Bashar Assad would negotiate a peaceful ending to the Syrian Civil War if there were enough financial benefits in it for his regime and rule. Many countries, such as Pakistan, have responded to calls for peace from the outside with vitriol until a financial deal was reached. Bashar Assad would, for the right price, end the Syrian war.
The only circumstance by which Bashar al-Assad can negotiate a peaceful endings to the Syrian civil war is an immediate and unconditional surrender to rebel forces. At that point, the rebels must bring him to justice rather than kill him. However, that scenario won't happen. Assad's regime will end with his death as opposed to his surrender.
I believe there are circumstances under which Bashar Assad would negotiate a peaceful ending to the Syrian Civil War. Personally, I believe he has made some bad choices in quelling the violence in his own country, but that doesn't mean that his arguments that they are caused by enemies is false. I believe if the government was sanctioned less and listened to more, this could probably be ended with little help from foreign countries.
No, I do not think that there are any circumstances under which Bashar Assad would negotiate a peaceful ending to the Syrian Civil War, because Assad knows that he is in for serious punishment when this is over. With some many human rights abuses, Assad cannot agree to end the war, because he knows there is judgment waiting for him.
No, this leader is after one thing and one thing only, war. He is a very violent man who wants to take power over his enemy, and will fight until he is either stopped by death or gets what he wants. He is not going to accept a peaceful ending.