Tomoyuki Ikai

Structure Control of Polysaccharide Derivatives for Efficient Separation of Enantiomers by Chromatography

Chirality is a ubiquitous feature in living systems. Today, it is widely recognized that many biologically interesting compounds, such as drugs, agrochemicals, food additives, and fragrances, are chiral and their physiological properties usually rely on their chirality due to the extremely high chiral discrimination ability of enzymes and receptors.1-6 Particularly, the different pharmacological effects between enantiomers are ultimately an important concern in the pharmaceutical field in which only one enantiomer of a chiral drug often exhibits the desirable therapeutic activity, while the other shows an antagonistic function, side effects, or even toxic effects.7-14 During the late 1950s and early 1960s, a racemate of N-phthalylglutamic acid imide, which is a sedative and hypnotic drug known as thalidomide, was administrated to pregnant women and caused the birth of approximately 10 000 babies with malformations. Later, Blaschke pointed out that this teratogenic effect was attributed to the S-(-)isomer.15 Even though the administration of the pure R-(+)isomer could not halt the disaster because thalidomide is unstable in the body and easily undergoes racemization,16-18 this tragedy undoubtedly brought a profound movement in the drug administration and industries. In 1992, the U.S. Food and Drug Administration issued a specific guideline for the production of new chiral drugs,19 which demanded a systematic investigation of the biological behavior of their individual enantiomers and significantly encouraged the development of single enantiomer drugs.20-22 Today, most of the best-selling drugs around the world are administered as single enantiomers with the desired therapeutic activity,23 and the annual sales of single enantiomer drugs are expected to exceed 200 billion dollars in 2008. Furthermore, the preparation of single enantiomers has also become important in the fields of functional materials, such as ferroelectric liquid crystals and organic nonlinear optical molecules.24-26 Based on this historical background, substantial efforts have been undertaken to develop practical techniques for the preparation of enantiomers with a high enantiomeric excess (ee). In general, two approaches are utilized for this objective, asymmetric synthesis and chiral separation. Asymmetric synthesis using chiral sources, such as chiral pools, chiral auxiliaries, asymmetric catalysts, and enzymes, has significantly progressed over the last few decades.27-41 Although the large-scale preparation of enantiomers can be economically attained using this approach, the products do not always show a high ee, and therefore, a further purification step may be inevitable. Additionally, nature produces only one of the enantiomers as a chiral source in most cases. This means that if both enantiomers of the target compounds are required, at least two kinds of chiral sources are necessary for each enantiomer. However, it is sometimes difficult to obtain both of them. On the other hand, the chiral separation approach can easily provide both enantiomers with a high ee. Since Pasteur first isolated two enantiomers of sodium ammonium tartrate using a magnifying glass and a pair of tweezers in 1848,42,43 the innovation of chiral separation techniques has attracted great interest. Basically, chiral separations44 are carried out by (i) crystallization,45-47 (ii) enzymatic kinetic resolution,48-50 and (iii) chromatographic separation.51-53 Specifically, direct chiral separation using chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) has significantly evolved during the past few decades and is recognized as the most popular and reliable tool for both the analysis of enantiomer compositions and the preparation of pure enantiomers.51-64 Chiral separations can * To whom correspondence should be addressed. Mailing address: EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 4648603, Japan. Phone: +81-52-789-4600. Fax: +81-52-789-3188. E-mail: okamoto@apchem.nagoya-u.ac.jp. † Nagoya University. ‡ Harbin Engineering University. Chem. Rev. 2009, 109, 6077–6101 6077

Immobilized-type chiral packing materials for HPLC based on polysaccharide derivatives

The polysaccharide-based chiral packing materials (CPMs) for high-performance liquid chromatography (HPLC) have been recognized as the most powerful ones for the analyzing and preparative separating of the chiral compounds. These CPMs have been conventionally prepared by coating polysaccharide derivatives on a silica gel support. This means that the solvents, which swell or dissolve the derivatives on the silica gel and reduce the performance of the chiral columns, do not allow to be applied as components of the eluents. Therefore, the polysaccharide-based CPMs can be used with a rather limited number of eluents. In order to enhance the versatility of the eluent selection for more practical and economical chromatographic enantioseparations, the polysaccharide derivatives must be immobilized onto the silica gel. This review summarizes our latest studies on the development of the immobilized-type CPMs via the radical copolymerization and the polycondensation of the polysaccharide derivatives bearing small amounts of vinyl groups and alkoxysilyl groups, respectively.

Synthesis and chiral recognition of novel amylose derivatives containing regioselectively benzoate and phenylcarbamate groups.

