• V N Rajasekharan Pillai

      Articles written in Journal of Chemical Sciences

    • Solid-phase synthesis of C-terminal peptide amides using a photoremovable α-methylphenacylamido anchoring linkage

      A Ajayaghosh V N Rajasekharan Pillai

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      Polymer-supported solid-phase synthetic procedures have been developed for the synthesis of C-terminal peptide amides using a new photolytically removable α-methylphenacylamido anchoring linkage between the polymeric support and the growing peptide. The preparation of this new polymeric support involves a four-step polymer-analogous reaction starting from 2%-divinylbenzene-crosslinked polystyrene resin. The steps involved are (i) Friedel-Crafts reaction with 2-bromopropionyl chloride to give the 2-bromopropionyl resin, (ii) reaction of the 2-bromopropionyl resin with potassium phthalimide to give the phthalimidomethyl resin, (iii) hydrolysis with alcoholic potash to give the partially hydrolysed phthalamido resin and (iv) treatment with alcoholic HCl to give the 2-aminopropionyl resin. N-protected amino acids undergo coupling with this amino resin by the dicyclohexylcarbodiimide-mediated coupling. The acylated resins on irradiation at 350 nm in DMF released the attached carboxyl function in the C-terminal amide form. The mechanism of the photolytic deprotection involves a radical scission of the amide linkage adjacent to the phenacyl group. The synthetic utility of the new resin has been illustrated by the preparation of several N-protected amino acid amides and the C-terminal peptide amides, Boc-Pro-Val-NH2, Boc-Gly-Phe-Pro-NH2 and Boc-Leu-Ala-Gly-Val-NH2 in 70–74% yield.

    • Topographical nature of the polymer matrix and reactivity of attached functional groups: Effect of crosslink density on the efficiency of solid-phase hypochlorite oxidations

      K Sreekumar V N Rajasekharan Pillai

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      A series of crosslinked polystyrene-supported hypohalite reagents with the reagent function in different microenvironments were prepared and their reactivities in oxidation and halogenation reactions on organic substrates were investigated. The reactivity of the hypohalite reagent function was observed to be dependent on the topographical nature of the polymer matrix. The reactivity in these cases depends on the degree of crosslinking which affects the extent of functionalisation and also the mechanical integrity and stability of the polymer support. Hypohalite reagents starting from 1%, 2%, 4% and 20% crosslinked polystyrenes were prepared and used for oxidation and halogenation reactions. The reactivity of the hypohalite function is reduced considerably when the degree of crosslinking is increased. A decrease in the degree of functionalisation is also observed with increase in the degree of crosslinking.

    • Regiospecific photochemical transformations of 4, 5-diaryl-Δ4imidazolin-2-ones: ketai formation and cyclodehydrogenation

      E Purushothaman V N Rajasekharan Pillai

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      4,5-Diaryl-Δ4-imidazolin-2-ones undergo conversion to 2,2-dialkoxy-4,5-diaryl-Δ4-imidazolines by a characteristic solvent interaction of the carbonyl group on irradiation in alcoholic solvents. The photochemical oxidative cyclization of the stilbene system in these compounds to the corresponding phenanthrene derivatives does not occur under the usual conditions. Acidic conditions hinder the ketal formation of the carbonyl group and favour the oxidative dehydrocyclization reaction to the phenanthrene derivatives. These reactions have been found to be general to the 4,5-diaryl-Δ4-imidazolin-2-ones and plausible mechanistic pathways have been suggested for the two different selective reactions.

    • Peptide synthesis catalysed by papain immobilised on polymer supports: Effect of the macromolecular structure and reaction conditions on synthesis

      V G Jayakumari V N Rajasekharan Pillai

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      Papain immobilised on different types of polymeric supports was used for the synthesis of peptides in aqueous-organic solvent mixtures. The effects of the nature of the polymer support, degree of crosslinking, nature and length of the spacer grouping between the polymer backbone and the point of attachment of the enzyme, and reaction conditions like pH, concentration of nucleophile and the immobilised enzyme content on the course of the synthesis were investigated. Divinylbenzene-crosslinked polystyrene, divinylbenzenecrosslinked polyacrylamide and N,N′-methylene-bis-acrylamide-crosslinked polyacrylamide systems immobilised with papain were used for these studies. An increase in the length of the spacer arm and an increase in hydrophilicity invariably resulted in an increase in the yield of the peptide synthesis. Papain immobilised on polystyrene-PEG supports and tetraethyleneglycol-crosslinked polystyrene supports was determined to be more efficient in effecting peptide synthesis when compared to other polystyrene-based supports.

    • Metal complexation of crosslinked polyacrylamide-supported dithiocarbamates: Effect of the molecular character and extent of crosslinking on complexation

      Beena Mathew V N Rajasekharan Pillai

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      Dithiocarbamate functions were incorporated into different polyacrylamide matrices crosslinked with a flexible and hydrophilic crosslinking agent, tetraethyleneglycol diacrylate (TEGDA), and their complexation behaviours were investigated. Crosslinked polyacrylamides with varying extents of the tetrafunctional TEGDA crosslinks were prepared by free radical solution polymerization at 60°C using potassium persulphate as initiator in ethanol. The dithiocarbamate functionality was incorporated into these polyacrylamides by a two-step polymer-analogous reaction involving (i)trans-amidation with ethylenediamine and (ii) dithiocarbamylation of the aminopolyacrylamide with carbon disulphide and alkali. The complexations of dithiocarbamate with Cu(II), Ni(II), Zn(II), Co(II) and Hg(II) ions were followed under different conditions. The metal ion intake varied with the extent of the crosslinking agent and the observed trend in complexation is Hg(II) > Cu(II)> Zn(II)> Co(II)> Ni (II). The time-course of complexation, the possibility of recycling, swelling characteristics, and spectral and thermal analyses were carried out. The thermal stability increases upon complexation with metal ions.

    • Tetraethyleneglycol diacrylate (TTEGDA)-crosslinked polystyrene resin as an efficient solid support for gel phase peptide synthesis

      M Renil V N Rajasekharan Pillai

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      Various applications of the newly developed tetraethyleneglycol diacrylate (TTEGDA)-crosslinked polystyrene resin are illustrated by the synthesis of model peptides, fully protected peptides, peptide amides and biologically important sequences. PS-TTEGDA resin was prepared by suspension polymerization of styrene and TTEGDA and functionalized with chloromethyl, 4-cholromethyl-3-nitro, aminomethyl, α-bromopropionyl, a-aminopropionyl, 4-bromomethyl 3-nitrobenzamido, 4-aminomethyl-3-nitrobenzamido groups. Peptide synthesis was carried out using these modified resins by standard solid phase methodology. Coupling and deprotection in this synthetic strategy went to near completion showing the positive role of hydrophilic and flexible crosslinking agent TTEGDA in facilitating gelphase reactions. The peptides were removed from the support by photolysis, trifluoroaceticacid (TFA) treatment,trans-esterification or ammonolysis in high purity and yield. The crude peptides were purified by column chromatography/FPLC and characterized by aminoacid analysis, sequencing or1H-NMR.

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