Figure 2. Removal of the thioamide directing group.
The directing group can be removed in two steps. The first step involves the reduction of 2a in presence of NiCl2 and NaBH4 to afford the dethiolated product 5. The second step involves the debenzylation of 5 in presence of BCl3 to afford the free amine, followed by Boc protection to give 6 in 58% yield and 98:2 er.