- Gérard Berry: "Real time programming: Special purpose or general purpose languages"

For the semantics, I like this paper focusing on abortion (for me, the most expressive construct of synchronous languages):

- Gérard Berry: "Preemption in Concurrent Systems"

There's also a well-known survey:

- Albert Benveniste: "The synchronous languages 12 years later"

This one is about bridging the gap between synchronous and asynchronous, something like Esterel+CSP:

- Gérard Berry: "Communicating reactive processes"

For a modern take (besides Céu), check ReactiveML:

- Louis Mandel: "ReactiveML: a reactive extension to ML"

You can also search for papers from professors Stephen A. Edwards and Reinhard von Hanxleden which still work actively on the subject.