Sunay V. Chankeshwara

Protic Acid Immobilized on Solid Support as an Extremely Efficient Recyclable Catalyst System for a Direct and Atom Economical Esterification of Carboxylic Acids with Alcohols

A convenient and clean procedure of esterification is reported by direct condensation of equimolar amounts of carboxylic acids with alcohols catalyzed by an easy to prepare catalyst system of perchloric acid immobilized on silica gel (HClO(4)-SiO(2)). The direct condensation of aryl, heteroaryl, styryl, aryl alkyl, alkyl, cycloalkyl, and long-chain aliphatic carboxylic acids with primary/secondary alkyl/cycloalkyl, allyl, propargyl, and long-chain aliphatic alcohols has been achieved to afford the corresponding esters in excellent yields. Chiral alcohol and N-t-Boc protected chiral amino acid also resulted in ester formation with the representative carboxylic acid or alcohol without competitive N-t-Boc deprotection and detrimental effect on the optical purity of the product demonstrating the mildness and chemoselectivity of the procedure. The esters of long-chain (>C(10)) acids and alcohols are obtained in high yields. The catalyst is recovered and recycled without significant loss of activity. The industrial application of the esterification process is demonstrated by the synthesis of prodrugs of ibuprofen and a few commercial flavoring agents. Other protic acids such as H(2)SO(4), HBr, TfOH, HBF(4), and TFA that were adsorbed on silica gel were less effective compared to HClO(4)-SiO(2) following the order HClO(4)-SiO(2) >> H(2)SO(4)-SiO(2) > HBr-SiO(2) > TfOH-SiO(2) >> HBF(4)-SiO(2) approximately TFA-SiO(2). When HClO(4) was immobilized on other solid supports the catalytic efficiency followed the order HClO(4)-SiO(2) > HClO(4)-K10 > HClO(4)-Al(2)O(3) (neutral) > HClO(4)-Al(2)O(3) (acidic) > HClO(4)-Al(2)O(3) (basic).

HClO4–SiO2 as a new, highly efficient, inexpensive and reusable catalyst for N-tert-butoxycarbonylation of amines

Perchloric acid adsorbed on silica-gel (HClO4-SiO2) was found to be a new, highly efficient, inexpensive and reusable catalyst for chemoselective N-tert-butoxycarbonylation of amines at room temperature and under solvent-free conditions.

Palladium-mediated chemistry in living cells

Bioorthogonal metal catalysed chemistry is the application of biocompatible transition metals to catalyse conventional synthetic organic chemistry reactions within a biological environment. Over the past decade, metals which were previously restricted to conventional organic synthesis have begun to be used in an increasing number of biological settings. This has been dominated by copper mediated catalysis of the azide-alkyne Huisgen cycloaddition (1,3-dipolar addition) chemistry but other, less toxic, metals such as palladium are now beginning to establish themselves in the chemical biology/chemical medicine arenas. The potential of palladium mediated chemistry in living systems now ranges from protein modifications to in cellulo synthesis or activation of drugs and suggests that palladium chemistry has the potential to become a powerful tool. In this review we highlight recent advances in Pd-mediated reactions in living systems.

Polymerizable Fluorescein Derivatives: Synthesis of Fluorescent Particles and Their Cellular Uptake

Fluorescent particles are used for a diverse number of biochemical assays including intracellular imaging, cellular tracking, as well as detection of a variety of biomolecules. They are typically prepared by postpolymerization conjugations of dyes onto preformed particles. Herein we report the synthesis of aminomethyl-functionalized fluorescent particles via the synthesis and application of polymerizable fluorescein monomers. These monomers allowed high and controllable fluorophore loading into the particles, resulting in enhanced fluorescence properties in comparison with more commonly used carboxyfluorescein conjugated particles. Furthermore, the particles were rapidly taken up by cells with enhanced fluorescence. The herein presented results demonstrate the advantages of dye polymerization in contrast with more conventional conjugation strategies for fluorescent particle generation with applications in the life sciences.

Organocatalytic Methods for Chemoselective O-tert-Butoxycarbonylation of Phenols and Their Regeneration from the O-t-Boc Derivatives

Carbon tetrabromide (CBr4) catalyzes O-tert-butoxycarbonylation of functionalized phenols without any side reactions (bromination, addition of CBr3 to a double bond, and formation of symmetrical diaryl carbonates, cyclic carbonates, or carbonic-carbonic anhydrides). The parent phenols are regenerated from the O-t-Boc derivatives by the catalyst system CBr4-PPh3 without affecting other protecting groups (aryl alkyl ether, alkyl ester, and thioacetal) or competitive side reaction such as bromination, nitrene (from NO2) and alpha,alpha-dibromoolefine (with CHO/COMe) formation, and transesterification (with CO2Me/Et) taking place.

