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