1: 1.) n-Bu2SnO / 1.) MeOH, reflux, 30 min.; 2.) DMF, 4 deg C 2: 85 percent / 4-dimethylaminopyridine, triethylamine / CH2Cl2 3: 85 percent / hexamethylphosphoric acid triamide / Ambient temperature 4: 68 percent / triethylamine / methanol / 4 h View Scheme

1: 1) dibutyltin oxide, 2) Et3N / 1) MeOH, 30 min, reflux, 2) acetone, 10 min 2: 94 percent / NH4OAc / dimethylformamide / 2 h / 80 - 85 °C 3: 72 percent / 3M NaOH / ethanol / 10 h / 75 - 80 °C / further reag.: NaOMe in MeOH, 95 deg C, 3 d View Scheme