A Short Synthesis of Conformationally Constrained Unnatural Bicyclic Amino Acids Containing a Nitrogen Atom at the Ring Juncture: (9aR)- and (9aS)-Octahydropyrido[1,2-a]pyrazine-(3S)-carboxylic Acids (Opc) - So, Soon Mog; Yeom, Chang-Eun; Cho, Seong Mi; Choi, Soo Young; Chung, Young Keun; Kim, B Moon (Synlett, 2008)