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

Article abstract of DOI:10.1021/jo0001277

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.