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.
Satires of Circumstance was created in 1914.
Circumstance was released on 08/26/2011.
The Production Budget for Circumstance was $900,000.
Victim of Circumstance was created in 1999.
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.