Combining Synthetic and Analytical Strategies for Preparative HPLC Enantioseparation of Monastrol Racemic Mixture

Role of the weak interactions in enantiorecognition of racemic dihydropyrimidinones by novel brush-type chiral stationary phases

The chiral discrimination ability of two recently prepared chiral stationary phases (CSP 1 and CSP 2), based on a leucine derived chiral selector, was tested for the enantiomers of dihydropyrimidone (DHPM) derivatives and compared with the commercially available Hyun-leucine CSP 3 and classical Pirkle-leucine CSP 4. By combining all of these CSPs, the enantiomers of all DHPM derivatives used in this study can be properly resolved. Particularly good enantioresolutions were achieved for thioureide derivatives, such as Monastrol. The results presented show that sulfur-aromatic interactions are meritorious for these very good separations.

Heterogeneity of adsorption mechanisms in chiral normal-phase liquid chromatography

The adsorption equilibria of two commonly employed strong mobile phase modifiers, ethyl acetate and 2-propanol, on a polysaccharide-based chiral stationary phase have been studied by modeling nonlinear perturbation peaks measured after equilibration of the column with hexane (the weak component of the binary mixture). The investigation of adsorption processes from dilute solutions for species that are strongly retained on the stationary phase could be performed by this approach. On the opposite, limits of traditional linear perturbation technique for isotherm determination, in presence of strong interactions, have been evidenced. Alcohol adsorption has been modeled by a single Langmuir isotherm, while the ester has required a BiLangmuir model. These findings have found to be in a semi-quantitative agreement with available spectroscopic data about 2-propanol and ethyl acetate adsorption on thin silica sol-gel films in contact with a weak solvent. Experimental features observed for racemic separation on polysaccharide-based chiral stationary phases, such as the dependence of the separation factor on the amount and type of the employed additive, have been explained in light of these measurements.

Rapid preparation of the mitotic kinesin Eg5 inhibitor monastrol using controlled microwave-assisted synthesis

We present here a protocol for the synthesis of the dihydropyrimidine (DHPM) derivative monastrol, which is known to be a specific mitotic kinesin Eg5 inhibitor. By applying controlled microwave heating under sealed-vessel conditions, the synthesis via the one-pot three-component Biginelli condensation can be performed in a shorter reaction time (30 min) compared with conventional heating methods that normally require several hours of reflux heating. For the purification of the crude target compound, two different methods are presented. The first protocol includes a simple precipitation/filtration step to provide monastrol in 76% isolated yield and high purity so that no recrystallization step is necessary. This can be ascribed to the microwave heating technology in which less side-product formation is typically one of the advantages. In an alternative purification step, column chromatography is performed, which provides the product in a slightly higher yield (86%). Monastrol synthesis can be conducted in approximately 2 h by employing the precipitation/filtration purification method.

High-performance liquid chromatographic separation of dihydropyrimidine racemates on polysaccharide-derived chiral stationary phases

The chromatographic behavior of a set of racemic dihydropyrimidines (DHPMs) has been evaluated on two polysaccharide-derived chiral stationary phases under normal phase conditions. One of these is coated, the other chemically immobilized. The outstanding solvent compatibility of the immobilized chiral stationary phase (CSP) permits the use of solvents such as ethyl acetate (EtOAc) that are unsuitable for coated supports, for which traditional 2-propanol:hexanes mixtures have been employed. Drastic changes in the chromatographic retention and resolution of DHPMs and, in general, in the separation performances have been observed for the two systems. From a practical point of view, EtOAc has been proven to be a better choice for the separation of this important class of compounds. By comparing molecules different in specific positions of their scaffolds, hypotheses concerning the role of individual chemical groups on retention and selectivity have been done. These effects have been quantified, in some cases, in terms of standard Gibbs energy variations. Even if no chromatographic measurements have been made under nonlinear conditions, clear indications of the potential use of immobilized chiral adsorptive media operated with EtOAc:hexanes mixtures for preparative separations of DHPMs have been evidenced.

Modeling of overloaded gradient elution of nociceptin/orphanin FQ in reversed-phase liquid chromatography

The Reversed-phase (RP) gradient elution chromatography of nociceptin/orphanin FQ (N/OFQ), a neuropeptide with many biological effects, has been modeled under linear and non-linear conditions. In order to do this, the chromatographic behavior has been studied under both linear and nonliner conditions under isocratic mode at different mobile phase compositions--ranging from 16 to 19% (v/v) acetonitrile (ACN) in aqueous trifluoracetic acid (TFA) 0.1% (v/v)-on a C-8 column. Although the range of mobile phase compositions investigated was quite narrow, the retention factor of this relatively small polypeptide (N/OFQ is a heptadecapeptide) has been found to change by more than 400%. In these conditions, gradient operation resulted thus to be the optimum approach for non-linear elution. As the available amount of N/OFQ was extremely reduced (only a few milligrams), the adsorption isotherms of the peptide, at the different mobile phase compositions examined, have been measured through the so-called inverse method (IM) on a 5 cm long column. The adsorption data at different mobile phase compositions have been fitted to several models of adsorption. The dependence of the isotherm parameters on the mobile phase composition was modeled by using the linear solvent strength (LSS) model and a generalized Langmuir isotherm that includes the mobile phase composition dependence. The overloaded gradient separation of N/OFQ has been modeled by numerically solving the equilibrium-dispersive (ED) model of chromatography under a selected gradient elution mode, on the basis of the previously determined generalized Langmuir isotherm. The agreement between theoretical calculations and experimental overloaded band profiles appeared reasonably accurate.