I'd ask an attorney to review the MoU and any corporate formation documents. As well as which state you are in and any obligations you may have if you remain in an equity position over a certain amount.
Overall though, from what I know and IANAL, you are not obligated to sign any documents on the way out. However, there could be ramifications like the forfeiture of your equity (if that was in the original agreement etc). So that's why I suggest letting an attorney review all the documents. My bet though is if you didn't have all the standard non-compete etc up front then they don't exist and you are probably safe to walk away without signing it, losing the board seat of course.
Overall though, from what I know and IANAL, you are not obligated to sign any documents on the way out. However, there could be ramifications like the forfeiture of your equity (if that was in the original agreement etc). So that's why I suggest letting an attorney review all the documents. My bet though is if you didn't have all the standard non-compete etc up front then they don't exist and you are probably safe to walk away without signing it, losing the board seat of course.
Good luck.