New Developments in Heterocyclic Tautomerism

Importance of tautomerism in drugs

Tautomerism is an important phenomenon exhibited by many drugs. As we discuss in this review, identifying the different tautomers of drugs and exploring their importance in the mechanisms of drug action are integral components of current drug discovery. Nuclear magnetic resonance (NMR), infrared (IR), ultraviolet (UV), Raman, and terahertz spectroscopic techniques, as well as X-ray diffraction, are useful for exploring drug tautomerism. Quantum chemical methods, in association with pharmacoinformatics tools, are being used to evaluate tautomeric preferences in terms of energy effects. Desmotropy (i.e., tautomeric polymorphism) of the drugs is particularly important in drug delivery studies.

The Structure of Biologically Active Functionalized Azoles: NMR Spectroscopy and Quantum Chemistry

This review summarizes the data on the stereochemical structure of functionalized azoles (pyrazoles, imidazoles, triazoles, thiazoles, and benzazoles) and related compounds obtained by multipulse and multinuclear 1H, 13C, 15N NMR spectroscopy and quantum chemistry. The stereochemistry of functionalized azoles is a challenging topic of theoretical research, as the correct interpretation of their chemical behavior and biological activity depends on understanding the factors that determine the stereochemical features and relative stability of their tautomers. NMR spectroscopy, in combination with quantum chemical calculations, is the most convenient and reliable approach to the evaluation of the stereochemical behavior of, in particular, nitrogen-containing heteroaromatic and heterocyclic compounds. Over the last decade, 15N NMR spectroscopy has become almost an express method for the determination of the structure of nitrogen-containing heterocycles.

Imidazolium-benzimidazolates as convenient sources of donor-functionalised normal and abnormal N-heterocyclic carbenes

Imidazolium-benzimidazolates are readily available conjugated mesomeric betaines, whose carbenic tautomers can form C,N_imine-metal chelates containing amino-functionalised normal or abnormal NHC ligands, depending on the degree of steric congestion. Deprotonation of the amino moiety gives rise to imine-functionalised C,N_amido-metal chelates, whose N_imine atom can be easily utilised for the construction of heteronuclear complexes.

Succinyl-β-cyclodextrin: Influence of the substitution degree on albendazole inclusion complexes probed by NMR

Succinyl-β-CD derivatives were obtained by green synthesis with degrees of substitution (DS) 1.3 and 2.9. The spray-drying technique was used to obtain albendazole (ABZ):succinyl-β-CD inclusion complexes. Phase solubility diagrams indicated that both succinyl-β-CD derivatives formed 1:1 molar ratio ABZ complexes, but the complex with DS 2.9 has a lower formation constant. The presence of stable inclusion complexes in aqueous solution was confirmed by NMR. For both complexes the aromatic moiety is encapsulated into the host cavity. In the solid-state, ¹³C and ¹⁵N NMR spectral differences between ABZ and ABZ included in spray-dried systems showed that strong structural changes occurred in the systems. At least two different ABZ amorphous species were identified based on DS. ABZ species were stable over more than six months based on spectral data. Finally, the influence of DS in the number and type of the inclusion complexes was elucidated.