Torquoselective Ring Opening of Fused Cyclobutenamides: Evidence for a <i>Cis,Trans</i>-Cyclooctadienone Intermediate

Electrocyclic ring opening of 4,6-fused cyclobutenamides <b>1</b> under thermal conditions leads to <i>cis,trans</i>-cyclooctadienones <b>2</b>-<i>E</i>,<i>E</i> as transient intermediates, en route to 5,5-bicyclic products <b>3</b>. Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving <i>cis,trans</i>-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.