Thiourea 4 (100 mg, 0.31 mmol, 1 equiv.) was stirred in dry CH2Cl2 (5 mL) under N2 atmosphere and acid 31 (106 mg, 0.46 mmol, 1.5 equiv.) was added, followed by bromo-tris-pyrrolidino phosphoniumhexafluorophosphate (214 mg, 0.46 mmol, 1.5 equiv.), and N,N-diisopropylethylamine (53 μL, 0.31 mmol, 1 equiv.). The reaction mixture was stirred at rt for 16 h then diluted with CH2Cl2 (20 mL) and washed with 1 M aqueous hydrochloric acid (1×10 mL), 1 M aqueous sodium hydroxide (1×10 mL), and saturated brine (1×10 mL). The organic phase was dried over MgSO4 and concentrated in vacuo to give a crude yellow oil which was purified with column chromatography on silica (25-50% ethyl acetate/petroleum ether) to give 32 (86 mg, 54%) as a colourless oil.
