Reference standard synthesis

For reference standard compounds 1–5, benzaldehyde (2 mmol), the respective carboxylic acid (p-toluic acid (2, 4) or acetic acid (1, 3, 5), 2 mmol), the respective isocyanide (tert-butyl isocyanide (4, 5), benzyl isocyanide (3) or 1,1,3,3-tetramethylbutyl isocyanide (1, 2), 2 mmol) and ammonia (excess, ~ 3 mmol) were combined in 2,2,2-trifluoroethanol (TFE, 2 mL) in a microwave tube. For reference standard compounds 6–7, levulinic acid (2 mmol), the respective isocyanide (benzyl isocyanide (6) or tert-butyl isocyanide (7), 2 mmol) and ammonia (excess, ~ 3 mmol) were combined in TFE (2 mL) in a microwave tube. The mixture was heated at 100 °C for 30 min in a microwave synthesis unit. The reaction mixture was cooled to room temperature and the solvent was removed under vacuum. The crude product was filtered and washed with ice-cold methanol. The final product was isolated using semi-preparative HPLC and lyophilised.