The predominant transformations of 4-methyl- and 4-phenyl-1,3,3-tris-carbethoxycyclobutenes to s-trans,trans-1,1,3-tris-carbethoxy-4-methyl- and s-trans,trans-1,1,3-tris-carbethoxy-4-phenyl-1,3-butadienes, respectively, are discussed to proceed through pathways entailing heterolytic cleavage of the sC3C4 bond rather than the usual conrotatory ring opening following the rules of torquoselectivity. The adventitious or in situ generated halogen acid from CDCl3 catalyzes the reaction by protonation

more »

... of the geminal ester group to weaken sC3C4 bond and allow its SN2 cleavage by chloride ion. This is followed by cisoid→transoid isomerization and loss of the elements of halogen acid to form the products. In the Lewis acid-catalyzed reaction of 4-phenyl-1,3,3-tris-carbethoxycyclobutene in CH2Cl2, coordination of Lewis acid with the geminal ester group is followed by heterolytic cleavage of the sC3C4 bond. The resultant species subsequently undergoes cisoid→transoid isomerization before losing the Lewis acid to form the products.