Assad "described opposition officials chosen for the constitutional committee as "agents" of Turkey, which backs anti-Assad rebel factions in northwest Syria." (Source)
He's right. What he didn't say is that he refuses to bargain with anyone over the future constitution. He simply has lines he isn't willing to cross and shouldn't.
He's not being unrealistic. He knows that the future constitution has to be generally acceptable under UN standards. Assad is not holding out for some framework making him dictator for life or anything of the kind. He's ready and willing to completely agree to a secular, democratic structure and to live by it.