A solution of tert-butyl N-[(benzyloxy)carbonyl]-l-methionyl-D-alaninate (11.9 g) in acetone (75 ml) was treated with methyl iodide (18 ml) and stirred at room temperature for 72 h. The reaction mixture was then concentrated under reduced pressure to give an orange solid which was dissolved in acetonitrile (200 ml). Dowex (OH− form) resin (19.42 g) was added and the mixture stirred for 18 h at room temperature. The mixture was filtered and the resin washed with ethyl acetate. The filtrate was evaporated under reduced pressure to afford a yellow oil which was purified by Biotage™ chromatography (eluting with cyclohexane/ethyl acetate 3:2) to give the title compound (2.92 g) as a colourless oil.
