative. The enediyne 1 readily forms a complex with Ni(II), which also lowered the onset temperature for Bergman cyclization of the parent enediyne by 60 °C. In the presence of a co-oxidant, MMPP, the Ni-complex can cleave ds-DNA into the nicked relaxed form at micromolar concentrations.     

ZrOCl2•8H2O-Catalyzed One-Pot Multicomponent Synthesis of β′-Acetamido-β-dicarbonyl Compounds with Special Reference to pref-Selective β′-Acetamido-β-ketoesters

ZrOCl₂ · 8H₂O catalyzes the one-pot synthesis of β′-acetamido-β-dicarbonyl compounds in high yields from aldehyde, enolizable β-dicarbonyl compound, acetyl chloride, and acetonitrile in solvent as well as in solvent-free conditions. β′-Acetamido-β-ketoesters are formed in moderate to exclusive pref-diastereoselectivity.     

Highly stereoselective synthesis of peracylated α-aldopyranosyl chlorides from aldopyranose peracetates and thionyl chloride catalyzed by BiCl3 generated in situ from the procatalyst BiOCl

Aldopyranose peracetates react with thionyl chloride and BiCl₃, generated in situ from a substoichiometric amount of the procatalyst BiOCl, producing the corresponding peracylated aldopyranosyl chlorides in very good to excellent yields (82–97%) with exclusive α-anomeric selectivity.     

Intramolecular fluorescence quenching in luminescent copolymers containing fluorenone and fluorene units: a direct measurement of intrachain exciton hopping rate

The quenching process of fluorescence emission in polyfluorene (PF) due to the presence of intramolecular 9-fluorenone (9 FL) moieties is studied in dilute toluene solution as a function of 9 FL content in eight copolymers containing both fluorene and fluorenone units (PF/FL(x)). The absorption spectrum of PF/FL(x) copolymers clearly shows a new absorption band, redshifted relatively to the PF and 9-fluorenone absorption, which increases in intensity when the fluorenone fraction increases and also decreases with solvent polarity. Fluorescence emission spectra of PF/FL(x) show that this redshifted and unstructured emission does not coincide with the 9-fluorenone emission and, with increasing solvent polarity, it further redshifts and decreases in intensity. An isoemissive point is clearly observed on the fluorescence emission spectra of PF/FL(x) as a function of fluorenone content, showing that the new emission band is formed at the expense of PF. We propose the formation of an intramolecular charge transfer complex (ICTC) between PF units and 9-fluorenone to explain the appearance of the new emission band. Global analysis of time resolved fluorescence decays collected at 415 nm (PF emission) and 580 nm (the ICTC emission) show that three exponentials are generally needed to achieve excellent fits. Two of the components (420 ps and 6.5 ns) are independent of 9-fluorenone fraction. A further fast component is strongly dependent on fluorenone fraction and ranges between 280 and 70 ps. This component appears as a decay time at 415 nm and as a rise time at 580 nm and is ascribed to the migration of exciton to quenching sites (formation of intramolecular CT complex or exciton ionization at CT complex). A kinetic mechanism involving three different kinetic species, quenched PF units kinetically coupled with the ICTC complex, and unquenched PF units is proposed to explain the experimental data and the quenching rate constant is obtained, k(1) congruent with 10(11) s(-1). This is an experimental measurement of the intrachain exciton hopping rate.     

Exploring DNA groove water dynamics through hydrogen bond lifetime and orientational relaxation

Dynamics of water molecules in the grooves of DNA are of great interest both for practical (functionality of DNA) and fundamental (as examples of confined systems) interest. Here the authors employ atomistic molecular dynamics simulations to understand varying water dynamics at the minor and the major grooves of a 38 base-pair long DNA duplex in water. In order to understand and quantify the diversity in the nature of hydrogen bond due to many hydrogen bond donors and acceptors present in the four bases, they have undertaken study of hydrogen bond lifetime (HBLT) correlation functions of all the specific hydrogen bonds between the base atoms and water molecules. They find that the HBLT correlation functions are in general multiexponential, with the average lifetime depending significantly on the specificity and may thus be biologically relevant. The average hydrogen bond lifetime is longer in the minor groove than that in the major groove by almost a factor of 2. Analysis further shows that water hydrogen bonds with phosphate oxygen have substantially shorter lifetimes than those with the groove atoms. They also compute two different orientational time correlation functions (OTCFs) of the water molecules present at the major and the minor grooves and attempt to correlate OTCF with HBLT correlation function. The OTCFs in the minor groove exhibit three time scales, with the time constant of the slowest component one to two orders of magnitude longer than what is observed for bulk water. A slow component is also present for the major groove water but with shorter time constant. Interestingly, correlation between reformations allowed HBLT correlation function [C(HB)(t)] and the OTCF markedly deviates from each other in the grooves, indicating enhanced rigidity of water molecules in the grooves.     

Vibration of rectangular plates with time-dependent boundary conditions

A general method of solution for the vibration of rectangular plates with any type of time-dependent boundary conditions is developed by an extension of the method of Mindlin and Goodman [1]. For illustration, the problems of a plate with different time-dependent boundary conditions are solved and the closed form solutions for the transverse deflections of the plate are obtained. The non-dimensionalized transverse deflections, ($\text{w}\text{a}$) at the middle of the plate are evaluated numerically for different dimensions of the plate and different forcing functions. These are presented graphically against the non-dimensionalized time, T, for three cases and tabulated for other cases.     

HSAB principle applied to the time evolution of chemical reactions

Time evolution of various reactivity parameters such as electronegativity, hardness, and polarizability associated with a collision process between a proton and an X- atom/ion (X = He, Li(+), Be(2+), B(3+), C(4+)) in its ground ((1)S) and excited((1)P,(1)D,(1)F) electronic states as well as various complexions of a two-state ensemble is studied using time-dependent and excited-state density functional theory. This collision process may be considered to be a model mimicking the actual chemical reaction between an X-atom/ion and a proton to give rise to an XH(+) molecule. A favorable dynamical process is characterized by maximum hardness and minimum polarizability values according to the dynamical variants of the principles of maximum hardness and minimum polarizability. An electronic excitation or an increase in the excited-state contribution in a two-state ensemble makes the system softer and more polarizable, and the proton, being a hard acid, gradually prefers less to interact with X as has been discerned through the drop in maximum hardness value and the increase in the minimum polarizability value when the actual chemical process occurs. Among the noble gas elements, Xe is the most reactive. During the reaction: H(2) + H(+) --> H(3)(+) hardness maximizes and polarizability minimizes and H(2) is more reactive in its excited state. Regioselectivity of proton attack in the O-site of CO is clearly delineated wherein HOC(+) may eventually rearrange itself to go to the thermodynamically more stable HCO(+).     

Cerium(IV) ammonium nitrate catalyzed synthesis of α-dehydro-β-amino esters

α-Dehydro-β-amino esters have been synthesized regioselectively from acetates of Baylis-Hillman adducts with amines in the presence of a catalytic amount of ceric ammonium nitrate (CAN) in good yield. The regioselectivity does not differ with respect to the polarity of the solvent.