Quinolinic acid as a reagent for the gravimetric and volumetric estimation of palladium

Summary Like -amino-picolinic acid, quinolinic acid behaves as a highly selective reagent for palladium with which it forms a complex of composition Pd(C₇H₄O₄N)₂. The complex being very stable is precipitated from solutions with an acidity of 0.25 N to a maximum p_H of 2.1 and thus is separated from all the common ions and other platinum metals except copper which, however, can be kept in solution in presence of EDTA. The palladium complex is either weighed after drying or dissolved in a standard potassium cyanide solution which is later back titrated with silver nitrate solution.     

Conformational studies of 3,4-dideoxy furanoid sugar amino acid containing analogs of the receptor binding inhibitor of vasoactive intestinal peptide

Conformational analysis of vasoactive intestinal peptide (VIP) receptor binding inhibitor Leu¹-Met²-Tyr³-Pro⁴-Thr⁵-Tyr⁶-Leu⁷-Lys⁸1 by various NMR techniques and constrained molecular dynamics (MD) simulation studies revealed that the molecule had a turn structure involving its Tyr³-Pro⁴-Thr⁵-Tyr⁶ moiety with intramolecular hydrogen bond between Tyr⁶NH→Tyr³CO. In order to mimic the structure of 1, peptidomimetic analogs 2-4 were synthesized using conformationally constrained scaffolds of 3,4-dideoxy furanoid sugar amino acids (2S,5R)-ddSaa1 5 and its enantiomer (2R,5S)-ddSaa2 6. All these analogs displayed well defined three-dimensional structures akin to that found in 1. Peptides 2 and 3, which differed only in the sugar amino acid stereochemistry, show propensity of structures with identical intramolecular hydrogen bonds between ThrNH→MetCO. A similar structure with a hydrogen bond between TyrNH→MetCO was observed in 4.     

Synthesis of a cyclic dinuclear organotin carboxylate via simultaneous debenzylation and decarbonylation reactions: X-ray crystal structure of [(PhCH2)2{O2CC6H4{N(H)N(C6H3-4(O)-5-O)}-o}Sn]2

The complex, [(PhCH₂)₂{O₂CC₆H₄{N(H)N(C₆H₃-4(O)-5-O)}-o}Sn]₂ (1), is obtained as the exclusive reaction product from the reaction of sodium 2-[(E)-2-(3-formyl-4-hydroxyphenyl)-1-diazenyl]benzoate and (PhCH₂)₃SnCl. The reaction possibly proceeds via Dakin type rearrangements where arylazosalicylaldehyde is oxidized to arylazocatechol, followed by facile Sn-C bond cleavage. Complete assignments were achieved by ¹H, ¹³C, 2D ¹H-¹¹⁹Sn HMQC (¹¹⁹Sn chemical shift), 1D gs ¹H-¹⁵N HMQC (¹J(¹⁵N, ¹H) coupling constant) NMR and ESI-MS. The crystal structure of compound 1 as determined by X-ray diffraction analyses shows a cyclic centrosymmetric dinuclear moiety linked into extended chains by pairs of long Sn?O contacts of approximately 3.2 Å. Two polymorphs were identified and their structures differ primarily in the packing arrangement afforded by the benzyl groups. In one polymorph, when viewed along the Sn?Sn vector, the benzyl groups at each Sn-atom are oriented to form an S-shape, while they form a U-shape in the second polymorph.     

Nucleation of β-hairpin structure in a pyrrole amino acid containing peptide

Synthesis and conformational studies of two short peptides containing pyrrole amino acids (1, Paa), Boc-Paa-Paa-d-Pro-Gly-Xaa-Paa-Paa-OMe (2: Xaa=Ala; 3: Xaa=Val), were carried out in which it was established that replacement of Ala in 2 with a Val residue helps peptide 3 to adopt a well-defined β-hairpin conformation in a nonpolar solvent, like CDCl₃.     

Synthesis of chiral α-amino acids

A novel method for the synthesis of chiral α-amino acids has been developed where the acid functionality was constructed by oxidizing a hydroxymethyl group introduced by Evans’ method in the α-position of an appropriate acid substrate and the amino part came from the amide of the original carboxyl group following a modified Hofmann rearrangement reaction.     

“Dial-In” Emission from a Unique Flexible Material with Polarization Tuneable Spectral Intensity

The development of organic photoluminescent materials, which show promising roles as catalysts, sensors, organic light-emitting diodes, logic gates, etc., is a major demand and challenge for the global scientific community. In this context, a photoclick polymerization method is adopted for the growth of a unique photoluminescent three-dimensional (3D) polymer film, E, as a model system that shows emission tunability over the range 350–650 nm against the excitation range 295–425 nm. The DFT analysis of energy calculations and π-stacking supports the spectroscopic observations for the material exhibiting a broad range of emission owing to newly formed chromophoric units within the film. Full polarization spectroscopic Mueller matrix studies were employed to extract and quantify the molecular orientational order of both the ground (excitation) and excited (emission) state anisotropies through a set of newly defined parameters, namely the fluorescence diattenuation and fluorescence polarizance. The information contained in the recorded fluorescence Mueller matrix of the organic polymer material provided a useful way to control the spectral intensity of emission by using pre- and post-selection of polarization states. The observation was based on the assumption that the longer lifetime of the excited dipolar orientation is attributed to the compactness of the film.     

Direct meta-C−H Perfluoroalkenylation of Arenes Enabled by a Cleavable Pyrimidine-Based Template

The development of efficient and mild methods for the synthesis of organofluorine compounds is of foremost interest in various fields of chemistry. A direct pyrimidine-based selective meta-C−H perfluoroalkenylation of arenes involving several commercially available perfluoroolefins is described. The synthetic versatility of the protocol is demonstrated by an extensive substrate scope including different benzylsulfonyl, alkylarene and phenylacetic acid scaffolds. The generality of this methodology including the meta-C−H perfluoroalkenylation of Ibuprofen, the facile cleavage of the directing group and gram-scale reactions are presented.     

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.