Some advice from author Mike Wells:
The Green Water Blog: A "Secret" Formula for Creating a Short Synopsis for Your Book
The synopsis isn't carved in stone. Go ahead and write one as early in the process as you can. Revisit it from time to time.
One advantage of doing it early: when people ask you what your story is about, you're more likely to tell them something interesting. More content, less hand-waving.
What do you do if there isn't a central hero?
How does this work if your book has subplots with different protagonists?