Controlled Release of Perfumery Aldehydes and Ketones by Norrish Type-II Photofragmentation of -Keto Esters in Undegassed Solution

Abstract

Alkyl or aryl α-keto esters of primary or secondary alcohols decompose upon irradiation at 350 – 370 nm from the intermediate triplet state into aldehydes or ketones in polar, as well as apolar solvents. The use of these keto esters as delivery systems for the controlled release of perfumery aldehydes and ketones was investigated by photoirradiation in the presence of oxygen with a Xe or UV lamp, as well as outdoor sunlight. Systematic GC/MS analysis of the irradiated solutions showed that, under these conditions, the desired Norrish type-II fragmentation of the ester side chain is the predominant reaction pathway in most of the cases. γ-H Abstraction from the alkyl side chain of alkyl keto esters, as well as an intramolecular Paternò-Büchi reaction or epoxidation of the alkene function in different citronellyl α-keto esters were identified as the most important side reactions. Some of the experimental findings have been rationalized on the basis of ab initio and density-functional calculations. (Cyclohexyl)oxoacetates and oxo(phenyl)acetates were found to be the most suitable precursors for the desired perfumery applications.