Synthesis of Oxygenated Carbazoles by Palladium-Mediated Oxidative Double C-H Activation of Diarylamines Assisted by Microwave Irradiation

Abstract

Microwave irradiation in the presence of palladium acetate and traces of dimethylformamide allows the fast and efficient cyclodehydrogenation of diphenylamines into carbazoles. The scope of the microwave-assisted reaction is broader than that of the one using conventional conditions in that it allows the preparation of oxygenated carbazoles without apparent loss in yield. The applicability of the method to the preparation of carbazole alkaloids has been demonstrated by the development of a total synthesis of ­murrayafoline A, which proceeds in 50% overall yield from commercially available materials and is the shortest and most efficient route for the preparation of this alkaloid to date.

Key words

carbazoles - transition metals - microwave-assisted ­synthesis - cyclizations - C-H activation

References and Notes

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19

General procedure: The suitable diarylamine 1 (1 equiv) and palladium acetate (2 equiv) were thoroughly mixed. After addition of three drops of DMF, the mixture was irradiated in a domestic microwave oven at 600 W for 1 min or 1.5 min, as specified in Table [2] . After completion of the reaction, carbazoles 2 were purified through silica gel via flash column chromatography, eluting with a PE-EtOAc (8:2) mixture.