NMR investigations on π-cyclopentadienyl manganese tricarbonyl oriented in the nematic phase

The NMR spectrum of π-cyclopentadienyl manganese tricarbonyl complex oriented in the liquid crystalline nematic phase has been studied. Values of the indirect spin couplings have been determined from the analysis of the spectrum. Information about the shape of the cyclopentadienyl moiety, the proton chemical shift anisotropy and molecular orientation has been obtained.     

Synthesis and structural studies of homooligomers of geminally disubstituted β(2,2)-amino acids with carbohydrate side chain

A new class of geminally disubstituted C-linked carbo-β(2,2)-amino acids (β(2,2)-Caa) were prepared from d-glucose. The structures of homooligomeric di-, tetra-, and hexapeptides prepared from (S)-β(2,2)-Caa were studied with NMR (in CDCl(3)), CD, and Molecular Dynamics calculations. These β(2,2)-peptides have shown the presence of stable 6-membered (6-mr) NH(i)···CO(i) intra-residue H-bonded (C(6)) strands. It was found that the strand structures realized in these systems were additionally stabilized by the electrostatic interaction arising due to the proximity of amide proton (NH(i)) to the oxygen of the preceding methoxy group (O(Me)(i-1)) at the C3 carbon of the carbohydrate ring. The new β(2,2)-Caa residues with additional support to H-bonding considerably expand the domain of foldamers.     

Chemometrics assisted spectrophotometric determination of pyridine in water and wastewater

The paper reports a direct method for the determination of pyridine in water and wastewater samples based on ultraviolet spectrophotometric measurements using multi-way modeling techniques. Parallel factor analysis (PARAFAC) and multi-way partial least squares (N-PLS) regression methods were employed for the decomposition of spectra and quantification of pyridine. The study was carried out in the pH range of 1.0-12.0 and concentration range of 0.67-51.7 μg mL⁻¹ of pyridine. Both the three-way PARAFAC and tri-PLS1 models successfully predicted the concentration of pyridine in synthetic (spiked) river water and field wastewater samples. The mean recovery obtained from PARAFAC regression model were 97.39% for the spiked and 99.84% for the field wastewater samples, respectively. The sensitivity and precision of the method for pyridine determination were 0.58% and 5.95%, respectively. The N-PLS regression model yielded mean recoveries of 99.29% and 100.18% for the spiked and field wastewater samples, respectively. The prediction accuracy of the methods was evaluated through the root mean square error of prediction (RMSEP). For PARAFAC, it was 0.65 and 0.82 μg mL⁻¹ for spiked river water and field wastewater samples, respectively, while for N-PLS, it was 0.25 and 0.37 μg mL⁻¹, respectively. Both the PARAFAC and N-PLS methods, thus, yielded satisfactory results for the prediction of pyridine concentration in water and wastewater samples.     

Three-component, one-pot diastereoselective synthesis of 4-amidotetrahydropyrans via the Prins-Ritter reaction sequence

Three-component coupling of carbonyl compounds, homoallylic alcohols, and nitriles has been achieved using 20 mol % of phosphomolybdic acid (PMA) at ambient temperature via the Prins-Ritter sequence to furnish 4-amidotetrahydropyrans in high yields with all cis selectivity. Spirocyclic-4-amidotetrahydropyrans are obtained using cyclic ketones.     

Synthesis and structural studies of peptides containing a mannose-derived furanoid sugar amino acid

A mannose-derived furanoid sugar amino acid (Maa) induced helical turns in peptides having repeat units of Maa(Bn(2))-Phe-Leu, which aggregated into head-to-tail duplexes in the longer oligomers.     

The stereoselective synthesis of cis-/trans-fused hexahydropyrano[4,3-b]chromenes via Prins cyclization trapping by a tethered nucleophile

A novel intramolecular Prins cyclization of (Z)-2-(5-hydroxypent-2-enyl)phenol with various aldehydes has been achieved using 10 mol% In(OTf)(3) and 30 mol% TsOH to produce the cis-fused hexahydropyrano[4,3-b]chromene derivatives in good yields, while the coupling of (E)-2-(5-hydroxypent-2-enyl)phenol with aldehydes under similar conditions affords the corresponding trans-fused hexahydropyrano[4,3-b]chromene derivatives.