...sorry: it's the Russian sponsored negotiations in Sochi, not Astana, they're refusing.

Of course, the Russians are going to stage a big conference there - including 'even' numerous 'members of the opposition to Assad'.

However, characters in question are going to be representatives of that part of the Syrian opposition, the Assadists are ready to tolerate already since 2011 - i.e. not a single representative of any of insurgent groups.