Cassia isn't a synthetic (though I suppose there may be a synthetic version of it; I've never seen one). It's Cinnamomum aromaticum, also called Chinese cinnamon.
It's also one of the oils that tends to be sensitising, more so than cinnamon, in fact. It's actually on the banned list for the International Fragrance Association. It is a very nice cinnamonny fragrance, a little sharper, I think, and I don't find it as deeply complex as cinnamon bark or cinnamon leaf. It's quite nice in something like a soap, that has lots of carrier material and is rinsed away.
I had the devil of a time finding any of it. It's impossible to get in Australia. It might be banned for sale here, I'm not sure (or, maybe just nobody carries it). I had to order it from overseas.