Scheme 5 Reagents and conditions: (a) MeOH, CH(OMe)₃ (14 equiv), TsOH (0.15 equiv), 50°C, 24 h; PTT (2.1 equiv), THF/MeOH/CH(OMe)₃ 2:1:0.1, 20°C, 2 h; AcOH(aq), 50°C, 12 h, 92%; (b) NaN(CHO)₂ (6 equiv), MeCN, 18 h, 20°C; (c) MeOH, HCl (1% v/v), 16 h; (d) H₂NCN (80 equiv), H₂O, 140°Cμwave, 3 min, 80%.PTT = phenyltrimethylammonium tribromide, THF = tetrahydrofuran.