Thioanisole → Methyl (3s,7as)-6-benzyloxycarbonylamino-5-oxohexahydropyrrolizine-3-carboxylate

1500 mg of 1-tert-butyl 2-methyl (2S,5S)-5-(2-benzyloxycarbonylamino-2-methoxycarbonylethyl)pyrrolidine-1,2-dicarboxylate were mixed with 0.2 ml of thioanisole. Then 10 ml of trifluoroacetic acid were added, and the mixture was stirred for 45 min. The solvents were then removed in vacuo, and the residue was taken up in 10 ml of pyridine. 1.1 ml of diisopropylethylamine were added, and the mixture was heated at 95° C. for 3 h. The solvents were removed in vacuo, and the residue was chromatographed on silica gel.