Methods have been developed for the stereocontrolled synthesis of bicyclic diaza [3.3.0] octane carboxylic acids as possible isosteres of piperazine 2S-carboxylic acid. In the first approach, l-pyroglutamic acid was functionalized adopting new as well as documented reaction sequences via Michael and aza-Michael reactions, leading to two of the four intended isosteres. An alternative shorter route relying on enolate chemistry starting with N-Pf 4-keto l-proline methyl ester led to two other isosteres. Calculated pKa values and density functional theory (DFT) calculations have provided some insights into the relative basicities of the nitrogen atoms in these diaza [3.3.0] octane carboxylic acids in relation to piperazine 2S-carboxylic acid.