Atilano Gutiérrez

New perspectives for boronic esters in macrocyclic chemistry

In the present contribution a tetrameric macrocyclic compound derived from 2,6-pyridinedimethanol and 3-nitrophenyl boronic acid, as well as 10 new dimeric boronates prepared from 2-salicylideneaminoethanol and different aryl boronic acids such as a 2-methylphenyl-, 3-methylphenyl-, 4-methylphenyl-, 3-methoxyphenyl-, 4-methoxyphenyl-, 3-chlorophenyl-, 4-chlorophenyl-, 3-nitrophenyl-, 3-trifluoromethylphenyl- and 4-fluorophenylboronic acid are described. The tetrameric and three of the dimeric structures have been analyzed by X-ray crystallography, and a series of parameters such as bond length, bond angles, deviation of the boron atom from the boronate mean plane and intermolecular interactions are discussed.

Determination of pKa values of tenoxicam from 1H NMR chemical shifts and of oxicams from electrophoretic mobilities (CZE) with the aid of programs SQUAD and HYPNMR

In this work it is explained, by the first time, the application of programs SQUAD and HYPNMR to refine equilibrium constant values through the fit of electrophoretic mobilities determined by capillary zone electrophoresis experiments, due to the mathematical isomorphism of UV-vis absorptivity coefficients, NMR chemical shifts and electrophoretic mobilities as a function of pH. Then, the pK(a) values of tenoxicam in H(2)O/DMSO 1:4 (v/v) have been obtained from (1)H NMR chemical shifts, as well as of oxicams in aqueous solution from electrophoretic mobilities determined by CZE, at 25 degrees C. These values are in very good agreement with those reported by spectrophotometric and potentiometric measurements.

Determination of pKa's for thymol blue in aqueous medium: evidence of dimer formation.

Formation constants for recrystallized thymol blue were determined in water, using the SQUAD and SUPERQUAD programs. The best model correlating spectrophotometric, potentiometric and conductimetric data was fitted with the dissociation of HL(-)=L(2-)+H(+)-log K=8.918+/-0.070 and H(3)L(2)(-)=2L(2-)+3H(+)-log K=29.806+/-0.133 with the SUPERQUAD program at variable low ionic strength (1.5x10(-4)-3.0x10(-4) M); and HL=L(2-)+H(+)-log K=8.9+/-0.000, H(3)L(2)(-) =2L(2-)+3H(+)-log K=30.730+/-0.032, H(4)L(2)=2L(2-)+4H(+)-log K=32.106+/-0.033 with SQUAD at 1.1 M ionic strength.

Study of cyclic borinates obtained from piperidine- and piperazine alcohols by spectroscopic methods and X-ray crystallography

Abstract A series of eleven new 2-aminoethyl- and 3-aminopropyl borinate derivatives with a coordinative N→B bond has been synthesized by condensation reactions between piperidine- as well as piperazine alcohols and diphenylborinic acid. The products obtained are analogous to N-spiro compounds and bicyclic systems and have been characterized by spectroscopic methods and X-ray crystallography. Thereby the N→B bond and the geometry of this new heterocyclic systems have been studied in more detail.

A new method for the preparation of unsymmetrical 1,4-substituted piperazine derivatives

Abstract Symmetrical and unsymmetrical N , N ′-piperazine derivatives of (−)-norephedrine and o -aminophenol were synthesized stereoselectively in yields >70% by reduction of the corresponding N , N ′-ethylenebisoxazolidine heterocycles. The stereochemistry at the ring fusion carbons was established by NMR spectroscopy and X-ray crystallography.

Furocoumarins of three species of the genus Dorstenia

Abstract Phytochemical studies of the roots of Dorstenia excentrica afforded a diastereoisomer of prandiol having the 2′ S ,1″ S configuration, 4-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxy]-7H-furo[3,2-g][1]1benzopyran-7-one, psoralen and 7-hydroxycoumarin. The furocoumarins 5-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxyl]-7H-[3,2-[1]benzopyran-7-one and bergapten were also present in the roots of D. drakena , while 7-hydroxycoumarin, psoralen and the psoralen dimer were isolated from D. lindeniana . The structure of the psoralen dimer was established by X-ray diffraction analysis.

Syntheses and Study of New 2-Hydroxy-5,6-Dihydro-2H-1,4-Oxazines by NMR and X-ray Crystallography

Abstract The preparation of six new 2-hydroxy-5,6-dihydro-2H-[1,4]-oxazines and four oxazino-oxazine type structures is described. The structures of four of these compounds were established by X-ray crystallographic analyses.

Syntheses of Bisoxazolidines and Morpholones

Abstract The preparation of two new bisoxazolidines, two N-(2-hydroxyethyl)- N-alkylglycine derivatives and two morpholones is described. The structure of (5S, 6R)-N-isopropyl-5-methyl-6-phenyl-l, 4-oxazin-2-one was established by X-ray crystallographic analysis. Supplementary Material available: Tables of atomic coordinates, thermal parameters, bond lengths and angles and observed and calculated structure factors have been deposited at the Cambridge Crystallographic Data Center.

13C NMR and Crystallographic Study of Thymolsulfonephthalein

Up to now the correct structure of the undissociated thymolsulfone- phthalein has not been determined unequivocally and three possible resonance structures have been proposed so far: a zwitterionic one, a quinoid one, and a lactonoid one. The present 13 C NMR and X-ray crystallographic study resolves this problem and it can be explicitly shown that the correct structure is zwitterionic with some delocalization of positive charge into the phenol rings. The correct assignment of the completely dissociated blue basic form is also included and permits the conclusion that a fourfold-charged resonance structure is as probable as a quinoid one.

A combined experimental–theoretical study of the acid–base behavior of mangiferin: implications for its antioxidant activity

Acidity constants of mangiferin (H4MGF) in DMSO/H2O (80%/20%, v/v) were determined by UV-Visible and 1H and 13C NMR spectroscopies. UV-Visible absorption spectra in the 4.2 ≤ pH ≤ 11.7 range were fitted using the computational program SQUAD, for the refining of pKa values, obtaining as results: pKa1 = 7.337 ± 0.001, pKa2 = 8.936 ± 0.001, and pKa3 = 10.297 ± 0.028. The sigmoidal curves of the chemical shifts of 1H and 13C in a similar pH range were fitted using the same model refined by SQUAD for UV-Visible data. The behavior of the chemical shifts as a function of pH allowed us to assign the deprotonation order. A theoretical DFT study has been followed confirming the deprotonation order determined experimentally, as well as a proposed mechanism of antioxidant activity, that considers the fractions of the different species at physiological pH. It was found that mangiferin is an excellent peroxyl radical scavenger in aqueous solution, significantly surpassing the activity of Trolox, which is a well-known antioxidant frequently used as a reference in this context.