To a stirred solution of 6-formyl-benzo[b]thiophene-2-carboxylic acid (1-isobutoxy-ethoxy) amide (Scheme 13) (0.128 g, 0.40 mmol) in DCE (5 ml) was added (S)-4-(4-aminomethyl-phenoxy)-2-tert-butoxycarbonylamino-butyric acid cyclopentyl ester (0.156 g, 0.40 mmol) followed by sodium triacetoxyborohydride (0.253 g, 1.1 g mmol) and glacial acetic acid (0.023 ml, 0.40 mmol). After stirring for 18 hours the mixture was quenched using saturated aqueous NaHCO3 solution (5 ml). The organic layer was separated, washed further with saturated aqueous NaHCO3 solution (2×5 ml), dried (MgSO4) and the solvent removed in vacuo to give a yellow residue. The residue was redissolved in a TFA/DCM solution (1:1, 5 ml). After stirring the solution for 45 minutes the solvent was removed in vacuo. Purification by preparative HPLC gave the product as a white solid (0.030 g, 15% yield). LCMS purity 96.1%, m/z 498 [M++H]+, 1H NMR (300 MHz, DMSO), δ: 1.50-1.90 (9H, m, CH2×4, CH), 2.25 (2H, m, CH2), 4.10-4.40 (7H, m, CH2×3, CH), 5.20 (1H, m, CH), 6.95 (2H, d, ArH×2, J=8.6 Hz), 7.45 (2H, d, ArH×2, J=8.6 Hz), 7.55 (1H, d, ArH, J=7.8 Hz), 8.00 (2H, m, ArH×2), 8.15 (1H, s, ArH), 8.40 (2H, br s, ArH×2), 9.20 (2H, m ArH×2).
