For contrasting ideas:
* But
* However
* On the other hand
* In contrast
* Nevertheless
* Conversely
* While
* Yet
* Despite
* Although
* Though
For introducing a statement of fact:
* While it is true that
* It is a fact that
* In light of the fact that
* Given that
For introducing a different point of view:
* On the contrary
* Conversely
* By comparison
* By contrast
For introducing a formal or legal statement:
* Whereas (this is often the most appropriate synonym in legal writing)
* Considering that
* Taking into account
The best synonym for "whereas" will depend on the specific context of your sentence. Consider the tone, the type of contrast you're trying to make, and the overall flow of your writing.