The photochemical
rearrangement of 2-methylcyclobutane-1,3-dione and the ethyl ether of its enol
(4) have been shown to follow the expected pathway for an α, β-unsaturated
cyclobutenone. The enol ether, when irradiated in moist cyclohexane, forms
3-ethoxy-2-methyl-trans-crotonic acid
(8) which is photo-isomerized to the cis-isomer
(7). This latter isomer is the major product when the solvent is moist diethyl
ether. It is suggested that the necessary rotation about a carbon-carbon single
bond is solvent-dependent,
and occurs in a β,γ-unsaturated carboxylic acid formed as a reaction
intermediate.
Mechanistic Analysis and Visualization of Solid-State Reactions of Unsaturated Carboxylic Acid Derivatives