The scheme below shows the synthesis of a late-stage intermediate towards PF-06826647, a compound identified by Pfizer as a potent and selective tyrosine kinase 2 (TYK-2) inhibitor that has entered phase 2 clinical trials for the treatment of autoimmune diseases. Can you identify intermediates A, B and D? Answers and a reference will be posted at the end of August, along with another problem. Enjoy!