Oxazolium-derived azomethine ylides. External oxazole activation and internal dipole trapping in the synthesis of an aziridinomitosene

Intermolecular alkylation of the aziridinyl oxazole 20 using PhSO2CH2CH2OTf is possible despite the presence of potentially nucleophilic aziridine nit rogen. The resulting oxazolium salt 22 reacts with BnNMe3(+)CN(-) to produc e the azomethine ylide 24b via electrocyclic ring opening of an oxazoline 2 3b. Internal cycloaddition affords 26 in 66% yield. After saponification an d base-induced cleavage of the N-phenylsulfonylethyl group, conventional cy clization provides access to 33. Deprotection and DDQ oxidation completes t he synthesis of the aziridinomitosene derivative 9b. The starting cis-disub stituted aziridine ester 16 can be prepared by the aza-Darzens reaction of 15 with tert-butyl chloroacetate.