Total synthesis of pyrano[3,2- e ]indole alkaloid fontanesine B by a double cyclization strategy.

The regioselective synthesis of pyrano[3,2- e ]indole alkaloid fontanesine B by two different cyclizations is described. The complete regioselectivity is controlled by the C4 Pictet-Spengler cyclization, in which an iminium ion acts as a transient directing (TDG) group. Furthermore, carbolines were constructed by a new Bischler-Napieralski-type cyclization, in which an unprecedented trichloromethyl carbamate serves as a reactive group.