TORQUOSELECTIVITY ON THE THERMAL ELECTROCYCLIC RING-CLOSURE OF VINYLALLENES TO ALKYLIDENECYCLOBUTENES

In addition to their described peri- and regioselectivity, the electrocyclic ring closure of (8E,13E)-12-tertbutyl-11,7-retroretinal 8 is also torquoselective, leading exclusively to alkylidenecyclobutenal E-14. Steric (and perhaps, electronic) factors govern the torquoselectivity of the process, as confirmed by ab initio study at the 6-31G*MP2//6-31G* level. For the structurally related silyl ether 6 and alcohol 7, the cyclization is moderately torquoselective (E/Z-alkylidenecyclobutenes are obtained in a similar to 5:1 and similar to 6:1 ratio, respectively). The process lacks torquoselectivity (a similar to 1:1 mixture of the E/Z alkylidenecyclobutenes) when the substituent at C-12 is a methyl group.