Regio and stereoselectivity in ionic cycloadditions

Acridizinium cation (ACR) and 2,3-dimethylisoquinolinium ion (DMIQ) undergo [2 + 4⁺] addition with dienophiles methyl vinyl ether (MVE) and propylene (PY) and the addition takes place across the diene unit containing the cationic centre and the dienophile acts as the electron donor. These reactions have two regiochemical paths and in each of them two possible stereo adducts could be formed. DFT calculations performed at B3LYP/6-31G(d) level have shown that the reactions pass through concerted mechanism and the TSs are highly asynchronous. Methoxy group in the dienophile can take up cis or trans orientation with respect to the double bond and in that trans orientation of the methoxy group is preferred. Calculations further show that syn 2 adduct is kinetically and thermodynamically more favoured in both the reactions in excellent agreement with the experimental observations. ACR is found to be more reactive than DMIQ as a diene and as a dienophile MVE is found to be more reactive than PY. Computed bond orders establish that the syn 2 transition states are the most ‘reactant’ like. Though the reactions have both electrostatic control and frontier orbital control the former dominates in the initial stages of the reaction. Presented in the Theoretical Chemistry Symposium 2006 held during 11–13 December 2006 in Tiruchirappalli, India     

Electrochemical behaviour of quinazoline in amphiprotic media

The electrochemical behaviour of quinazoline in aqueous methanolic solutions has been studied in detail. In acid solutions two single-electron waves of equal height are obtained whereas in neutral solutions a single two-electron wave is observed. Quinazoline gets reduced to tetrahydroquinazoline in alkaline media and very interestingly gives reverse cathodic peaks during cyclic voltammetric experiments. The polarographic waves are diffusion-controlled and irreversible on the basis of the usual criteria. Based on the results obtained, reduction schemes are given for quinazoline in various buffered media. A comparison of the electrochemical behaviour, between quinazoline and 4-(2-thienyl)quinazoline (4-TQ) has been attempted.

---

Das elektrochemische Verhalten von Chinazolin in amphiprotischen Medien

Zusammenfassung Das elektrochemische Verhalten von Chinazolin in wäßrigen methanolischen Lösungen wurde im Detail untersucht. In sauren Lösungen wurden zwei Einelektronenwellen gleicher Höhe erhalten, währenddessen in neutralen Lösungen eine einzige Zweielektronenwelle beobachtet wurde. Chinazolin wird in alkalischem Medium zu Tetrahydrochinazolin reduziert und ergibt interessanterweise kathodische Umkehrpeaks während cyclischer voltammetrischer Experimente. Die polarographischen Wellen sind diffusionskontrolliert und nach den üblichen Kriterien irreversibel. Aufgrund dieser Ergebnisse werden Reduktionsschemata für Chinazolin in verschiedenen Puffermedien angegeben. Es wird ein Vergleich zwischen dem elektrochemischen Verhalten von Chinazolin und 4-(2-Thienyl)-chinazolin (4-TQ) angestellt.

     

Underpotential deposition of copper on electrochemically treated polycrystalline Au surfaces

Cyclic voltammetric responses of electrochemically treated polycrystalline gold surfaces have been examined using Cu-UPD probe reactions (UPD-underpotential deposition). The treatment procedure used appears to induce preferential crystal orientation of the Au surface with a very small roughness factor and affords a simple method to obtain well-defined surfaces for electrochemical studies.     

Polarography of 4-nitrodiphenylamine (DPA) in methanol-water mixtures

The polarographic reduction of 4-nitro DPA in methanol-water mixtures of varying composition (30–80% v/v) in the pH range 1–12 was investigated. A single wave was observed in the reduction in these media at all pH values and solvent compositions. The number of electrons involved in the reduction was found to be six in all solutions indicating that 4-amino DPA is the final product. A mechanism consistent with these observations which involves the formation of 4-hydroxylaminodiphenylamine as an intermediate has been proposed.     

Insitu dehydro-oligomerization of castor oil,fatty acid ester into esters of dimer and oligomer acids over molybdenum oxide on silica-alumina catalyst

A process is described for the conversion of methylricinoleate, the major castor oil, fatty acid ester, directly into esters of dimer and oligomer acids by using molybdenum oxide on a silica–alumina catalyst. The dehydration of the methylricinoleate into a corresponding mixture of conjugated and nonconjugated olefins and the oligomerization reaction took place side by side in a continuous column, fixed-bed reactor. The products are characterized by IR, ¹H NMR, and MS. The fragmentation pattern in the mass spectrum of the dimeric species distilled from the mixture reveals that the product contains cyclic and acyclic isomeric structures. The mechanism of dimerization and oligomerization reactions to transition metal complexes and transition metal oxide catalyst systems is reviewed briefly and the possibility of a common mechanism for all similar systems is suggested. A general mechanism for oligomerization reactions to similar catalysts is proposed and the products of the present reaction are explained on the basis of the proposed mechanism.     

Synthesis and structure of tris(N-methylaminoethanoldithiocarbamato)arsenic(III)

The title compound was synthesized by the interaction of arsenic(III)chloride with freshly prepared N-methylaminoethanoldithiocarbamic acid in methanol. The compound crystallized in the triclinic system, space groupP with two molecules per unit cell, the dimensions area=6.509(10)Å,b=14.077(3)Å,c=14.195(3)Å, =64.16(2)^o, =84.67(2)^o and =84.00(2)^o. The molecule is monomeric with three short (mean 2.320(3)Å) and three long (mean 2.927(2)Å) As-S bonds. Asymmetry in the As-S bonds is due to the presence of stereochemically active lone pair on arsenic and the coordination polyhedron deviates from the usual D₃ symmetry to C₃ symmetry. Disorder exists in one of the dithiocarbamate ligands probably due to the O...O interaction arising from the water molecule. The anisobidenticity of the As-S bond distances is as a consequence of stereochemical requirement of the ligand and the lone pair of electrons on arsenic.     

Targeted and modular architectural polymers employing bioorthogonal chemistry for quantitative therapeutic delivery

There remain several key challenges to existing therapeutic systems for cancer therapy, such as quantitatively determining the true, tissue-specific drug release profile in vivo, as well as reducing side-effects for an increased standard of care. Hence, it is crucial to engineer new materials that allow for a better understanding of the in vivo pharmacokinetic/pharmacodynamic behaviours of therapeutics. We have expanded on recent “click-to-release” bioorthogonal pro-drug activation of antibody-drug conjugates (ADCs) to develop a modular and controlled theranostic system for quantitatively assessing site-specific drug activation and deposition from a nanocarrier molecule, by employing defined chemistries. The exploitation of quantitative imaging using positron emission tomography (PET) together with pre-targeted bioorthogonal chemistries in our system provided an effective means to assess in real-time the exact amount of active drug administered at precise sites in the animal; our methodology introduces flexibility in both the targeting and therapeutic components that is specific to nanomedicines and offers unique advantages over other technologies. In this approach, the in vivo click reaction facilitates pro-drug activation as well as provides a quantitative means to investigate the dynamic behaviour of the therapeutic agent.

There remain several key challenges to existing therapeutic systems for cancer therapy, such as quantitatively determining the true, tissue-specific drug release profile in vivo, as well as reducing side-effects for an increased standard of care.