In organic chemistry lecture we discussed the beginning of the Woodward Hoffmann Rules with a key step of the vitamin B12 synthesis.
(The initial paper from their work can be found here: http://pubs.acs.org/doi/pdf/10.1021/ja01080a054)
Let's first see the forward reaction (the electrocyclic ring closure) as Woodward and Hoffmann found (which goes by disrotatory cyclization; 6 electrons, thermal).
I'm interested in the reverse reaction, the ring opening (6 electrons, and therefore disrotatory:
I'm thinking now about the torquoselectivity of the ring-opening (because in an exercise we had an example with 1-Ethyl-1-ethoxy(EDG)-cyclobut-2-ene).
Is there a rule how to see why the E (E: Electron withdrawing group) is in trans position when I start with the product?
I'm thinking if they are some stereoelectronic effects here which explain the obtained selectivity:
See also: http://pubs.acs.org/doi/abs/10.1021/jo00244a058 (Electronic control of the stereoselectivities of electrocyclic reactions of cyclobutenes against incredible steric odds)
Well here we have a conrotatory ring-opening. I don't know if they are still valid in our 6e case for disrotatory ring-opening of the reaction from W. and H..