To a solution of 2a (280 mg, 0.50 mmol) in DMF (4.8 mL) and methanol (4.8 mL) were added successively palladium (II) acetate (12.0 mg, 0.05 mmol), dppf (54.0 mg, 0.10 mmol) and triethylamine (0.15 mL, 1.10 mmol). The reaction mixture was saturated with carbon monoxide and then heated to 80° C. under a carbon monoxide atmosphere (balloon) for 18 h. After cooling to room temperature, the reaction mixture was poured into water (20 mL) and extracted three times with EtOAc. The combined organic extracts were washed three times with water, dried (MgSO4) and concentrated. Flash chromatography of the residue (silica gel; hexane/EtOAc 9:1) give a solid (122 mg) that was triturated with ether/hexane to provide 2b as a solid (62 mg), m.p. 106-107° C.
