## 4 Conclusions  
Through the investigations done with _Yarrowia_, _Candida_, _Hansenula_, and _Saccharomyces_ strains for biotransformation of \(\alpha\)-acetylbutyrateolactone **1**, we have determined that _Saccharomyces_ strains do not transform this substrate and _H. anomalous_ C2 transform it to a small degree. _C. viswanathi_ AM120 transformed \(\alpha\)-acetylbutyrateolactone **1** preferentially to (3\(S\),1'S)-\(\alpha\)'-1'-hydroxyethyl-\(\gamma\)-butylrolactone (**2a**). With reference to _Y. lipolytica_ strains, we found that substrate was converted highly enantioselective to (3\(R\),1'_R_)-\(\alpha\)'-1'-hydroxyethyl-\(\gamma\)-butylrolactone (**2b**) alone by all used strains. The initiation of biotransformation with resting cell cultures decreased the enantioselectivity of the reduction. The modification of the reaction medium by addition of small amounts of organic solvents resulted in an increase in the substrate's reactivity but a decrease in the enantioselectivity of the process. Consequently, the addition of DES to the cultures of _C. viswanathi_ AM120 at 10% and 25% decreased the time of transformation, and the stereoselectivity of the process was enhanced. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**DES [\%]**} & \multirow{2}{*}{**Time [days]**} & \multirow{2}{*}{**1 [\%]**} & \multicolumn{4}{c}{_anti_} & \multicolumn{2}{c}{_sym_} \\ \cline{3-8}  & & & **2a (3\(S\),1’_S) & **2b (3\(R\),1’_R) & **ee [\%]** & **2c (3\(S\),1’_R) & **2d (3\(R\),1’_S)** & **ee [\%]** \\  & & & **[\%]** & **[\%]** & & **[\%]** & **[\%]** & **[\%]** \\ \hline \multirow{2}{*}{10} & 1 & 1 & 81 & 11 & 76 & 0 & 7 & \textgreater{}99 \\  & 2 & 0 & 81 & 11 & 76 & 0 & 8 & \textgreater{}99 \\ \hline \multirow{2}{*}{25} & 1 & 34 & 49 & 15 & 64 & 0 & 2 & \textgreater{}99 \\  & 2 & 0 & 80 & 16 & 66 & 0 & 4 & \textgreater{}99 \\ \hline \multirow{4}{*}{50} & 1 & 100 & 0 & 0 & - & 0 & 0 & - \\  & 2 & 72 & 17 & 11 & 20 & 0 & 2 & \textgreater{}99 \\  & 3 & 50 & 27 & 20 & 16 & 0 & 4 & \textgreater{}99 \\  & 7 & 37 & 34 & 25 & 14 & 0 & 4 & \textgreater{}99 \\ \hline \end{tabular}
\end{table} 
Table 6: The biotransformation of \(\alpha\)-acetylbutyrateolactone **1** by _Candida viswanathi_ AM120 strain in presence of different amount of deep eutectic solvent (DES) (choline chloride and glycerol 1:2) (in % according to GC). **Supplementary Materials:** Supplementary materials can be found at [[http://www.mdpi.com/2076-3417/8/8/1334/s1](http://www.mdpi.com/2076-3417/8/8/1334/s1)]([http://www.mdpi.com/2076-3417/8/8/1334/s1](http://www.mdpi.com/2076-3417/8/8/1334/s1)). Figure S1: GC chromatogram of biotransformation of \(\alpha\)-acetylbutylrolactone **1** by _Rhodotorula marina_ AM77, ((3\(R\), 1'S)-\(\alpha\)-'1'-hydroxyethyl-\(\gamma\)-butylrolactone **2**d R\({}_{\text{t}}\) = 44.3 min), Figure S2: 1H NMR (500 MHz, CD3 OD) spectrum of \(\alpha\)-'1'-hydroxyethyl-\(\gamma\)-butylrolactone **2**, Figure S3: 1\({}^{3}\)C NMR (500 MHz, CD3 OD) spectrum of \(\alpha\)-'1'- hydroxyethyl-\(\gamma\)-butylrolactone **2**, Figure S4: MS spectrum of _anti_ stereoisomer of \(\alpha\)-'1'-hydroxyethyl-\(\gamma\)-butylrolactone **2**, Figure S5: MS spectrum of _syn_ stereoisomer of \(\alpha\)-'1'-hydroxyethyl-\(\gamma\)-butylrolactone **2**, Figure S6: GC chromatogram of biotransformation of \(\alpha\)-acetylbutylrolactone **1** by _Candida viswanathi_ AM120. (Substrate **1** R\({}_{\text{t}}\) = 27.9 min; (3\(S\),1'S)-**2a** R\({}_{\text{t}}\) = 34.9 min; (3\(R\),1'R_)-**2b** R\({}_{\text{t}}\) = 35.5 min; (3\(R\),1'S)-**2d** R\({}_{\text{t}}\) = 44.3 min), Figure S7: GC chromatogram of biotransformation of \(\alpha\)-acetylbutylrolactone **1** by _Yarrovia lipolytica_ P26A ((3\(R\),1'R_)-\(\alpha\)-'1'-hydroxyethyl-\(\gamma\)-butylolactone **2**b** R\({}_{\text{t}}\) = 35.5 min).