Efficient construction of pyrano[3,2-a]carbazoles: application to a biomimetic total synthesis of cyclized monoterpenoid pyrano[3,2-a]carbazole alkaloids.

We have developed a highly efficient route to 2-hydroxy-3-methylcarbazole (1) via a palladium-catalyzed construction of the carbazole skeleton. Using 1 as relay compound, different methods for annulations of pyran rings by reaction with terpenoid building blocks have been tested. The Lewis acid promoted reaction of 1 with prenal (21) opened up an efficient route to girinimbine (3) and the corresponding reaction with citral (25) afforded mahanimbine (5).

Total synthesis of the biscarbazole alkaloids murrafoline A-D by a domino sonogashira coupling/claisen rearrangement/electrocyclization reaction.

Why take things one step at a time? Aryl-pyran-linked biscarbazole alkaloids of the murrafoline group (see crystal structure of murrafoline A; dark gray: C, red: O, blue: N) were accessed readily by a novel domino reaction sequence involving Sonogashira coupling, a Claisen rearrangement, and electrocyclization. The one-pot procedure enables the straightforward synthesis of these structurally challenging alkaloids in only a few steps.

Efficient iron-mediated approach to pyrano[3,2-a]carbazole alkaloids--first total syntheses of O-methylmurrayamine A and 7-methoxymurrayacine, first asymmetric synthesis and assignment of the absolute configuration of (-)-trans-dihydroxygirinimbine.

Iron-mediated oxidative cyclisation provides an efficient approach to pyrano[3,2-a]carbazole alkaloids. Thus, improved routes to girinimbine and murrayacine as well as the first total syntheses of O-methylmurrayamine A and 7-methoxymurrayacine are reported. Asymmetric epoxidation of girinimbine led to (-)-trans-dihydroxygirinimbine and the assignment of its absolute configuration.

Palladium-catalyzed total synthesis of euchrestifoline using a one-pot Wacker oxidation and double aromatic C-H bond activation.

We describe the total synthesis of euchrestifoline featuring an unprecedented one-pot Wacker oxidation and double aromatic C-H bond activation.