## 1 Introduction  
In biological testing it is important to use compounds in the form of pure enantiomers, especially for their use in the synthesis of pharmaceuticals. This necessity is evident when a drug has a high enantiomeric excess to avoid undesired side effects by the second enantiomer [1]. The product of the reduction of \(\alpha\)-acetylbutyrateolactone **1**--\(\alpha\)'-1'-hydroxyethyl-\(\gamma\)-butyrolactone **2** possesses two stereogenic centres and may be classified as a \(\beta\)-hydroxyester. This compound is a structural analog of \(\gamma\)-butyrolactone (GLB), which exerts several inhibitory action over the central nervous system (CNS) through the interaction with neuronal high-affinity receptors. Isomers of \(\alpha\)-1'-hydroxyethyl-\(\gamma\)-butyrolactone could be potential ligands of the same receptors [2].  
Microbial reduction of the carbonyl group is a convenient method for obtaining enantiomerically enriched \(\beta\)-hydroxyesters [3], which encouraged us to use a yeast strain in the biotransformation of \(\alpha\)-acetylbutyrateolactone. Whole-cell biocatalysts, in comparison to isolated enzymes, exhibit increased stability due to the protective cell matrix and are more profitable due to lower production costs [1]. Additionally, cofactors of enzymes are already present in the cells and can, therefore, be recycled via the oxidation of the second substrate (i.e., glucose or glycerol) [4]. Unfortunately, biotransformation can be ineffective when substrates are toxic or have low solubility in water. Overcoming these limitations can be accomplished through the use of a biphasic system, in this case both an organic and aqueous phase. A buffer and a biocatalyst compose the aqueous phase, while the organic phase is water immiscible, which both the substrate and the product seem to strongly prefer. The organic phase acts as substrate reservoir during the reaction and as an in situ extractant for the product [5]. Deep eutectic solvent (DES) has been recently identified as an innovative solvent because it forms a eutectic mixture of two or more components with a melting point lower than that of both of the individual components [6; 7; 8]. The charge delocalization that occurs through hydrogen bonding between the salt anion and hydrogen bond donor (HBD) moiety explains their lower melting point [9]. The most popular DES in biocatalysis today is a combination of choline chloride (ChCl) and glycerol [10]. Choline chloride is similar to B vitamins and is a biodegradable and non-toxic salt [8]. In turn, glycerol is a hydrogen bond donor in DES. Glycerol is a conventional solvent used extensively in the food and pharmaceutical industries [11]. DES based on ChCl and glycerol is hydrophilic but its polarity does not influence the catalytic activity. Viscosity has the ability to change the enzyme activity in the reaction systems by changing the mass-transfer limitations. DES are inexpensive, nontoxic, biodegradable, and do not require further purification [9]. So far, cholinium-based deep eutectic solvents have been used in asymmetric bioreduction of ketoesters by baker's yeast [9; 12; 13] and recombinant _E. coli_ CCZU-T15 [14]. Baker's yeast [15] and enzymes like _Ralstonia_ sp. ADH (alcohol dehydrogenase) and horse liver ADH overexpressed in _E. coli_[16] established better selectivity in presence of DES in biotransformation of acetophenones. The aim of the work presented was to obtain pure \(\alpha\)-1'-hydroxyethyl-\(\gamma\)-butyrolactone enantiomers using biotransformation by means of yeast culture. To increase the stereoselectivity and efficiency of biotransformation, different conditions (growing and resting cell cultures, addition of different solvents including DES) were used.