Total Synthesis of 7-and 8-Oxygenated Pyrano[3,2-a]carbazole and Pyrano[2,3-a]carbazole Alkaloids via Boronic Acid-Catalyzed Annulation of the Pyran Ring

The boronic acid-catalyzed annulation of citral opens up a short route to oxygenated cyclized monoterpenoid pyranocarbazole alkaloids. Thus, murrayamine-D is available in only three steps and 55% overall yield from the corresponding carbazole precursor.