It is mainly Kant who is the thinker of synthesis. For Kant, synthesis is partly a moral ideal, and partly something we are always already caught up in, and need to deconstruct.
The Žižekians are right that Hegel specialized in pointing out how Kantian syntheses break. I’ve written at greater length about my own Brandomian take on this latter point under Error.
So-called absolute knowledge is actually a deconstruction, not a synthesis.