You can find all the genuine replacement parts you need for your Cannondale at Escapa.
The Cannondale 28.6mm Seat Clamp is an original replacement part for the Synapse Alloy.
Cannondale Synapse Neo, Synapse Alloy Saddle Clamp, 28.6mm
Fits the Synapse Alloy with a 25.4mm seatpost