Scheme 4. On-DNA “Recipe” Synthesis of 20 Using the THP Linker Followed by Release of Compound 21.

Reagents and conditions: (a) 4-fluoro-3-nitrobenzoic acid, DMTMM, DMA, pH 9.4 sodium borate buffer, rt, 2 h; (b) 2-methoxyehtan-1-amine, acetonitrile/water = 50:50, pH 9.4 sodium borate buffer, 60 °C, 1 h; (c) FeSO₄, pH 9.4 sodium borate buffer, 10% v/v 1 M NaOH, 80 °C, 2 h; (d) DMA, pH 5.5 sodium phosphate buffer, rt, o/n; (e) formic acid (0.1%), 60 °C, 1 h. (%RBA = Relative Binding Affinity by AS-MS.)