In a 25 mL round bottom flask equipped with a magnetic stir bar 4 (1.2 eq.) was dissolved in dry methanol (0.2 M) and triethylamine in presence of molecular sieves (4 Å). To this solution paraformaldehyde (1.2 eq.) was added and the mixture was stirred for 30 minutes before the appropriate carboxylic acid (1.0 eq.) and after 10 more minutes the appropriate isocyanide (1.0 eq.) was added. After 24-72 hours the reaction mixture was filtered by vacuum filtration and the residue was washed with MeOH and CH2Cl2. The filtrate was evaporated under reduced pressure and the residue was dissolved in EtOAc and washed three times with NaHCO3 solution (sat.). After drying over anhydrous Na2SO4 and filtration, solvents were removed under reduced pressure and the crude product was purified by flash chromatography or column chromatography.