Structural Effects Affecting the Thermal Electrocyclic Ring Closure of Vinylallenes to Alkylidenecyclobutenes

The thermal electrocyclic ring closure of
(2E,7E)-3,4,7-trialkylnona-2,4,5,7-tetraenes
(divinyl-4,5-allenes)
is regioselective, occurring at the most sterically congested
vinylallene subunit to afford the trisubstituted
alkylidenecyclobutene. Ring closure of both divinylallenes and
vinylallenes displays high torquoselectivity (exclusive
formation of the (E)-alkylidenecyclobutenes) when the
substituent at C4 is a sterically demanding alkyl group
and
the substituent at C2 is a formyl group. Ab
initio calculations clearly demonstrate the dominant steric
influence of
the bulky C4 substituent in these selectivities.
However, torquoselectivity appears to be enhanced if
cyclization
involves loss of conjugation by a π system encompassing the
C2 substituent.