Methanol is the organic solvent closest to water and able to dissolve a huge amount of organic compounds. Therefore, it is a good candidate for pKa determination of drugs sparingly soluble in water or a basic drug presented as a salt which pKa is close to that of its counter-acid. In this work, the acidic dissociation constants in pure methanol of the most common acids used in pharmaceutical preparations (lactic, tartaric, fumaric, maleic and citric) were determined. In addition, the pKa values of the antipsychotic quetiapine presented as hemifumarate (Seroquel) and the very insoluble antiarrhythmic amiodarone were also determined by potentiometry. From these values, the aqueous pKa of these drugs were estimated by means of previously established equations. Estimated values are consistent with those from literature and show the interest of methanol for drug discovery pKa measurements. 相似文献
The ability to form a gel through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH‐sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (= [? CH?O]/[? NH2]), of 1.61 and 3.22 × 10?2. Variation in swelling as a result of stepwise changes in temperature between 40 and 2 °C at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 °C results in contraction of the gel network structure. While raising the temperature from 2 to 40 °C leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake). Subsequent cooling to 2 °C is accompanied by a new contraction cycle. At pH ≥ 7.6 the temperature dependence of the swelling–contraction behavior is exactly the opposite of that observed at pH 7.0. Very similar trends were observed for the gels at both degrees of crosslinking. The swelling–shrinking behavior observed in gels of pH ≥ 7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the ? NH groups leads to destruction of the hydrophobic hydration thus effectively reversing the negative thermal shrinking.
Evolution of the swelling ratio, S, as a function of time and temperature for crosslinked chitosan hydrogels. Circles represent S values recorded at pH 7.0 and triangles those at pH 7.6. 相似文献
The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn2bdc2dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction. A pressure-induced structural phase transition is found for the EtOH- and DMF-included frameworks during compression in a hydrostatic medium of the guest species, which is dependent upon the nature and quantity of the guest in the channels. The EtOH-included material undergoes a phase transition from P4/mmm to C2/m at 0.69 GPa, which is accompanied by a change in the pore shape from square to rhombus via super-filling of the pores. The DMF-included material undergoes a guest-mediated phase transition from I4/mcm to P4/mmm at 0.33 GPa via disordering of the DMF guest. In contrast, the benzene-included framework features a structure with rhombus-shaped channels at ambient pressure and shows direct compression under hydrostatic pressure. These results demonstrate the large influence of guest molecules on the high-pressure phase behavior of flexible MOFs. Guest-mediated framework flexibility is useful for engineering MOFs with bespoke pore shapes and compressibility.The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn2bdc2dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction.相似文献
Mesoporous iron phosphate containing CTAB as templating agent was synthesized and characterized by means of X-ray diffraction, Fourier transform infrared spectroscopy and TGA techniques. The mesoporous material shows a highly ordered structure, that collapses when submitted to extraction with acetate ions. The treatment of the exchanged samples at 573 K under nitrogen atmosphere leads to amorphous phases with an electrochemical behaviour typical of carbon-coated iron phosphate electrodes. The existence of this coating, proceeding from incomplete pyrolysis of the organic exchange agent, enhances the electronic properties of the system, as evidenced by galvanostatic experiments and impedance spectroscopy measurements. 相似文献
The first diastereoselective synthesis of fulleropyrrolidines endowed with diastereomerically pure functionalized cyclobutanes is reported. The new C(60)-based cyclobutane derivatives 7a,b and 9 are suitably functionalized for further incorporation into peptide surrogates. 相似文献
Co-polymerised 4-bromopolystyrene has been converted to a range of polymer-supported reagents and scavengers by bromine-magnesium exchange using Oshima's trialkylmagnesate complex followed by quenching with a variety of electrophiles. Mitsunobu, halogenation and Wittig reactions, were explored to assess the utility of the resins for target oriented and diversity oriented synthesis. 相似文献
Summary. The chiral diphosphine ligand xylophos (1) was tested as an auxiliary in palladium catalyzed allylic substitution reactions. Whereas its activity was found to be generally
good only in the case of 1,3-diphenylprop-2-en-1-yl acetate, a fair level of asymmetric induction was achieved with sodium
dimethyl malonate (83%ee) and benzylamine (66%ee) as nucleophiles.
Received June 13, 2000. Accepted July 3, 2000 相似文献
Two classical results characterizing regularity of a convergence space in terms of continuous extensions of maps on one hand, and in terms of continuity of limits for the continuous convergence on the other, are extended to convergence-approach spaces. Characterizations are obtained for two alternative extensions of regularity to convergence-approach spaces: regularity and strong regularity. The results improve upon what is known even in the convergence case. On the way, a new notion of strictness for convergence-approach spaces is introduced. 相似文献
We developed a novel and convenient stereoselective path for the preparation of pyrrolo-1,5-benzoxazepinones (PBOs). This innovative route envisaged the employment of (−)-menthol as convenient chiral auxiliary and a key SNAr for the stereoselective preparation of a tertiary aryl–alkyl ether. As a further advancement, we exploited this newly conceived synthetic route for the preparation of 2-substituted PBO analogues to either undergo biological evaluation themselves or give access to a variety of further functionalization options. 相似文献