Enantioselective extraction of terbutaline enantiomers with β-cyclodextrin derivatives as hydrophilic selectors
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Hydrophilic β-cyclodextrin (β-CD) and its derivatives are not soluble in organic liquids but they are highly soluble in water and can interact with enantiomers selectively to form diastereomeric complexes which enable their use as chiral selectors in chiral solvent extraction. In this paper, terbutaline enantiomers were extracted by hydrophilic β-CD derivatives in an aqueous/organic biphasic solvent system with racemic terbutaline in the organic phase and β-CD in the aqueous phase. Five β-CD derivatives, namely: methyl-β-cyclodextrin (Me-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD), 2-hydroxyethyl-β-cyclodextrin (2-HE-β-CD), (2-hydroxypropyl)-β-cyclodextrin (HP-β-CD) and (4-sulfobutylether)-β-cyclodextrin (SBE-β-CD) were used as hydrophilic selectors, respectively. Process variables affecting extraction efficiency were investigated, namely influence of the type of organic solvents and β-CD derivatives, concentrations of selectors and terbutaline enantiomers, pH, and temperature. Experimental results show that the efficiency of extraction depends, often strongly, on process variables. All five β-CD derivatives studied preferentially extract the more biologically active (R)-terbutaline from the organic phase; HP-β-CD has the strongest recognition ability. The maximum enantioselectivity (α) of 1.42 was achieved under optimal conditions: pH 7.0 and temperature of 5°C. Utilization of the extraction method for separation of terbutaline enantiomers is expected to be cheap and easy to scale up to commercial scale.
Keywordschiral separation reactive extraction β-cyclodextrin derivatives terbutaline enantiomers
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