(S)-5-allyl-4-tert-butoxycarbonyl-1-(2,4-dimethoxybenzyl)-2-piperazinone

(S)-4-tert-butoxycarbonyl-1-(2,4-dimethoxybenzyl)-5-(2-oxoethyl)-2-piperazinone

1(S),5(S)-1-benzyl-8-tert-butoxycarbonyl-3-(2,4-dimethoxybenzyl)-3,8-diaza-2-oxobicyclo[3.2.1]octane

5(S),3(S)-3-benzyl-4-tert-butoxycarbonyl-1-(2,4-dimethoxybenzyl)-5-(2-oxoethyl)-2-piperazinone

(S)-4-tert-butoxycarbonyl-1-(2,4-dimethoxybenzyl)-5-(2-hydroxyethyl)-2-piperazinone