Total synthesis of glycomaurrol and eustifoline-C by DIBAL-H promoted reductive ring opening of pyrano[2,3-c]carbazoles

Abstract

The palladium-catalyzed construction of the carbazole framework provides an efficient access to 3-hydroxy-6-methylcarbazole (glycozolinine) (5). Regioselective annulation of a pyran ring at glycozolinine (5) using either a C5- or a C10-building block leads to the pyrano[2,3-c]carbazole alkaloids glycomaurin (5,6-pyranoglycozoline, eustifoline-A) (2) and eustifoline-B (4), respectively. Reductive opening of the pyran ring with DIBAL-H in the presence of an additional Lewis acid transformed 2 into the 5-prenyl-substituted carbazole alkaloid glycomaurrol (1) and 4 into the 5-geranyl-substituted homologue eustifoline-C (3).