In a two-neck 50 mL flask was combined 0.55 mL (8.42 mmol, 1.2 equiv.) of methanesulfonic acid and 20 mL of anhydrous toluene under N2 and a Dean-Stark trap. The solution was heated to reflux for 90 minutes, then cooled, and the trap removed. Triphenylphosphine (2.21 g, 8.42 mmol, 1.2 equiv.) was added with stirring, the solution was cooled to 0° C. (ice water bath), and 1.93 mL (9.83 mmol, 1.4 equiv.) of diisopropyl azidodicarboxylate was added slowly via syringe over 15 minutes. The bright yellow slurry was warmed to RT, compound 15 added (2.0 g, 7.02 mmol), then triethylamine (0.39 mL, 2.81 mmol, 0.4 equiv.), and the resulting yellow slurry was heated to ˜70° C. for two hours. The mixture was then cooled to room temperature, diluted with ˜25 mL EtOAc, and a white precipitate filtered off. The filtrate was concentrated, redissolved in EtOAc, loaded onto silica, and eluted with 2:1 EtOAc/hexanes to give 0.63 g (1.73 mmol, 59% yield based on 1.16 g of recovered starting material) of 16 as a colorless glass which crystallized on standing. 1H-NMR (400 MHz, CDCl3, δ ppm) 7.89 (dt, J=2 Hz, 7 Hz, 2H), 7.60 (tt, J=1 Hz, 8 Hz, 1H), 7.53 (td, J=2 Hz, 7 Hz, 2H), 5.14 (m, 1H), 4.57 (dd, J=3 Hz, 10 Hz, 1H), 3.71 (m, 1H), 3.69 (s, 3H), 3.62 (dq, J=1 Hz, 12 Hz, 1H), 2.94 (s, 3H), 2.52 (dq, J=1 Hz, 15 Hz, 1H), 2.32 (m, 1H); 13C-NMR (400 MHz, CDCl3, δ ppm) 171.47, 138.42, 133.67, 129.61, 127.94, 78.23, 59.33, 53.88, 53.07, 39.30, 37.41; m/z: 364 (M+1)+. See, for example, Anderson, N. G., et al.; J. Org. Chem.; 61(22), pp. 7955–7958 (1996), which is incorporated by reference in its entirety.
