Are complex, synthesis, compound, composite all synonyms?

No, I cannot think of a single instance in which they are all synonymous.

But do they all carry atleast somewhat the same meaning?

Generally, yes-- they all speak of mixtures. 'Synthesis' stands as the odd man out in that it carries a greater meaning of A+B = C, where the others are just A+B = A,B (so to speak).

Can you explain what you mean, do you mean that synthesis is a uniuely formed compound, and the other words imply compounds that are not unique?

No; only 'synthesis' suggests that a 3rd different substance or entity is created; the others speak only to the mixture of substances. (I am speaking generally, of course.)