In a 10 mL round-bottomed flask, (S)-methyl 7-((S)-2-(methoxycarbonylamino)-3-methylbutanoyl)-1,4-dioxa-7-azaspiro[4.4]nonane-8-carboxylate (150 mg, 436 μmol) was combined with tert-butanol (250 μl) to give a colorless solution. LiOH (1M, 871 μl, Eq: 2) was added and stirred at room temperature for 2 hr. It was acidified with 1N HCl to pH 3 and diluted with 100 ml of EtOAc. It was washed with brine (5 ml), dried with MgSO4, filtered and concentrated in vacuo to afford (S)-7-((S)-2-(methoxycarbonylamino)-3-methylbutanoyl)-1,4-dioxa-7-azaspiro[4.4]nonane-8-carboxylic acid as a white solid which was used for the next reaction without further purification. (75 mg, 52.1%).
