Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-03-29
2002-02-12
Therkorn, Ernest G. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S656000, C210S659000, C210S198200, C210S502100
Reexamination Certificate
active
06346194
ABSTRACT:
BACKGROUND OF THE INVENTION
Ever since Pasteur discovered the property of optical activity displayed by chiral compounds, the resolution of racemic mixtures into their enantiomeric components has posed a challenge. Substantial progress in separating enantiomeric pairs has been achieved since Pasteur's laborious hand separation of the enantiomeric crystals of racemic sodium ammonium tartrate, yet methods of resolution, and the materials used therefor, remain a formidable obstacle to commercial production of optically active organic substances.
A traditional method of resolution comprises reacting a racemic mixture with a second optically active substance to form a pair of diastereomeric derivatives. Such derivatives generally have different physical properties which permit their separation by conventional means. For example, fractional crystallization often permits substantial separation to afford at least one of the diastereomers in a pure state, or largely so. An appropriate chemical transformation then converts the purified derivative, which was formed initially solely to prepare a diastereomeric pair, into one enantiomer of the originally racemic compound. This traditional method is exemplified by the reaction of naturally occurring optically active alkaloids, for example, brucine, with racemic acids to form diastereomeric salts, with release of an optically active organic acid from a purified diastereomer upon acidification of the latter.
Such traditional methods suffer from many limitations. Generally, only one of the enantiomeric pairs can be obtained, so yields are necessarily less than 50%. The separation of the material so obtained usually is incomplete, leading to materials with enhanced rather than complete optical purity. The optically active materials used to form the diastereomers frequently are expensive and quite toxic—the alkaloids as a class are good examples—and are only partially recoverable. Regeneration of optically active material from its derivative may itself cause racemization of the desired compound, leading to diminution of optical purity. For example if optically active benzyl alcohols are prepared through their diastereomeric ester derivatives, subsequent acid hydrolysis of the latter to regenerate the alcohol may be accompanied by appreciable racemization.
With the advent of chromatography diverse variations on the basic theme of separating diastereomers became possible. These approaches undeniably represent substantial advances in the art, yet fail to surmount the basic need, and associated problems, to prepare diastereomeric derivatives of the desired compound and to transform such derivatives after separation to the optically active compounds of interest.
Chromatographic methods of separating diastereomers offer advantages of general application, mild conditions which generally preclude chemical or physical transformation, efficiency of recovery and separation which are limited only by the number of theoretical plates employed and the capability of utilization from a milligram to kilogram scale. Translation from a laboratory to industrial scale has proved feasible, and commercial processes employing chromatographic separation occupy an important position in the arsenal of available industrial methods. For such reasons, methods based on chromatographic separation remain under intensive exploration.
To circumvent the disadvantage of separating diastereomeric derivatives of a compound while retaining the advantage of chromatographic separation, recent advances in the art have employed chiral, optically active compounds in association with the chromatographic support. The theory underlying this approach is that chiral material will have differential weak interactions with enantiomers, for example, hydrogen bonding, or acid-base interactions generally. Such weak interactions lead to reversible formation of entities which we refer to as complexes, and the equilibrium constant characterizing complex formation will be different for each member of the enantiomeric pair. The different equilibrium constants manifest themselves as a differing partition coefficient among the phases in a chromatographic process, leading ultimately to separation of enantiomers.
Thus, enantiomers of some chromium complexes were resolved by chromatography on powdered quartz, a naturally occurring chiral material. Karagounis and Coumolos,
Nature,
142, 162 (1938). Lactose, another naturally occurring chiral material, was used to separate p-phenylene-bis-iminocamphor. Henderson and Rule,
Nature,
141, 917 (1938). However, despite this knowledge substantiating theoretical considerations, advances in the art have been tortuous at best.
A major obstacle has been development of a chiral solid phase capable of resolving, at least in principle, a broad class of racemic organic compounds, with a stability which permits repeated usage, and with adequate capacity to make separation feasible on a preparative scale. Gil-Av has made a major contribution toward one kind of solution by gas-liquid phase chromatographic resolution of enantiomers using columns coated with N-trifluoroacetyl derivatives of amino acids, di-and tri-peptides. Gil-Av and Nurok, “Advances in Chromatography”, Volume 10, Marcel Dekker (New York), 1974. However, the advances suffer practical limitations originating from the need to have volatile substrates and the inability to scale up the methods employed.
Another advance is represented by the work of Baczuk and coworkers,
J. Chromatogr.,
60, 351 (1971), who covalently bonded an optically active amino acid through a cyanuric acid linkage to a modified dextran support and utilized the resulting material in column chromatography to resolve 3,4-dihydroxyphenylalanine. A different approach is exemplified by polymerization of optically active amides with the resulting polymer used as a solid phase in liquid-solid chromatography. Blaschke and Schwanghart,
Chemische Berichte,
109, 1967 (1976).
More recently it has become an accepted reality that enantiomeric medicinals may have radically different pharmacological activity. For example, the (R)-isomer of propranolol is a contraceptive whereas the (S)-isomer is a beta-blocker. An even more dramatic and tragic difference is furnished by thalidomide where the (R)-enantiomer is a safe and effective sedative when prescribed for the control of morning sickness during pregnancy whereas the (S)-enantiomer was discovered to be a potent teratogen leaving in its wake a multitude of infants deformed at birth. This has, in part, provided the motivation for developing additional tools for chiral separations. Chromatographic processes, especially liquid chromatography, appear to offer the best prospects for chiral separations. One variant of the latter utilizes achiral eluents in combination with chiral stationary phases (CSPs), which has the critical aspect that a variety of chiral stationary phases be available to the practitioner. In recent years substantial progress has been made by developing a class of chiral stationary phases based upon derivatized polysaccharides, especially cellulose, adsorbed on a carrier such as silica gel or a modified silica gel. This recently has been summarized by Y. Okamoto,
J. Chromatog.,
666 (1994), 403-19.
However effective may be the aforedescribed supports based on polysaccharides, there remains a need for chiral stationary phases where chirality is imparted by a monomer rather than by oligomers or polymers as represented by the polysaccharides. To be optimally useful the chiral monomer should have a plurality of chiral sites, so as to offer several chiral recognition sites and afford the potential of being broadly used in chiral separations. An appropriate monomer should afford a CSP based on covalent linkage of the monomer to the underlying carrier; covalently attaching the chiral monomer to a carrier virtually eliminates leaching, regardless of the mobile phase, and permits one to use many more types of mobile phases and to switch from forward to reverse phase eluents using the same column
Molinaro Frank S.
Naas Maryann
Therkorn Ernest G.
Tolomei John G.
UOP LLC
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