don't think there is one...
Circumstances are things that are set in place or situations that occur. An antonym would be Volition.
You could say an antonym for circumstances is "plans"; circumstances generally refers to the position you are in given "the circumstances" - like variables that were subject to change. Whereas a plan is considered to be concrete, a course of action that has been predetermined.
The antonym of the word 'synonym' is 'antonym'.
That's a completely different circumstance.
Circumstance was released on 08/26/2011.
The Production Budget for Circumstance was $900,000.
Victim of Circumstance was created in 1999.
Satires of Circumstance was created in 1914.
An antonym means "opposite". So an antonym for the word synonym is antonym.
"They happened to be partnered together by circumstance."
I will make an exception in this circumstance, with certain reservations.
The antonym for from is to. Because antonym means opposite.