(S)-1-((S)-3-tert-Butoxycarbonyl-3-{tert-butoxycarbonyl-[(S)-3-tert-butoxycarbonyl-3-(tert-butyl-dimethyl-silanyloxy)-propyl]-amino}-propyl)-azetidine-2-carboxylic acid tert-butyl ester

tert-butyl (S)-2-(tert-butyldimethylsiloxy)-4-hydroxybutanoate

tert-butyl (S)-2-tert-butyldimethylsilyloxy-4-oxobutanoate

tert-butyl (S)-2,4-di-tert-butyldimethylsilyloxybutanoate

tert-butyl (S)-1-[(S)-3-benzyloxycarbonylamino-4-tert-butoxy-4-oxobutyl]azetidine-2-carboxylate