Counterattack mode differential acetylative deprotection of phenylmethyl ethers: applications to solid phase organic reactions.

A counterattack protocol for differential acetylative cleavage of phenylmethyl ether has been developed. The phenylmethyl moiety is liberated as benzyl bromide that is isolated and reused providing advantages in terms of waste minimization/utilization and atom economy. The applicability of this methodology has been extended for solid phase organic reactions with the feasibility of reuse of the solid support.

Surface charge determines the lung inflammogenicity: A study with polystyrene nanoparticles.

Abstract Surface functionalization is a routine process to improve the behavior of nanoparticles (NPs), but the induced surface properties, such as surface charge, can produce differential toxicity profiles. Here, we synthesized a library of covalently functionalized fluorescent polymeric NPs (F-PLNPs) to evaluate the role of surface charge on the acute inflammation and the localization in the lung. Guanidinium-, acetylated-, zwitterionic-, hydroxylated-, PEGylated-, carboxylated- and sulfated-F-PLNPs were synthesized from aminated-F-PLNP. The primary particle sizes were identical, but the hydrodynamic sizes ranged from 210 to 345 nm. Following surface functionalization, the F-PLNPs showed diverse zeta potentials from −41.2 to 31.0 mV, and each F-PLNP showed a single, narrow peak. Pharyngeal aspiration with these eight types of F-PLNPs into rats produced diverse acute lung inflammation, with zeta potentials of the F-PLNPs showing excellent correlation with acute pulmonary inflammation parameters including the percentage of polymorphonuclear leukocytes (R2 = 0.90, p < 0.0001) and the levels of interleukin-1β (R2 = 0.83, p < 0.0001) and of cytokine-induced neutrophil chemoattractant-3 (R2 = 0.86, p < 0.0001). These results imply that surface charge is a key factor influencing lung inflammation by functionalized polymeric NPs, which further confirms and extends the surface charge paradigm that we reported for pristine metal oxide NPs. This demonstrates that the surface charge paradigm is a valuable tool to predict the toxicity of NPs.

In‐Cell Dual Drug Synthesis by Cancer‐Targeting Palladium Catalysts

Transition metals have been successfully applied to catalyze non-natural chemical transformations within living cells, with the highly efficient labeling of subcellular components and the activation of prodrugs. In vivo applications, however, have been scarce, with a need for the specific cellular targeting of the active transition metals. Here, we show the design and application of cancer-targeting palladium catalysts, with their specific uptake in brain cancer (glioblastoma) cells, while maintaining their catalytic activity. In these cells, for the first time, two different anticancer agents were synthesized simultaneously intracellularly, by two totally different mechanisms (in situ synthesis and decaging), enhancing the therapeutic effect of the drugs. Tumor specificity of the catalysts together with their ability to perform simultaneous multiple bioorthogonal transformations will empower the application of in vivo transition metals for drug activation strategies.

Palladium-mediated bioorthogonal conjugation of dual-functionalised nanoparticles and their cellular delivery

Nanoparticles have gained considerable significance in the life sciences due to their ability to be internalised by living cells and the relative ease with which they can be functionalised with cargos ranging from molecular sensors to biomacromolecules. However, the scope of available bioconjugation methods is limited and new bioorthogonal methods are much sought after. Herein, we present dual functionalised (HO)2B/H2N-polymeric nanoparticles which can be conjugated via amide bond formation and/or Pd-mediated Suzuki–Miyaura cross-coupling in a chemoselective and bioorthogonal manner. These dual-functionalised particles were found to be efficiently taken up by mammalian cells without toxicity and were successfully employed in the cellular delivery and intracellular release of a “turn-on” molecular probe thereby demonstrating the potential use of the new particles for chemical biology applications.

Palladium-mediated in situ synthesis of an anticancer agent

As a novel prodrug activation strategy Pd(0) nanoparticles, entrapped within a modular polymeric support, were used in cell culture, to synthesise the anticancer agent PP-121 from two non-toxic precursors, thereby inducing cell death in the first example of in situ mediated drug synthesis.