Singlet oxygen conversion of indoles into α,β-unsaturated oxindoles in model compounds related to the welwitindolinone alkaloids

A series of conformationally restricted bridged indoles, having structures related to the natural alkaloid N-methylwelwitindolinone C isothiocyanate (welwistatin), are oxidised to the corresponding α,β-unsaturated oxindoles by treatment with singlet oxygen.