Torquoselectivity

"[1] Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic reactions, as it represents a further level of selectivity beyond the Woodward-Hoffman rules.

In a typical electrocyclic ring closing, selection for either conrotatory or disrotatory reactions modes still produces two enantiomers.

Torquoselectivity is also used to describe selective electrocyclic ring openings, in which different directions of rotation produce distinct structural isomers.

[2] Other mechanisms by which torquoselectivity can operate include chiral Lewis acid catalysts, induction via neighboring stereocenters (in which case the torquoselectivity is a case of diastereoselectivity), and axial-to-tetrahedral chirality transfer.

An example of the latter case is shown below for the torquoselective Nazarov cyclization reaction of a chiral allenyl vinyl ketone.

Conrotatory and disrotatory modes of rotation each showing two possible directions of rotation that result in pairs of enantiomers for a generic hexatriene system. Note: in the case shown, there is no reason for the reaction to be torquoselective and both products would be expected for any particular set of conditions.
Thermal electrocyclic ring opening of a cyclobutene giving selectivity via steric strain
Torquoselectivity via axial to tetrahedral chirality transfer in the Nazarov cyclization of allenyl vinyl ketones