[orgo814]

I know you can easily predict whether something undergoes a conrotatory or disrotatory mechanism by simply looking at the reactants and products and if one (either reactant or product) has substituents in the plane of the molecule and the other has them with some designated stereochemistry. However, is it possible to tell what mechanism something went by if both the reactant and products have some stereochemistry on the carbon the substituents are attached to (in other words, they're both out of the plane) so you can't just "use your thumbs" to see which pathway it went by.

Hopefully I didn't make this sound too confusing- it's hard to ask over a forum. I can try and rephrase if unclear.

[clarkstill]

Can you give an example? As far as I'm aware, conrotatory and disrotatory is a nomenclature relating only to electrocyclic reactions. By definition, these always involve formation of a new sigma bond from a delocalized pi system, while shortening the pi system by 2 atoms. This means that the terminal atoms prior to cyclization must always be sp or sp2 hybridized so I don't see how "both reactants and products [can] have stereochemistry on the carbon the substituents are attached to" (assuming you don't mean E/Z stereoisomers or chiral allenes). But maybe I'm just not understanding your question...