2,5-Dimethylphenacyl Esters: A Photoremovable Protecting Group for Carboxylic Acids†

Irradiation of 2,5-dimethylphenacyl (DMP) esters (1a−c) in benzene or cyclohexane solutions produces the corresponding free carboxylic acids (2a−c) in high chemical yields, along with 6-methyl-1-indanone (3). In methanol, 2-(methoxymethyl)-5-methylacetophenone (4) is formed as a coproduct. Quantum yields for the photorelease of the DMP group are higher in nonpolar solvents, φ ≈ 0.2, than in methanol, φ ≈ 0.1. The photoreaction is initiated by efficient photoenolization, φ ≈ 1. Three transient intermediates were identified by laser flash photolysis of 1a, the triplet-state photoenol and the two ground-state photoenols of Z and E configuration. Release of the acids 2a−c occurs predominantly from the E isomer, which has a lifetime of about 2 ms in methanol and 0.5 s in benzene solution. The present data are essential for potential applications of the DMP moiety as a photoremovable protecting group in organic synthesis or biochemistry (“caged compounds”).