## 2 Results and Discussion
### Spectroscopic and Crystallographic Characterization of the Cycloadducts **5**  
The structure and the relative configuration of the spiro-adducts **5** was deduced by NMR spectroscopy. As a representative example, **5b** has been chosen and selected \({}^{1}\)H and \({}^{13}\)C chemical shifts along with some characteristic coupling constants are shown in Figure 2.  
The \({}^{1}\)H NMR spectrum of **5b** shows a singlet at \(\delta\) 2.64 ppm due to the \(N\)-methyl protons. Two mutually coupled doublets are observed at \(\delta\) 2.10 and 2.31 ppm (\(J\) = 16 Hz) corresponding to the diastereotopic4\({}^{\prime}\)-CH\({}_{2}\) group of the pyrrolide-2,5-dione cycle. A doublet at \(\delta\) 3.67 ppm (\(J\) = 8 Hz) and quadruplet-like multiplet at \(\delta\) 4.15-4.19 ppm (\(J\) = 6.8 Hz, \(J\) = 14 Hz) are assigned to the pyrrolidine H-4 and H-5 protons, respectively. The multiplicity of these signals clearly demonstrates the regiochemistry of the 1,3-dipolar cycloaddition reaction. If the alternative other regioisomer **6b** (Scheme 3) would have been formed, the pyrrolidinyl H-3 and H-5 protons should give rise to a singlet and doublet of doublet pattern in the \({}^{1}\)H NMR spectrum. The aromatic protons give rise to a multiplet in the region \(\delta\) 6.77-7.31 ppm and a broad singlet at \(\delta\) 10.62 ppm is attributed to the NH proton of the oxindole motif. In addition, a doublet at \(\delta\) 3.80 ppm can be assigned to the NH proton of the pyrrolidine ring. The proton decoupled \({}^{13}\)C NMR spectrum of **5b** exhibits signals at \(\delta\) 41.1 and 72.8 ppm corresponding to the C-4 and C-5 carbons. The two spiranic carbons C-3 and C-2 resonate at \(\delta\) 60.2 and 78.8 ppm, respectively. In addition, the oxindole carbonyl carbon is recognized at 178.8 and two further signals resonating at \(\delta\) 184.0 and 184.4 ppm are attributed to the succinimide carbonyl carbons. The regio- and stereochemical outcome of the reaction was finally ascertained by an X-ray study, allowing elucidation of the molecular structure of **5b**, and confirming that the isopropyl and aryl group stemming from the dipolarophile are in a transoid configuration. The crystallographic data and the structural refinement details are reported in Table S1 (Supplementary Materials). The asymmetric unit contains two independent molecules. A thermal ellipsoid plot with the atom numbering is shown in Figure 3, and selected bond distances and angles are listed in Table S2. The solid-state structure of compound **5b** is stabilized through significant intermolecular interactions such as N\(-\)H\(\cdots\)N, N\(-\)H\(\cdots\)O, C-H. \(\cdots\)Cl and C-H\(\cdots\)O hydrogen bonding (Table S2). Within the supramolecular crystal structure of **5b** (Figure 4), the two independent molecules are linked together via weak non-conventional intermolecular H-bonding interactions. Thus, the N2 atom of the pyrrolidine ring of the first molecule is involved in a N4\(-\)H4\(\cdots\)N2 H-bond with the H4 atom of the pyrrolidine ring of the second molecule with a separation of 2.871(2) A. The H5A atom of the pyrrolidine ring in the second molecule is also involved in another N5\(-\)H5A \(\ldots\) O6 H-bond with the O6 oxygen atom of pyrrolidine-2,5-donering of another nearby molecule with a length of 2.957(2) A (Figure 4a). In addition, this crystal packing is reinforced by several weak C\(-\)H\(\cdots\)O interactions between the H16A  
Figure 2: \({}^{1}\)H (black) chemical shifts with coupling constants and \({}^{13}\)C NMR (pink) data (ppm) of **5b**. atom of the pyrrolidine-2,5-dione group of one molecule and the O5 oxygen atom of the same group of another molecule, with a C16-H16A-O5 distance of 2.943 A. A similar contact also occurs between the H46 atom of the phenyl ring of one molecule and the _p_-chloroaryl substituent of a neighbored molecule (C46-H46-O1, 2.871 A (Figure 4b). Further details of the intermolecular interactions present in **5b** are listed in Table S2.