Solution-state NMR spectroscopy of famotidine revisited: spectral assignment, protonation sites, and their structural consequences.

Multinuclear one (1D-) and two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopic investigations of famotidine, the most potent and widely used histamine H(2)-receptor antagonist, were carried out in dimethyl sulfoxide-d(6) (DMSO-d(6)) and water. Previous NMR assignments were either incomplete or full assignment was based only on 1D spectra and quantum-chemical calculations. Our work revealed several literature misassignments of the (1)H, (13)C, and (15)N NMR signals and clarified the acid-base properties of the compound at the site-specific level. The erroneous assignment of Baranska et al. (J. Mol. Struct. 2001, 563) probably originates from an incorrect hypothesis about the major conformation of famotidine in DMSO-d(6). A folded conformation similar to that observed in the solid-state was also assumed in solution, stabilized by an intramolecular hydrogen bond involving one of the sulphonamide NH(2) protons and the thiazole nitrogen. Our detailed 1D and 2D NMR experiments enabled complete ab initio (1)H, (13)C, and (15)N assignments and disproved the existence of the sulphonamide NH hydrogen bond in the major conformer. Rather, the molecule is predominantly present in an extended conformation in DMSO-d(6). The aqueous acid-base properties of famotidine were studied by 1D (1)H- and 2D (1)H/(13)C heteronuclear multiple-bond correlation (HMBC) NMR-pH titrations. The experiments identified its basic centers including a new protonation step at highly acidic conditions, which was also confirmed by titrations and quantum-chemical calculations on a model compound, 2-[4-(sulfanylmethyl)-1,3-thiazol-2-yl]guanidine. Famotidine is now proved to have four protonation steps in the following basicity order: the sulfonamidate anion protonates at pH = 11.3, followed by the protonation of the guanidine group at pH = 6.8, whereas, in strong acidic solutions, two overlapping protonation processes occur involving the amidine and thiazole moieties.

[Solid-state NMR spectroscopy and its pharmaceutical use]. / Sziárdfázisú NMR spektroszkópia es gyógyszerészeti alkalmazása.

Liquid-state NMR spectroscopy has become an essential analytical tool in almost all fields of chemical research. However, the scope of NMR spectroscopy is not confined to the analysis of fluids. The progress in the investigation of solid samples is remarkably fast and solid-state NMR has developed to a high performance method. The majority of drug substances and products manufactured in the pharmaceutical industry are formulated in solid state, their analysis gains the increasing potential of solid-state NMR spectroscopy. The aim of this work is to survey the basics of solid-state NMR and to highlight some pharmaceutical applications focusing on polymorphism.

Quantifying low levels of polymorphic impurity in clopidogrel bisulphate by vibrational spectroscopy and chemometrics.

Vibrational spectroscopic methods were developed for quantitative analysis of Form II of clopidogrel bisulphate in Form I and Form II polymorphic mixtures. Results show that both IR and Raman spectroscopy combined with chemometrics are suitable to quantify low levels of Form II in Form I, down to 2 and 3%, respectively, with less than 1% limit of detection. Different preprocessing and multivariate methods were applied for spectral processing and were compared to find the best chemometric model. Common problems of quantitative vibrational spectroscopy in the solid phase are discussed; and procedures appropriate to eliminate them are proposed.

Quantitative determination of famotidine polymorphs: X-ray powder diffractometric and Raman spectrometric study.

X-ray powder diffractometric and Raman spectrometric methods were developed for quantitative measurement of the polymorphic forms of famotidine in their mixtures. This study aims to deduce some useful conclusions regarding quantitative polymorph analysis, which could also be utilized in industrial practice. Both form A and form B of famotidine possess specific X-ray diffraction reflections as well as characteristic Raman vibrational bands, which permits simple determination of the phases in their mixtures. Keeping in mind that multivariate data processing by chemometric approach is thought of nowadays as superior over univariate one, the results of the two evaluation methods were compared by precision, accuracy as well as robustness. It was found that both approaches provide similar results provided analytically useful data regions are properly selected. Overcoming the common problems of quantitative X-ray powder diffractometry and solid state Raman spectrometry both permit accurate quantification of famotidine polymorphs; the latter, however, seems to be more favourable in regular laboratory practice.

Selective NR1/2B N-methyl-D-aspartate receptor antagonists among indole-2-carboxamides and benzimidazole-2-carboxamides.

(4-Benzylpiperidine-1-yl)-(6-hydroxy-1H-indole-2-yl)-methanone (6a) derived from (E)-1-(4-benzylpiperidin-1-yl)-3-(4-hydroxy-phenyl)-propenone (5) was identified as a potent NR2B subunit-selective antagonist of the NMDA receptor. To establish the structure-activity relationship (SAR) and to attempt the improvement of the ADME properties of the lead, a series of compounds were prepared and tested. Several derivatives showed low nanomolar activity both in the binding and in the functional assay. In a formalin-induced hyperalgesia model in mice, 6a and (4-benzylpiperidine-1-yl)-[5(6)-hydroxy-1H-benzimidazol-2-yl]-methanone (60a) were as active as besonprodil (2) after oral administration. A CoMSIA model was developed based on binding data of a series of indole- and benzimidazole-2-carboxamides.

Synthesis of 4-cyano and 4-nitrophenyl 1,6-dithio-D-manno-, L-ido- and D-glucoseptanosides possessing antithrombotic activity.

