Aromatic hydrazones derived from nicotinic acid hydrazide as fluorimetric pH sensing molecules: Structural analysis by computational and spectroscopic methods in solid phase and in solution.

Structural analyses of aroylhydrazones were performed by computational and spectroscopic methods (solid state NMR, 1 and 2D NMR spectroscopy, FT-IR (ATR) spectroscopy, Raman spectroscopy, UV-Vis spectrometry and spectrofluorimetry) in solid state and in solution. The studied compounds were N'-(2,3-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide (1), N'-(2,5-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide (2), N'-(3-chloro-2-hydroxy-phenylmethylidene)-3-pyridinecarbohydrazide (3), and N'-(2-hydroxy-4-methoxyphenyl-methylidene)-3-pyridinecarbohydrazide (4). Both in solid state and in solution, all compounds were in ketoamine form (form I, CONHNC), stabilized by intramolecular H-bond between hydroxyl proton and nitrogen atom of the CN group. In solid state, the CO group of 1-4 were involved in additional intermolecular H-bond between closely packed molecules. Among hydrazones studied, the chloro- and methoxy-derivatives have shown pH dependent and reversible fluorescence emission connected to deprotonation/protonation of salicylidene part of the molecules. All findings acquired by experimental methods (NMR, IR, Raman, and UV-Vis spectra) were in excellent agreement with those obtained by computational methods.

Mechanism of chiral recognition in the enantioseparation of 2-aryloxypropionic acids on new brush-type chiral stationary phases.

New brush-type chiral stationary phases (CSP I-IV) comprising N-3,5,6-trichloro-2,4-dicyanophenyl-L-alpha-amino acids (1-4) were prepared by binding of chiral selectors 1-4 to gamma-aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N-3,5,6-trichloro-2,4-dicyanophenyl-L-alanyl-(3-triethoxysilyl)propylamide to unmodified silica gel. The best separation of racemic 2-aryloxypropionic acids (TR-1-13) was obtained with CSP I; the -(-)-S enantiomer were regularly eluted first, as determined by a CD detector. The mechanism of chiral recognition implies a synergistic interaction of carboxylic acid analyte with the chiral selector and achiral free gamma-aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separation ability for 2-aryloxypropionic acids, but a similar enantioselective discrimination of esters TR-19-20, in comparison with CSP I. CSP I-IV retain unaltered separation ability after a few months of continuous work using a large number of various mobile phases.

Solid-phase synthesis of chiral stationary phases based on 2,4,5,6-tetrachloro-1,3-dicyanobenzene derivatives spaced from N-3,5-dinitrobenzoyl alpha-amino acids: comparative study of their resolution efficacy.

Two new chiral stationary phases, 3-[5-chloro-1,3-dicyano-2,4-[2'-(N'-1,3-dinitrobenzoyl-D-phenylglycinyl) aminoethyl]aminophen-1-yl] aminopropyl silica (CSP-1) and 3-[5-chloro-1,3-dicyano-2,4-[2'-(N'-1,3-dinitrobenzoyl-L-leucinyl) aminoethyl] aminophen-1-yl] aminopropyl silica (CSP-2), were prepared by solid-phase synthesis. They comprise chiral unit, 3,5-dinitrobenzoyl derivative of the amino acid, D-PhGly or L-Leu, bound via spacer 1,2-diaminoethane to 2,4-positions of the persubstituted benzene ring, derived from compound 1, and possess pseudo-C2 symmetry. Preparation of model compounds 6 and 7 confirmed the structure of chiral selectors, which comprise pi-donor persubsituted aromatic ring and two strong pi-acceptor 3,5-dinitrobenzoyl amido units. CD spectra of model selectors 6 and 7, run in DMSO above 250 nm, exhibit negative exciton coupling (EC) between pi-acceptor and pi-donor chromophores, C(1) symmetric model compound 8 exhibited much weaker EC and 9, devoid of pi-donor unit, does not exhibit any significant CD. Combined pi-donor and pi-acceptor properties enable the new CSPs to separate a broad range of racemates. The columns with CSP-1 and CSP-2 were tested for the separation of 22 racemates by HPLC with two different mobile phase systems and the results are compared with those obtained by using a structurally related commercial column.

Preparation and evaluation of chiral stationary phases based on N, N-2,4-(or 4,6)-disubstituted 4,5-(or 2,5)-dichloro-1, 3-dicyanobenzene.

