## 3 Results and Discussion
### Formation of Oxazolidin-5-One Oligomers  
The metal ion-stabilized Schiff bases, in addition to promoting oxazolidin-5-one formation, can also undergo old addition with short chain _aldo_-sugars, such as glycolaldehyde (Figure 4a), or _keto_-sugars, such as dihydroxyacetone (Figure 4b), and serve as molecular scaffolds for the generation of oligomeric structures through the intramolecular cyclization and formation of a putative oxazolidin-5-one with a carbonyl-containing side chains, such as the ion at [M + H]\({}^{+}\) 144.0648 shown in Figure 4a. Subsequently, this carbonyl group can undergo nucleophilic addition with the most abundant 4-methyl-oxazolidin-3-one to form oxazolidine-5-one oligomers or react with alanine to form the ion at [M + H]\({}^{+}\) 215.1032, as shown in Figure 4a. Similarly, the carbonyl group of the ion observed at [M + H]\({}^{+}\) 174.0766 shown in Figure 4b can undergo nucleophilic addition with the most abundant 4-methyl-oxazolidin-3-one to form ions at [M + H]\({}^{+}\) 257.1138 and 328.1491.  
Generation of Dimeric and Trimeric Oxazolidin-5-Ones Observed at [M + H]\({}^{+}\) 227.1016 and 328.1491  
According to Figure 4a, oligomerization can be initiated by the ald addition of glycolaldehyde to an formaldehyde-alanine Schiff base followed by the migration of the carbonyl double bond and intramolecular cyclization to form an oxazolidine-5-one moiety with a carbonyl-containing side chain after a dehydration step. The subsequent carbonyl amine reaction between 4-methyl-oxzolidin-5-one (the most abundant oxzolidin-5-one in the reaction mixture) and the newly formed carbonyl side chain can generate the observed ion at [M + H]\({}^{+}\) 227.1016, which can be considered as dimeric oxazolidine-5-one. Alternatively, the newly formed carbonyl group can also react with free alanine and form a dimeric structure observed at [M + H]\({}^{+}\) 215.1032. As expected, these ions incorporated the expected number of carbon atoms from [13C-U] glucose and C-3 atoms from [13C-3] alanine (see Table 5). A similar oligomerization reaction can be initiated by the aldol addition of dihydroxyacetone with formaldehyde-alanine Schiff base followed by the migration of the carbonyl bond and intramolecular cyclization to form an oxazolidine-5-one moiety with a carbonyl-containing side chain after a dehydration reaction. The carbonyl amine reaction between 4-methyl-oxzolidin-5-one (the most abundant oxzolidin-5-one in the reaction mixture) and the newly formed carbonyl side chain on structure at [M + H]+ 174.0766 can generate a new structure with two oxazolidinone moieties with a carbonyl side chain after dehydration step observed at [M + H]+ 257.1138. This newly formed carbonyl moiety can subsequently undergo Schiff base formation with alanine, be stabilized as trimeric oxazolidine-5-one, and be observed at [M + H]+ 328.1491. As expected, this ion incorporated five carbon atoms from [13C-U] glucose and three C-3 atoms from [13C-3] alanine. Table 5 shows isotope incorporation data, and the elemental composition of MS/MS fragments are shown in Figure 4a,b. One of the characteristic properties of oxazolidinones is their ability to undergo facile decarboxylation due to the formation of relatively stable azomethine ylides [24; 25; 26] and this property is well demonstrated in their MS/MS fragmentations discussed below.  
Figure 4: Proposed formation pathways of oxazoline-5-one oligomers observed at (**a**) _m/z_ 227 and (**b**) _m/z_ 328 detected in the Ala/Glu/FeCl\({}_{2}\) model system (see Table 5).