You know... If I put on a big tin foil hat:
I have little doubt Putin would not hesitate to arrange the gassing of civilians in order to embarrass Obama and weaken the U.S. The situation in Syria is so chaotic, I suspect such an attack could be pulled off without Russian involvement being found out. Just add a few chem shells to an ongoing Syrian bombardment or one the Russkies knew was going to occur. Cheese in the trap Obama built for himself...
Sticking point is that the so called rebels have already used gas.
Three or four times already.
Chlorine was the gas of choice for two of those.
The media trumpeted each gassing, then dumped it down the memory hole the second credible data began showing the so called rebels doing the deed.
If the Rus did it, it would have looked like Assad personally urinated chem weapons from the side door of a helicopter while raining flaming puppies down on the stricken.