Selective functionalization synthesizes chemotherapeutic natural products

Dictyodendrins share a common structure, consisting of a highly substituted pyrrole core, i.e. a five-membered aromatic ring containing a nitrogen atom. Arising from their complex structure and potent biological activity, this family of molecules has attracted much attention from the scientific community. The groups of Davies and Itami have worked on synthesizing dictyodendrins through combining their expertise on C-H functionalization chemistry, which involves substituting the hydrogen atom attached to a carbon by another functional group. This was achieved by the collaboration between two graduate students, Katie Chepiga from the Davies group and Atsushi Yamaguchi from the Itami group, who both participated in an exchange program at each other's institute. The two graduate students speak of their experience on accomplishing the total synthesis of the two dictyodendrins.

"Originally, Atsushi had started to work on the synthesis of dictyodendrins at Emory. We wanted to functionalize the molecules at all four of the C-H bonds on the pyrrole ring to make the core of dictyodendrin. Atsushi had first optimized the first C-H arylation reaction with the Itami group chemistry and he had worked on optimizing some Davies group chemistry to apply the first carbenoid C-H functionalization reaction. My role was to optimize the two rhodium carbenoid C-H functionalization reactions, which is the Davies group chemistry," says Chepiga.

"I was involved in carrying out the direct C-H arylation of the pyrrole ring and the total syntheses of the dictyodendrins. The double C-H insertion reaction optimized by Katie was necessary for the synthesis of these compounds, so without her help, I couldn't have completed this project," says Atsushi Yamaguchi.

In order to attach different functional groups at specific positions on the pyrrole core, the sequence and selection of coupling reactions are highly significant. As the molecule becomes more complex, the molecule changes its reactivity and spatial environment, making the following reactions more challenging. Therefore, developing appropriate conditions with high reactivity and regiocontrol were crucial to achieve the total synthesis of dictyodendrins. "After extensive screening of catalysts and conditions, it was found that the Davies' group and our catalyst were the best to achieve the two types of C-H functionalizations that enabled selective installation of the desired functional groups," say Profs. Itami and Junichiro Yamaguchi, who led this research. Through sequential C-H functionalization of the pyrrole core, an efficient formal synthesis of dictyodendrin A and F was achieved in relatively a short number of linear steps.

This work is an outcome from the collaboration between Profs. Davies and Itami. Prof. Davies' lab at Emory University is a member site of the National Science Foundation's Center for Selective C-H Functionalization (CCHF) in the United States and Prof. Itami's lab at Nagoya University is a member site of the World Premier International Research Center Initiative's Institute of Transformative Bio-Molecules (ITbM). The CCHF and ITbM are currently partner institutes in the Virtual Institute for C-H Functionalization (VICHF), exchanging students/faculty and carrying out collaborative projects. Atsushi, Katie and Prof. Davies comment on this international collaboration that led to this study.

"Prof. Itami suggested this exchange, as he knew Prof. Davies well. At that time, the connection between ITbM and CCHF was not as strong and this was the first opportunity for collaboration between the two centers," says Atsushi, who was the first member to be involved in this exchange program.