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

Abstract

Alkyl or aryl a-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. g-H Abstraction from the alkyl side chain of alkyl keto esters, as well as an intramolecular PaternoÁ-Büchi reaction or epoxidation of the alkene function in different citronellyl a-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.1. Introduction. ± Aldehydes and ketones are important classes of fragrances that are present in all kinds of perfumes. However, many of them are very volatile and can, after application, only be perceived over a relatively short period of time. Furthermore, as constituents of perfumes for a variety of different bodycare or household applications, such as shampoos, soaps, all purpose cleaners, fabric softeners, or detergent powders, they are often too hydrophilic and thus easily carried away by water during various rinsing processes instead of staying on substrates like hair, skin, or fabrics.To prolong the desired odor perception of this class of compounds, we have prepared a series of photolabile, hydrophobic, non-volatile fragrance precursors as potential delivery systems for the controlled release of perfumery aldehydes and ketones in typical bodycare and household applications. In this publication, we describe the fragrance release from a-keto esters upon irradiation with artificial light sources, as well as natural sunlight in the presence of O 2 [1]. Some interesting fragrances for release in functional perfumery are depicted in Fig. 1 [2].The photolysis of alkyl or aryl a-keto esters of primary or secondary alcohols has been intensively studied since the early sixties [3 ± 7]. It was found that these esters decompose from their intermediate triplet state into aldehydes or ketones upon irradiation at 350 ± 370 nm in polar, as well as apolar solvents [7 ± 11]. Most of the photoirradiations described in the literature were carried out in degassed solutions in the absence of O 2 . The relatively good yields of the fragmentation process have allowed the use of a-keto esters as intermediates for the transformation of primary or secondary alcohols to aldehydes and ketones, respectively [12].

Profragrances and Properfumes

“…Both alkyl and aryl 2-oxoacetates were successfully used to release a series of fragrance aldehydes and ketones under realistic application conditions. 43,46 Understanding the mechanistic pathway of the photoreaction allows an optimization of the structure of the profragrance substrates on both sides of the photoreactive keto group, and thus to minimize side reactions. In addition to the desired ester chain fragmentation to generate the carbonyl compounds, alkyl a-keto esters such as 43 can also undergo alkyl chain fragmentation, resulting in another keto ester with a shorter alkyl chain (44).…”

Kontrollierte Freisetzung flüchtiger Verbindungen unter milden Reaktionsbedingungen: von der Natur zu Alltagsprodukten

“…61, 62 Die Ketoester 16 – 20 setzen die entsprechenden Aldehyde oder Ketone in guten Ausbeuten durch Photooxidation mit einer Xenonlampe oder Sonnenlicht frei 63. 64 Eine systematische Untersuchung der Photoreaktion in nicht entgaster Lösung zeigte, dass die Spaltung der Alkylkette mit der Esterkettenspaltung konkurriert. Während die gewünschte Esterkettenspaltung von 16 direkt Citronellal freisetzt, bildet die Alkylkettenspaltung zunächst Ketoester 17 , der dann in einer weiteren Esterkettenfragmentierung Citronellal freisetzen kann 64.…”

Homogeneous Catalytic Oxidation of Unactivated Primary and Secondary Alcohols Employing a Versatile “Helmet” Phthalocyaninato Iron Complex Catalyst Without Added Organic Solvent

“…Controlled oxidation reactions of primary and secondary alcohols to produce aldehydes and ketones occupy a place of fundamental importance in the field of organic synthesis, within both the academic and commercial/industrial realms . Much of the emphasis placed upon these transformations has derived from the utility of the resulting carbonyl compounds as synthons in the production of commercial products important to the fine chemicals, pharmaceutical, fragrance, and other industries . In recent years, a major point of emphasis for many investigators concerned with alcohol oxidation reactions has been the search for sustainable, environmentally benign methodologies known as “green” chemistry.…”