Axial Chirality or Centre of Symmetry in Diamides (diketopiperazine)

The syn diastereoisomers (SS and RR) have no plane of symmetry but have a C2 axis of symmetry running straight through the middle of the ring.

Click on the structures to display them in 3D and try to arrange the structures so that they can be superimposed

The anti diastereoisomer (RS) has no plane of symmetry, nor does it have an axis. Instead it has a centre of symmetry.