Mild and Efficient Winterfeldt Oxidation of 1,2,3,4-Tetrahydro-γ-carbolines for the Synthesis of Dihydropyrrolo[3,2-b]-quinolones and Pyrrolo[3,2-b]quinolones

The Winterfeldt oxidation (NaOH, DMF, air, rt) of substituted 1,2,3,4-tetrahydro-γ-carbolines has been developed, which provides a convenient and efficient method for the synthesis of the corresponding dihydropyrrolo[3,2-b]quinolones in moderate to excellent yields (38–94%). The generality and substrate scope of this reaction are explored and a possible mechanism is proposed. The results imply that electron-withdrawing groups on N(2) of tetrahydro-γ-carbolines and N(5)-H are necessary. The synthesis of 5 or 7-substituted pyrrolo[3,2-b]quinolones in near quantitative yields was also achieved through deprotection and aromatization of N(1)-boc-dihydropyrrolo[3,2-b]quinolones.