## 2 Results and Discussion  
### Synthesis of (\(\pm\))-Ectoine  
The (\(\pm\))-ectoine, (\(\pm\))-2-methyl-1,4,5,6-tetrahydropropylidine-4-carboxylic acid 7, was synthesized by combining different synthetic approaches and improved the method. The overall conducted synthesis is shown in Scheme 2. **Scheme 2. ** Synthesis of (\(\pm\))-2-methyl-1,4,5,6-tetrahydropyrimidine-4-carboxylic acid (7): (a) PBr\({}_{3}\), Br\({}_{2}\); (b) potassium phthalimide, DMF; (c) (**1**) potassium phthalimide, DMF, (**2**) acetic acid (conc. ); (d) HCl (aq) (1:1, _p_/_p_); (e) Ac\({}_{2}\)O; (f) (**1**) NaOH, Cu(CH\({}_{3}\)COO)\({}_{2}\)\(\times\) H\({}_{2}\)O, acetone, compound **5**, (2) quinolin-8-ol, acetone, water; (g) Et\({}_{3}\)N, _n_-butanol. Compound **1** was prepared from the \(\gamma\)-butylrolactone (dihydrofuran-2(3_H_)-one) by reaction with elemental bromine and with a catalytic amount of phosphorus tribromide. In this reaction 2,4-dibromobutyric acid was formed as intermediate with the release of HBr. In regular conditions only a small amount of 2,4-dibromobutyric acid changes to product, and some interventions in form of vacuum distillation or addition of base KOH are needed [18]. When the reaction was completed, the residual acid was neutralized with Na\({}_{2}\)CO\({}_{3}\). Compound **1** was isolated after extraction with dichloromethane (DCM). The synthesis of compound **4** includes three steps: (i) nucleophilic substitution of bromine with potassium phthalimide, (ii) lactone ring opening with the second molecule of potassium phthalimide, and (iii) hydrolysis. The first step involves the \(\mathsf{S}_{\text{N}}2\) reaction of compound **1** with potassium phthalimide resulting in substitution of bromine with phthalimide and the formation of 2-(oxotetrahydrofuran-3-yl)isoundolin-1,3-dione (**2**) (Scheme 2) [16]. The second step involves the reaction of compound **2** with another molecule of potassium phthalimide and the formation of intermediate potassium salt, which with concentrated acetic acid gave 2,4-bis(1,3-dioxoisoinoldin-2-yl)butyric acid (**3**) in 79% yield (Scheme 2). The third step was hydrolysis of compound **3** with HCl to remove the phthalate groups. Compound **4** was obtained in 71% yield (Scheme 2). The reaction of 4-nitrophenol and acetic anhydride, in which the oxygen from hydroxyl group of 4-nitrophenol nucleophilically attacks the carbonyl carbon in the acetic anhydride, gives compound **5** in 98% yield (Scheme 2). The synthesis of compound **6** was obtained in two different methods (A and B). Method A is the general procedure for the preparation of compound **6**. We examined six modifications (A1-A6) of this method, which are described in the Experimental section. According to [PERSON] et al. [19], the expected products are 4-acetamido-2-aminobutyric acid and 2-acetamido-4-aminobutyric acid in different ratios depending on the base added. We observed that in each of the six modifications of the general procedure, three products were observed, i.e., 4-acetamido-2-aminobutyric acid and 2-acetamido-4-aminobutyric acid, which were expected, but another compound not described in the literature was also found: 2,4-diacetamidobutyric acid, as well as a small amount of the starting compound **4**. Isolation of the desired product is difficult due to the similar nature of the compounds. For this reason, a different synthetic route was discussed, involving complexation with copper(II)-ions and then subsequent acetylation with 4-nitrophenyl acetate (**5**) (Scheme 3). acid copper(II)complex with 4-nitrophenylacetate (**5**) resulting in release of 4-nitrophenol and acetylation of amino groups of copper(II) complex; (iii) reaction with 8-hydroxyquinoline results in copper(II)-quinoline complex and two molecules of monoacetylated diaminobutyric acid (**6**). Complexation of copper(II)-ion was obtained with two molecules of 2,4-diamondobutyric acid where copper(II)-ion coordinate hydroxyl and \(\alpha\)-amino group from two 2,4-diaminobutyric acids.