## 2 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Solvent**} & **DFT/** & \multirow{2}{*}{**1H-4**} & **9H-4** & **5** & **Salt 4** & \multicolumn{2}{c}{**Iminium Salt 4**} & \multicolumn{2}{c}{**Transition State 4**} & \multirow{2}{*}{**6**} \\  & **Basis Set** & & & & & & & & \\ \hline \multirow{4}{*}{H\({}_{2}\)O} & B3 LYP/ & 1.9 & 0 & –11.8 & 2.9 & 6.5 & –5.1 & 7.4 & –17.3 \\  & 6-31+G(d) & 1.9 & \(0\) & _2.7_ & _3.6_ & _8.5_ & _11.0_ & _23.0_ & –0.8 \\  & B3 LYP-D3/ & 2.4 & 0 & –14.7 & 3.0 & 5.7 & –13.5 & –1.4 & –26.7 \\  & 6-31+G(d) & 2.3 & \(0\) & 0 & 3.9 & _7.6_ & _3.0_ & _14.3_ & –9.0 \\ \hline \multirow{4}{*}{MeOH} & B3 LYP/ & 2.2 & 0 & –11.9 & 5.0 & 8.5 & –4.7 & 8.0 & –16.1 \\  & 6-31+G(d) & 2.1 & \(0\) & _2.6_ & _6.9_ & 10.5 & _11.4_ & _23.6_ & _0.5_ \\  & B3 LYP-D3/ & 2.2 & 0 & –14.9 & 4.7 & 7.3 & –25.9 & –1.2 & –25.9 \\  & 6-31+G(d) & 2.2 & \(0\) & –_0.2_ & _5.9_ & _9.2_ & –8.2_ & _14.4_ & –8.2_ \\ \hline \multirow{4}{*}{H\({}_{2}\)O} & \(\omega\)B97X-Dand/ & 2.1 & 0 & –17.5 & 3.2 & 5.7 & –18.3 & –0.1 & –26.7 \\  & 6-31+G(d)- & 2.1 & \(0\) & –_2.7_ & _4.0_ & _7.5_ & –1.3 & _16.2_ & –9.0 \\ \hline \multirow{4}{*}{MeOH} & \(\omega\)B97X-Dand/ & 2.4 & 0 & –17.4 & 4.2 & 7.6 & –18.1 & 0.3 & –31.5 \\  & 6-31+G(d)- & 2.4 & \(0\) & –_2.5_ & _6.1_ & _9.4_ & –1.0 & 16.7 & –13.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The relative electronic (plain text) and Gibbs free (italics) energies (kcal/mol) of subsequent stages of the tandem Mannich—electrophilic amination reaction calculated by DFT/PCM methods. Figure 2: The lowest unoccupied orbital (LUMO) (isoval = 0.02) and electron density (isoval = 0.0004) mapped with LUMO density calculated for the iminium zwitterion **1c**. It should be pointed out that the obtained theoretical evaluations match the observed chemical experiments. According to our previous report, reactions involving isoxazoline **1** are rather slow and require heat [4; 19]. Conversely, tandem transformations involving isoxazoline **4** are fast (Figure 3). Finally, we have evaluated the scope of the tandem reaction in terms of steric factors that would limit its applications. As shown in Scheme 4, isoxazoline **4** has been subjected to reactions with piperazine, homopiperazine and two \(N\),\(N^{\prime}\)-dialkylethylenediamines. Hence, the reaction with the most sterically constrained piperazine gave rise to the formation of a mono-derivative, i.e., 1\(H\)-spiro[1,2,4]triazoleol[4,3-4]quinoline-2,1\({}^{\prime}\)-piperazin]-2-ium-4-carboxylate (**7**) as a single product. On the contrary, the reactions with less constrained diamines produced double Mannich-amination products **8a,b** and **9**. In order to rationalize the difference in reactivity of piperazine and homopiperazine we have performed theoretical computations, analogical to the experiments presented above. Figure 3: The formation of Safirinium Q (**6**) from equimolar quantities (0.6 mmol/L) of isoxazoline **4**, formaldehyde and diethylamine (absorbance at 370 nm vs. time). Albeit, the energy barriers for transformations **A** -> **9** and **7** -> **B** were found to be comparable, the formation of product **B** (4.8 and \(-\)0.5 kcal/mol) was estimated to be thermodynamically unfavored in comparison to the homopiperazine derivative **9** (1.3 and \(-\)3.6 kcal/mol) (Table 4).