A new class of regioselectively substituted amylose derivatives bearing three different substituents at 2-, 3- and 6-positions, and two different substituents at 2-position and 3-, 6-positions were synthesized by a sequential process based on the esterification of 2-position of a glucose unit. Their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Each derivative had its own characteristic recognition ability depending on the arrangement of side chains at the three positions. Among the derivatives, amylose 2-(4-t-butylbenzoate) and amylose 2-(4-chlorobenzoate) series exhibited high chiral recognition. Some racemates can be efficiently separated on these derivatives as well as on the amylose tris-3,5-dimethylphenylcarbamate, which is commercially available as Chiralpak AD and one of the most powerful CSPs. The structures of the amylose derivatives were also investigated by circular dichroism spectroscopy.

Organic-Inorganic Hybrid Materials for Efficient Enantioseparation Using Cellulose 3,5-Dimethylphenylcarbamate and Tetraethyl Orthosilicate

The hybrid bead-type chiral packing material (CPM) for preparative enantioseparation has been prepared from the cellulose 3,5-dimethylphenylcarbamate containing a small number of 3-(triethoxysilyl)propyl groups in the presence of tetraethyl orthosilicate, by a sol-gel reaction in an aqueous surfactant solution. The obtained hybrid bead-type CPM was packed into a column and evaluated by high-performance liquid chromatography. When compared with the commercially available Chiralpak IB, which is prepared by the immobilization of cellulose 3,5-dimethylphenylcarbamate on silica gel, the hybrid bead-type CPM was shown to exhibit a similar chiral recognition and possess a higher loading capacity.

Immobilization and chiral recognition of 3,5-dimethylphenylcarbamates of cellulose and amylose bearing 4-(trimethoxysilyl)phenylcarbamate groups.

A small amount of 4-(trimethoxysilyl)phenyl groups was randomly introduced onto the 3,5-dimethylphenylcarbamates of cellulose and amylose by a one-pot method. The obtained derivatives were then effectively immobilized onto silica gel as chiral packing materials (CPMs) for high-performance liquid chromatography through intermolecular polycondensation of the trimethoxysilyl groups. The effects of the amount of 4-(trimethoxysilyl)phenyl groups on immobilization and enantioseparation were investigated. Also, the solvent durability of the immobilized-type CPMs was examined with the eluents containing chloroform and tetrahydrofuran. When these eluents were used, the chiral recognition abilities of the CPMs for most of the tested racemates were improved to some extent depending on the compounds.

Immobilization of 3,5-dimethylphenylcarbamates of cellulose and amylose onto silica gel using (3-glycidoxypropyl)triethoxysilane as linker

The 3,5-dimethylphenylcarbamates of cellulose and amylose were effectively immobilized onto plain silica gels as chiral packing materials (CPMs) for HPLC by means of intermolecular polycondensation of triethoxysilyl groups introduced with (3-glycidoxypropyl)triethoxysilane. The immobilization and chiral recognition abilities of the obtained CPMs prepared with different amounts of (3-glycidoxypropyl)triethoxysilane were investigated. In addition, the solvent compatibilities of the immobilized-type CPMs were examined with eluents containing chloroform and THF. When these eluents were used, for most of the tested racemates, the chiral resolving abilities of the obtained CPMs were improved.

Preparation and HPLC application of chiral stationary phase from 4-tert-butylphenylcarbamates of cellulose and amylose immobilized onto silica gel

The 4-tert-butylphenylcarbamates of cellulose and amylose bearing a small amount of 3-(triethoxysilyl)propyl residues were synthesized by a one-pot process and efficiently immobilized onto a silica gel through intermolecular polycondensation of the triethoxysilyl groups. The obtained chiral packing materials (CPMs) were evaluated by HPLC. The polysaccharide derivatives containing about 1-2% of the 3-(triethoxysilyl)propyl residue were efficiently immobilized with a high chiral recognition ability. The immobilized CPMs could be used with the eluents containing chloroform and tetrahydrofuran (THF), which cannot be used with the conventional coated-type CPMs. By using these eluents, the chiral recognition for many racemates was improved.

Preparation and Chiral Recognition of Polysaccharide-Based Selectors

Among more than one hundred commercially available CSPs, those based on the phenylcarbamates of polysaccharides including cellulose and amylose have been recognized as the most powerful for the resolution of a wide range of racemates, and nearly 90% of chiral compounds can be resolved at the analytical level using the polysaccharide-based CSPs. Although the qualitative understanding of the chiral recognition mechanism of polysaccharide-based CSPs is rather difficult in contrast to the small molecule-based CSPs, several attempts have made for comprehension of the chromatographic behavior on the polysaccharide-based CSPs. In this chapter, after describing the development of the polysaccharide-based CSPs with high recognition ability, special emphasis is placed on the mechanistic study of the polysaccharide-based CSPs on the basis of spectroscopic and computational methods.