Publisher Summary This chapter provides information on the chemistry and pharmacology and physicochemical properties of barbiturates, along with the tautomerism and solvation of the barbituric acid ring. Barbituric acid exists in the solid state in the trioxo structure. NMR investigation of the oxo-hydroxy equilibrium indicates that only the oxo form is present in a solution in anhydrous DMSO. Several spectral properties of the barbituric acid are discussed using UV spectra, infrared and Raman spectroscopy, 1 H-NMR spectroscopy, 13 C-NMR spectroscopy, and mass spectrometry. In the crystalline state, barbiturates show several modes of intermolecular hydrogen bonding, which differ by the number of hydrogen bonds formed between the NH and CO groups. These hydrogen bonds are responsible for layer packing in the crystal lattice. Weak intermolecular van der Waals interactions of carbonyl groups are also responsible for the three-dimensional packing of the molecules in the crystal structure.

Recent Progress in Barbituric Acid Chemistry

The results of NMR spectroscopic and X-ray crystallographic studies are critically discussed with respect to the structure of hydantoins, their tautomerism, and their acidity. The imide NH proton of the preferred, nearly planar 2,4-imidazolidine-dione tautomer proved to be more acidic than the corresponding amide NH proton. Phenyl substituents at the ring nitrogen atoms and at C-5 are twisted from the plane of the hydantoin ring; in case ofortho substituents restricted rotation about the N-aryl bond was found and the barrier to rotation determined by dynamic NMR spectroscopy. For 5-benzyl substituents, afolded conformation of the two rings, due to intramolecular interactions, was found and for 5-exo-methylene substituted hydantoins the relevant E/Z isomerism at theexo-cyclic C, C double bond was studied. In addition, the1H and13C chemical shifts of the hydantoins proved to excellently indicate the electronic distribution along the hydantoin ring moiety. Finally, the mass spectrometric fragmentation of the hydantoins is critically discussed.

The structure of hydantoins in solution and in the solid state

Pre- and postcolumn derivatization with a suitable chromophore or fluorophore have often been utilized to enhance the sensitivity and selectivity of detection for analytes possessing a carboxyl group. The major classes of UV and fluorescent derivatization reagents include the coumarin analogues, alkyl halides, diazoalkanes, and amines. The derivatization reaction conditions, HPLC systems for separation of the conjugates and detection limits of various analytes are presented in this review. High mass sensitivity for various carboxylic acids have been achieved with the hydrazides, anthracenes, and diazoalkanes.

Ultraviolet and fluorescence derivatization reagents for carboxylic acids suitable for high performance liquid chromatography: a review.

This article describes both pre- and postcolumn derivatization chemistry used in conjunction with either chromatography or capillary electrophoresis (CE) to facilitate the determination of drugs. Generally, only prederivatization is used in gas chromatography (GC), principally to enhance the volatility, temperature stability, and/or detectability. The GC section considers derivatization of drugs by reagent class: alkylation, acylation, and silylation. The GC sample-handling section describes an approach that combines the extraction and derivatization steps together. Both pre- and postcolumn derivatization are common approaches for high-performance liquid chromatography (HPLC) and many of these methods have been adapted for CE. Derivatization for HPLC is often directed toward aliphatic amines, carboxylic acids, or alcohols that are difficult to detect at low levels by absorbance, luminescence, or electrochemical means. In addition, small hydrophilic molecules upon prederivatization are often converted into larger more hydrophobic compounds, making reversed-phase HPLC easier or even feasible. The HPLC section considers derivatization of drugs based on the following types: alkaloids, amines, antibiotics, barbiturates, carbonyl compounds and carboxylic acids, catecholamines, hydroxy compounds, steroids, and sulfur compounds. For the GC and HPLC sections, tables are included, giving the structures of the more important derivatizing agents, the analytes, and the corresponding reaction products. A brief rationale for derivatization chemistry with CE concludes this article. For CE, derivatization is often done to improve detectability as the path length for absorbance detection is very short and fluorescence can be effective using laser-based systems.

Chemical Reagents and Derivatization Procedures in Drug Analysis

Quantitative analysis of common anaesthetic agents

Improved detectability of barbiturates in high-performance liquid chromatography by pre-column labelling and ultraviolet detection

Gas chromatographic alkylation studies of phenytoin, mephenytoin and primidone: investigation of butylated derivatives.

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Improved detectability of barbiturates in high-performance liquid chromatography by pre-column labelling and ultraviolet detection

Gas chromatographic alkylation studies of phenytoin, mephenytoin and primidone: investigation of butylated derivatives.