The U.S. in turn promised to separate the "moderate rebels" it supported from their close al-Qaeda ally. But that promise was never fulfilled. The U.S. delayed and delayed and enabled new attacks by al-Qaeda and U.S. proxy forces on Aleppo and elsewhere. It also instigated Kurdish YPG forces to attack the Syrian government in Hasakah. This was the strategic overreach that doomed the Kurds to a minor role in any upcoming solution.
Russia finally had enough and pulled the plug on further negotiations with the U.S. The Syrian forces and their allies besieged Aleppo and liberated it. At the same time Turkey was allowed to invade Syria in the north eastern Aleppo governate to prevent a connection of Kurdish areas in the north-east and north-west of Syria. The probably U.S. instigated putsch attempt against the Turkish government contributed to this change of alliances. Turkey decided to work with Russia and Iran to end the Syrian crisis (and to get as much out of it as possible.)
Before Christmas Erdogan met with Putin followed by a meeting of the Russian, Turkish and Iranian foreign and defense ministers.