Synthesis of Dragmacidin D via Direct C-H Couplings
Debashis Mandal, Atsushi D. Yamaguchi, Junichiro Yamaguchi, and Kenichiro Itami
I thought about writing this review a few months ago but never found the time to get it done. But here it is and I hope you enjoy this cool paper as I did. I am a big fan of “flat” chemistry and C-H activation so naturally this piece had to be reviewed. Dragmacidin D itself shows some promising activity in the treatment of neurodegenerative diseases like Alzheimer’s or Parkinson’s disease. Only one total synthesis has been published by the Stoltz group in 2002 so there is still room for improvement. Retrosynthetically the group planned to stick all parts together via C-H activation/C-C coupling reactions.
The “sticky” –positions are marked in blue. As can be seen from this picture almost all crucial bonds can be formed through C-H activation (except the iodide).
Indole 1 was carboxylated to block the 3 position of the indole which would normally undergo iodination in the presence of NIS. Removal of the carboxyl group after halogenation gave indole 3. Tosylation was accomplished under standard conditions yielding the first coupling partner 4. Thiophene boronic acid 5 was oxidized to the corresponding alcohol to furnish after TIPS protection coupling partner 6. In the presence of PdII and silver(I) as the re-oxidant thiophene 7 formed in good yield on a gram scale. Desilylation and reductive desulfuration with Raney-Ni was followed by global deprotection and double MOM-protection to produce ketone 8. 
Next the 3 position of the indole moiety was again functionalized. This time pyrazine N-oxide was used in the presence of PdII to give 9. A TFAA mediated Polonovski-Potier rearrangement gave pyrazinone 10 which was used in a Friedel-Crafts-like acylation with 6-bromoindole to furnish 11 in good yield. Bromination of the ketone was accomplished via the TMS-enolate and NBS mediated bromination yielding 12. In the presence of Boc-guanidine Dragmacidin D was formed after deprotection.
Short and very efficient I would say. The only drawback might be that Dragmacidin D is formed in both enantiomeric forms. I was wondering how much silver the group stores in their laboratories J.
 Really a nice method to introduce the side chain on the indole. The net result is the reaction of an umpoled ketone with the aromatic ring.