1,6-Anhydro-3,4-O-isopropylidene-1-thio-D-mannitol was converted into its sulfoxide which after hydrolysis, acetylation and subsequent Pummerer rearrangement gave the penta-O-acetyl-1-thio-D-mannoseptanose anomers in excellent yield. This anomeric mixture was used as donor for the glycosylation of 4-nitro- and 4-cyanobenzenethiol in the presence of boron trifluoride etherate and trimethylsilyl triflate, respectively, to yield the corresponding thioseptanosides in high yield. The same strategy was applied for the synthesis of the corresponding L-idothioseptanosides using 1,6-anhydro-3,4-O-isopropylidene-1-thio-L-iditol as starting material. The penta-O-acetyl-D-glucothioseptanose donors could not be synthesised the same way, as the Pummerer reaction of the corresponding tetra-O-acetyl-1,6-thioanhydro-1-thio-D-glucitol sulfoxides led to an inseparable mixture of the corresponding L-gulo- and D-glucothioseptanose anomers. Therefore, D-glucose diethyl dithioacetal was converted via its 2,3,4,5-tetra-O-acetyl-6-S-acetyl derivative into an anomeric mixture of its 6-thio-septanose and -furanose peracetates which could be separated by column chromatography. Condensation of the 6-thio-glucoseptanose peracetates with 4-cyano- and 4-nitrobenezenethiol in the presence of boron trifluoride etherate afforded anomeric mixtures of the corresponding thioseptanosides. The D-manno-, L-ido- and D-glucothioseptanosides obtained after Zemplén deacetylation of these mixtures were tested for their oral antithrombotic activity.

[From routine acylation towards stable sigma-complexes of pyrimidine: carbon protonation of the pyrimdine-ring]. / Rutin acilezéstol stabilis pirimidin sigma-komplexekig: A pirimidin-gyuru szén protonálódása.

It is well known, that the ring nitrogen of pyrimidine possesses basic property, electron donating groups increase basicity. According to literature data published so far pyrimidines protonate at the ring nitrogen. The present paper gives brief account of the C(5) carbon-protonation observed among triaminopyrimidine derivatives, which in some cases results in stable sigma-complexes. Steric and electronic effects responsible for carbon-protonation are investigated in simple C(5) substituted 2,4,6-triaminopyrimidine derivatives. We show new stable sigma-complexes in the triaminopyrimidine series. The paper summarizes, in Hungarian, our recently published results (J. Amer. Chem. Soc., 125, 2535-2540, 2003) as well as our new results presented in the symposium ,,Gyógyszerkémiai Gyógyszertechnológiai Szimpózium 2003".

Rietveld refinement in the routine quantitative analysis of famotidine polymorphs.

Accurate, precise and reliable X-ray powder diffraction method was developed for the quantitative determination of famotidine polymorphic forms in their binary mixtures, which slightly outperforms the previously established Raman method. The study highlights the advantage of focused beam transmission geometry in diminishing the effect of preferred orientation in general, and the straightforward transmission foil sample preparation technique in facilitating high-throughput measurements in particular. This combination can provide good quality data for Rietveld refinement which assures more reliable quantitative results than utilizing intensity ratios of selected single reflections. After careful adjustment of profile parameters, simple routine application of the method was achieved.

Polymorph transitions of bicalutamide: a remarkable example of mechanical activation.

Bicalutamide, an active pharmaceutical ingredient possessing antiandrogenic activity, is known to exhibit polymorphism. The higher melting Form I relates monotropically to the lower melting Form II. The amorphous form can be easily produced by quench cooling the melt, but it is known to crystallize spontaneously to Form II at room temperature within days. Our results show that crystallization of amorphous bicalutamide is greatly influenced by experimental conditions and sample treatment. The effect of mechanical activation on the polymorph transitions is investigated in detail. Seeds of Form I can be formed in the amorphous phase even due to gentle mechanical treatment, which results in crystallization to the more stable structure at elevated temperature. The crystalline Form II may as well be transformed to the stable modification through mechanical activation at elevated temperature.

Carbon protonation of 2,4,6-triaminopyrimidines: synthesis, NMR studies, and theoretical calculations.

Several C5-substituted 2,4,6-triaminopyrimidine derivatives and their HBF4 salts were synthesized to study the carbon protonation of the pyrimidine ring. NMR investigations in DMSO-d6 prove experimentally that, in addition to the usual protonation at N1, the compounds can be protonated at C5 as well. We present several new stable cationic sigma-complexes in the pyrimidine series, where C5 protonation predominates over N1 protonation. Quantum chemical calculations using the B3LYP/cc-pVDZ method were utilized in the gas phase and also in DMSO solvent with the polarized continuum model (PCM) method to rationalize the observed protonation behavior. Results of the calculations accord with the experimental observations and prove that combined steric and electronic effects are responsible for the observed C5 protonation and for sigma-complex stability. We demonstrate that C5 protonation is a general feature of the 2,4,6-triaminopyrimidine system.

Protonation of the pyrimidine ring at the C(5) position: formation of a stable cationic sigma-complex.

NMR studies showed that, in addition to the expected N(1) protonation, 2,4,6-pyrimidinetriamine, N,N,N',N',N",N"-hexamethyl- (1) could also be protonated at the C(5) position in water, leading to an equilibrium between the C(5) and N(1) protonated forms. Analysis of the NMR titration data gives 6.87 and 6.89 for the pK(a) of the C(5) and N(1) protonation equilibria. Moreover, the reaction of 1 with chloroacetyl chloride leads to a novel 1,1-bis(pyrimidin-5-yl)-2-chloroethene type derivative (4) that is, peculiarly, fully monoprotonated at the C(5) position in either of the pyrimidine rings, forming a stable cationic sigma-complex.