To a stirred solution of the peptide of Example 83 (536 mg, 1.00 mmol) in DMF (3 mL) was sequentially added phosphonoacetic acid 79 (1.00 mmol), HOBt (135 mg, 1.00 mmol) and PYBOP (520 mg, 1.00 mmol) at room temperature. DIPEA (523 μL, 3.00 mmol) was then added and the mixture was stirred at room temperature for 18 h. The reaction mixture was poured onto an aqueous solution of NaHCO3 (5%) and extracted several times with ethyl acetate. The combined organic extracts were washed with NaHCO3 (5%), H2O, KHSO4 (5%) and H2O. Drying (Na2SO4) and evaporation gave the crude product (780 mg) as a light oil. Flash chromatography (SiO2, ethyl acetate:hexane (2:1) to 100% ethyl acetate) gave a separable pair of diastereoisomers in a combined yield of 71%. The less polar diastereoisomer had the following spectroscopic data: 1H NMR (CDCl3, 400 MHz) □ 8.03 (br d, 2H), 7.61 (br d, 1H), 7.40–7.11 (m, 19H), 6.82 (d, 1H), 5.15–4.82 (m, 5H), 4.87 (dd, 1H), 4.01 (m, 4H), 3.31 (d, 2H), 3.08 (dd, 1H), 2.78–2.43 (m, 4H), 2.35 (m, 1H), 2.00 (m, 1H), 1.26 (dt, 6H); 31P NMR (CDCl3) δ 25.1; MS (Pos. FAB) 780.8 (M+).
