Synthesis of (S)-(-)-Cucurbitine and Conformation of Its Homopeptides.

A chiral cyclic α,α-disubstituted α-amino acid, (S)-(-)-cucurbitine, which has a pyrrolidine ring with a chiral center at the α-position, was synthesized, and its homopeptides were prepared. (S)-(-)-Cucurbitine homopeptides with a Boc-protecting group formed helical structures with slight control of the helical screw sense to the left-handed form. The state of the pyrrolidine ring in (S)-(-)-cucurbitine was important for the control of the helical structures and helical screw sense of its homopeptides.