## Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Yeast Strain**} & \multirow{2}{*}{**Time [days]**} & \multicolumn{3}{c}{**Substrate**} & \multicolumn{3}{c}{**Stereoisomers of Product**} \\ \cline{3-6}  & & **1 [\%]** & **2a + 2b (anti)** & **2c + 2d (syn)** & **de [\%]** \\  & & & **[\%]** & **[\%]** & **[\%]** & **de [\%]** \\ \hline _Yarrovia lipolytica_ P26A & 1 & - & 100 & - & \(>\)99 \\ \hline _Yarrovia lipolytica_ AM71 & 1 & 15 & 85 & - & \(>\)99 \\  & 2 & - & 100 & - & \(>\)99 \\ \hline _Yarrovia lipolytica_ AM72 & 1 & 19 & 80 & - & \(>\)99 \\  & 2 & 10 & 90 & - & \(>\)99 \\  & 3 & - & 100 & - & \(>\)99 \\ \hline _Candidia viswanathi_ AM120 & 1 & 7 & 89 & 3.9 & 92 \\  & 2 & - & 95 & 4.8 & 90 \\ \hline _Hansenula anomala_ C2 & 1 & 80 & 20 & - & \(>\)99 \\  & 2 & 68 & 30 & 2 & 89 \\  & 3 & 60 & 38 & 2 & 89 \\  & 4 & 46 & 51 & 3 & 90 \\ \hline _Saccharomyces cerevisiae_ K1 & 1 & 100 & - & - & - \\  & 2 & 100 & - & - & - \\  & 3 & 100 & - & - & - \\  & 4 & 100 & - & - & - \\ \hline _Saccharomyces pombe_ C1 & 1 & 100 & - & - & - \\  & 2 & 100 & - & - & - \\  & 3 & 100 & - & - & - \\  & 4 & 100 & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The results of screening biotransformation of \(\alpha\)-acetylbutyrateolactone **1** (in % according to GC). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Strain**} & \multirow{2}{*}{**Time**} & \multirow{2}{*}{**1 [\%]**} & \multicolumn{3}{c}{_anti_} & \multicolumn{3}{c}{_syn_} \\ \cline{3-6}  & & & **2a (3S,1’S)** & **2b** & **ee [\%]** & **2c (3S,1’R)** & **2d (3R,1’S)** \\  & & & **[\%]** & **(3R,1’R)[\%]** & & **[\%]** & **[\%]** \\ \hline _Yarrovia lipolytica_ & & & & & & & \\ P26A & 1 & 0 & 0 & 100 & \(>\)99 & 0 & 0 & - \\ \hline _Yarrovia lipolytica_ & 1 & 10 & 0 & 90 & \(>\)99 & 0 & 0 & - \\ AM71 & 2 & 0 & 0 & 100 & \(>\)99 & 0 & 0 & - \\ \hline _Candidia viswanathi_ & 1 & 3 & 81 & 12 & 73 & 0 & 4.0 & \(>\)99 \\ AM120 & 2 & 0 & 83 & 13 & 73 & 0 & 4.3 & \(>\)99 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The enantioselectivity of biotransformation of \(\alpha\)-acetylbutyrateolactone **1** (in % according to GC). rotation (\([\alpha]_{20}^{\rm D}=-20.4\) (c = 0.9; CHCl\({}_{3}\)) was compared with literature data to confirm configuration of the biotransformation product [18]. Stereoselective transformation of \(\alpha\)-acetylbutyrateolactone **1** can take place either by reducing the carbonyl group or by reducing the double bond in the enol that is readily formed by keto-enol tautomerism. Enol reduction could be catalysed by ene-reductases (flavin-dependent oxidoreductase belonging to the Old Yellow Enzyme family), which require NADPH as a cofactor. If the structure of the enol hydroxyl group is present, it could function as an electron-withdrawing group (EWG) [21]. In turn, stereoselective reduction of the carbonyl group can be catalysed by dehydrogenases [22] or ketoreductases [23,24]. In the _Y. lipolytica_ strains, stereoselective dehydrogenases have been identified [25]. We assumed based on this statement and [PERSON]'s work [2], that the reduction of **1** would therefore be catalysed by dehydrogenase. In turn, reduction of **1** was carried out by _C. viswanathi_ AM120 with less stereoselectivity, hence the presence of both enzymes, ene-reductase and dehydrogenase, is possible. The following phase of our research was to use resting cells in place of growing cultures (Table 3). The resting cell suspension, in comparison to the growing cultures, seems to have had a negative effect on bioreduction time. In all three cases, the time needed for transformation was extended by one day. In addition, the formation of an undesired stereoisomer **2a**, in biotransformation by means of _Yarrowia_ strains was observed. In contrast, for _C. viswanathi_ AM120, the enantiomeric excess of the expected isomer **2a** decreased by about 20%.