Leveraging flow chemistry for the synthesis of trisubstituted isoxazoles

The synthesis of trisubstituted isoxazoles generally requires multiple individual chemical steps, making them amenable to improvements in efficiency by telescoping as a multistep flow process. Three steps (oximation, chlorination and cycloaddition) were developed in continuous flow mode, aiming to function as an high-yielding and efficient sequence. We demonstrate this sequence using two aldehyde starting materials of interest: one carbocyclic and one heterocyclic. Between these two substrates, significant differences in solubility and reactivity necessitated modifications to the route. Most notably, the chlorination step could be carried out using either an organic N-Cl source (applicable for the carbocyclic aldehyde) or Cl₂ generated on-demand in a flow setup (applicable for the heterocyclic aldehyde). By selecting the most effective method for each substrate, good yields could be achieved over the telescoped sequence.