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 nitrogen. The resulting oxazolium salt 22 reacts with BnNMe3(+)CN(-) to produce the azomethine ylide 24b via electrocyclic ring opening of an oxazoline 23b. Internal cycloaddition affords 26 in 66% yield. After saponification and base-induced cleavage of the N-phenylsulfonylethyl group, conventional cyclization provides access to 33. Deprotection and DDQ oxidation completes the synthesis of the aziridinomitosene derivative 9b. The starting cis-disubstituted aziridine ester 16 can be prepared by the aza-Darzens reaction of 15 with tert-butyl chloroacetate.