Synthesis of pyrinodemins A and B. Assignment of the double bond position of pyrinodemin A

Condensation of aldehyde 3a with hydroxylamine 4b afforded nitrone 2, which underwent an intramolecular cycloaddition to give 1b, the proposed structure of pyrinodemin A. A similar condensation of aldehyde 3a and hydroxylamine 4a provided pyrinodemin A (1a), which has the double bond one carbon further away from the isoxazolidine. An analogous sequence gave pyrinodemin B (21).