Xiande Shen

Synthesis, curing kinetics and thermal properties of novel difunctional chiral and achiral benzoxazines with double chiral centers

Novel difunctional chiral and achiral benzoxazine monomers were synthesized from the reaction of bisphenol A with paraformaldehyde and primary amines, including S-(+)-3-methyl-2-butylamine and rac-(±)-3-methyl-2-butylamine, by solventless method. The chemical structures of chiral and achiral benzoxazines were identified by fourier transform infrared, nuclear magnetic resonance (1H NMR and 13C NMR). The curing behavior and non-isothermal curing kinetics of chiral and achiral benzoxazine monomers were investigated by differential scanning calorimeter (DSC). Isoconversional methods based on Friedman and Kissinger–Akahira–Sunose were applied to analyze the curing process of chiral and achiral benzoxazines. The thermal properties of cured polymers were characterized by DSC and thermogravimetry. The results suggested that the optical purity and stereo-configuration for chiral and achiral benzoxazines have definite influence on curing behavior and thermal properties despite the same chemical structure. Chiral benzoxazine displayed typical characteristics of difunctional benzoxazines. Achiral benzoxazine showed distinctly double peaks in DSC exotherms due to the presence of racemic and mesomeric isomers. The thermal properties of achiral polybenzoxazine were slightly higher than those of chiral polybenzoxazine, and were much higher than those of other bisphenol A-C3–C8 linear aliphatic amine-based polybenzoxazines because of tight packing, low free volume, and abundant intramolecular and intermolecular hydrogen bonds in network structure of polymers.

Controlled synthesis and chiral recognition of immobilized cellulose and amylose tris(cyclohexylcarbamate)s/3-(triethoxysilyl)propylcarbamates as chiral packing materials for high-performance liquid chromatography

The cyclohexylcarbamates of cellulose and amylose bearing a controlled amount of 3-(triethoxysilyl)propyl residue were synthesized by a one-pot process and efficiently immobilized onto silica gel through the intermolecular polycondensation of triethoxysilyl group. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high-performance liquid chromatography (HPLC). The immobilized CPMs exhibited comparable or higher recognition abilities than the conventional coated-type CPMs. The universal solvent compatibility of the immobilized CPMs clearly contributes to the improvement of chiral recognition for most racemates used in the present study. Interestingly, a significantly improved resolution for racemic trans-stilbene oxide (α=2.23) could be attained on the immobilized CPM using the eluent containing 30 vol.% chloroform in hexane, which cannot be used for the conventional coated-type CPMs. On the CPMs, almost no resolution of trans-stilbene oxide was attained by a typical eluent, hexane-2-propanol mixture (90/10, v/v). The novel immobilized CPM can also be used in thin-layer chromatography (TLC) due to the absence of an aromatic group.

Preparation and chiral recognition in HPLC of cellulose 3,5-dichlorophenylcarbamates immobilized onto silica gel.

The 3,5-dichlorophenylcarbamates (2) of cellulose bearing a small amount of 3-(triethoxysilyl)propyl residues were synthesized by a one-pot process and immobilized onto a silica gel through intermolecular polycondensation of the triethoxysilyl groups. The obtained cellulose derivatives were characterized by (1) H NMR and elemental analysis (EA), and their recognition abilities were evaluated by high-performance liquid chromatography (HPLC). The cellulose derivatives containing about 1-5% of the 3-(triethoxysilyl)propyl residue were efficiently immobilized with a high chiral recognition ability. The immobilized chiral packing materials (CPMs) could be used with the eluents containing chloroform and tetrahydrofuran (THF), which cannot be used with the conventional coated-type chiral packing materials. By using these eluents, the chiral recognition for many racemates was improved.

Immobilization and chromatographic evaluation of novel regioselectively substituted amylose‐based chiral packing materials for HPLC

The regioselectively substituted amylose derivatives bearing a 4-tert-butylbenzoate or 4-chlorobenzoate group at 2-position, and 3,5-dichlorophenylcarbamate and a small amount of 3-(triethoxysilyl)propylcarbamate groups at 3- and 6-positions were synthesized by a two-step process based on the esterification of 2-position of a glucose unit. The obtained derivatives were effectively immobilized onto macroporous silica gel by intermolecular polycondensation of triethoxysilyl groups. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high-performance liquid chromatography. These CPMs showed high chiral recognition as well as the conventional coated-type CPM, and can be used with the eluents-containing chloroform and tetrahydrofuran. With the extended use of these eluents, improvement of chiral recognition and reversed elution orders were realized. For some racemates, the immobilized CPM exhibited ability comparable or better to the commercial immobilized amylose- or cellulose-based columns, Chiralpak IA, IB, and IC.

Optical resolution of racemic compounds by chiral HPLC on immobilized amylose derivatives with regioselective substituents

The optical resolution of various racemic compounds are evaluated by high-performance liquid chromatography (HPLC) using a new series of immobilized amylose derivatives as the chiral stationary phases (CSPs), which regioselectively carry two different substituents at 2-position and 3-, 6-positions. Each derivative exhibited its own characteristic recognition ability depending on the arrangement of side chains at different positions, indicating an attractive potential for the development of chiral drug industry. Some racemates can be efficiently separated on these derivatives as well as on the immobilized amylose tris(3,5-dimethylphenylcarbamate)s, which is commercially available as Chiralpak IA and one of the most powerful CSPs.