electrocyclic reactions
There is a type of reaction called Electrocyclic. It is of the pericyclic reactions, known for their conserted mechinism. An electrocyclic reaction can form a ring or break a ring. Can also make a ring into a double ring.
These reactions are stereospecific- the Woodward-Hoffman rules predict which products are "allowed".
Though these reactions disallow certain products, stereoisomers can still form. That prefferece can be inferred form substituent effects - for that, read some papers by K.N. Houk. He, along with others, have studied these reactions (and more) using computational chemistry tools.
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