Regioselective functionalization of 2,4,5,6-tetrachloro-1, 3-dicyanobenzene (TCDCB) by nucleophilic substitution of the chlorine at C(4) with L-Ala, L-Phe or L-Pro, followed by amide-bond formation to lipophilic amines containing strong pi-donor group, and by final introduction of the spacer 3-aminopropyltriethoxysilyl (APTES), provided a number of new brush-type chiral selectors in the form of 1-2:1 mixture of 2,4 and 4,6-di(alkyl)amino regioisomers (8/9, 10/11, 12/13, 14/15, 20/21, 23/24). Linking these to silica gel (Nucleosil 100-5) gave new chiral stationary phases for HPLC columns (CSP I-CSP VI). Being strong pi-basic selectors, most of these columns exhibited good resolution properties for pi-acid test racemates (TR 1-TR 9), specifically rac 3, 5-dinitrobenzoyl-alpha-amino acid isopropyl-esters (DNB-AA). CSP V [1,3-dicyano-2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-¿[N-butyl]-N'-[(1R)-cyclohexylethyl]-N'-[napht hylmet hyl]acetamido¿-aminobenzene] and particularly the dipeptide-containing CSP VI [2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-(3', 5'-dimethylanilido)-L-alanyl-L-prolyl-aminobenzene] proved to have the highest efficiency, comparable with the best commercial brush-type columns with pi-donor properties. Further evidence revealed that multiple hydrogen bonding via the amide group in the chiral environment and pi-pi interaction play a major role in chiral recognition, whereas steric perturbations via nonbonding VDW interactions contribute substantially only to the resolution of CSP III [2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-(cyclohexylamido)-L-alanyl-aminobenzene]. This contribution is minor for the other CSPs.

Novel chiral stationary phases comprising 2,4-(or 2,6)-diamino-5,6-(or 2,5)-dichlorobenzene-1,3-dicarbonitrile and 1-acyl (1R,2R)-diaminocyclohexane.

Novel chiral selectors 3-5 were prepared by regioselective nucleophilic substitution of 2,4,5,6-tetrachlorobenzene-1,3-dicarbonitrile (TCBDC, 1) at C(4) by (1R,2R)-trans-diaminocyclohexane, followed by acylation of the intermediary 2 with carboxylic acids containing pi-acid or pi-basic unit. On substitution of the second chlorine atom by the spacer 3-aminopropyltriethoxysilane (APTES), a 1:1 mixture of regioisomers of N-(([3, 6-dichloro-2,4-dicyano-5-(4,4,4-triethoxy-4-silabutyl)-amino]phenyl)amino) cyclo-hexylcarboxamides and N-(5,6-dichloro-2,4-dicyano-3-(4,4,4-triethoxy-4-silabutyl)-amino]pheny) amino)cyclohexylcarboxamides (6/7, 8/9, 10/11) was obtained. Their covalent binding to Nucleosil 100-5 provided three new chiral stationary phases (CSP-1-CSP-3). NMR spectra of model compounds 12-14 and MM2 calculations on model compounds 15,16 revealed pi-pi interactions between persubstituted benzene ring and second aromatic ring. The results of the evaluation of new CSPs in the separation of 23 test racemates by HPLC are reported. CSP-2 and CSP-3, that have lower conformational freedom than CSP-1, allow for better separation. In particular, good results are obtained in the separation of some 1,4-benzodiazepines and open-chain aromatic amides by CSP-2 and CSP-3.

New chiral stationary phases based on (R)-1-naphthylethylamine bound to 2,4,5,6-tetrachloro-1,3-dicyanobenzene

Chiral functionalization of 2,4,5,6-tetrachloro-1,3-dicyanobenzene (1) by regioselective nucleophilic substitution of one or two chlorine atoms by optically pure (R)-(+)-1-naphthylethylamine (NEA), or by a glycine unit as a spacer to (R)-NEA, enables the preparation of brush-type chiral selectors (2, 3, 9, 13). By the introduction of the 3-aminopropyltriethoxysilyl (APTES) group, reactive intermediates 4a/b, 5, 10a/b, and 14a/b are obtained (a/b indicate a mixture of regioisomers with APTES in 6- and 2-position). Binding of these to silica gel afforded four novel chiral stationary phases (CSPs) 6, 7, 15, and 16. HPLC columns containing CSPs with (R)-NEA directly linked to polysubstituted aromatic ring (6, 7) are not very effective in resolution of most of the 23 racemic analytes, whereas the columns with distant pi-basic subunits (15, 16) exhibited higher resolving efficacy, in particular towards the isopropyl esters of racemic N-3,5-dinitrobenzoyl-alpha-amino acids. Effective resolution of test racemates reveals the importance of the presence of the hydrogen bond donor amido group and the distance between the persubstituted benzene ring in 1 and the pi-basic naphthalene ring of (R)-NEA. Copyright 1999 Wiley-Liss, Inc.