Stereochemical selectivities in the electrocyclic valence isomerizations of cyclobutenones and 2,4-cyclohexadienones

Theoretical Predictions of Substituent Effects on the Thermal Electrocyclic Ring Openings of Cyclobutenones

“…Later that year, Baldwin and McDaniel studied the thermal and photochemical reactions of 2,4-dichloro-3-phenylcyclobutenone, 2 . The vinylketene intermediate was trapped by nucleophilic addition of methanol.…”

Cyclobutenones and Benzocyclobutenones: Versatile Synthons in Organic Synthesis

“…In 1967, Baldwin and co‐workers studied the ring opening of 2,4‐dichloro‐3‐phenylcyclobutenone 73 (Scheme ) . Under thermal conditions, E ‐β,γ‐unsaturated ester 74 was provided; in contrast, under irradiation by a high‐pressure mercury lamp, Z ‐β,γ‐unsaturated ester 75 was afforded.…”

“…In 1967, Baldwin and co-workers studied the ring opening of 2,4-dichloro-3-phenylcyclobutenone 73 (Scheme 25). [30,31] Undert hermalc onditions, E-b,g-unsaturated ester 74 was provided;incontrast, under irradiationbyahigh-pressuremercury lamp, Z-b,g-unsaturated ester 75 was afforded.The stereoselectivity was tentatively explained by the calculated excited state of the reaction. Based on Kikuchi's work, [32] the n-p*s ingletexcited state could lead to ap erpendicular diradical, which caused an inward roation.…”

On the mechanism of ring opening in 2-pyrone irradiations

“…This electrocyclic process would seein to be taking place under very mild conditions, but such transformations under not much more severe conditions have already been noted (7,12,13). Since one cannot be certain that rotational isomerization is not rapid relative to protonatioil on carbon, the stereochemistry of the final product reveals nothing about the stereochemistry of the ring opening.…”