Solerone

Sotolon (4,5-dimethyl-3-hydroxy-2(5H)-furanone) and solerone (4-acetyl- y-butirrolactone) were claimed to be responsible for some aroma characteristic of flor sherries wines. These compounds are present only as traces, and are chemically unstable. A system of two gas chromatographs coupled with a four-port switching valve was used to quantitate these components without previous fractionation. The first chromatograph was equipped with an on-column injector, in order to avoid thermal degradation of sotolon in the heated injector, a DB-5 column and an FID. The second chromatograph was equipped with an on-column injector, a DB-1701 column and an FID. The method allowed quantification of solerone and sotolon at concentrations as low as a few ppb (29). 

B. Martin and P. Etievant, Quantitative determination of solerone and sotolon in flor shenies by two dimensional capillar y GC , 7. High Resolut. Chromatogr. 14 133-135 (1991). 

Other lactones detected include a-butyrolactone and pantolactone (2,4-hydroxy-3,3-dimethylbutyrolactone), both of which are typical of sherries. Besides the duration of aging, their concentration is closely linked to yeast strain (Zea et al., 1995). Another lactone, solerone (4-... 

Wines aged under a flor film typically contain various compounds especially important among which are lactones. Thus, solerone (4-acetyl-y-butyrolactone) abounds mfino and Jura wines and has for years been deem one of the greatest contributors to their aroma profiles (Augustyn et al. 1971 Muller et al. 1973). However, subsequent studies have suggested that this lactone has no impact on wine aroma (Martin and Etievant 1991). 

Augustyn, O.P.H., Van Wyk, C.J., Muller, C.J., Kepner, R.E., Webb, A.D. (1971). The structure of solerone (5-acetyldihydro-2-(3H)furanone) a substituted gamma-lactone involved in wine aroma. J. Agric. Food Chem, 19, 1128-1130. 

Martin, B., Etievant, P.X. (1991). Quantitative determination of solerone and sotolona in flor sherries by two dimensional-capillary GC. HRC CC., 14, 133-135. 

A biomimetic synthesis of solerone (5-oxo-4-hexanolide) 1 using both enzymatic and acid-catalyzed reactions was performed. Starting from L-glutamic acid 5-ethyl ester 2 enzymatic oxidative deamination followed by subsequent decarboxylation of the corresponding 2-oxoglutaric acid 5-ethyl ester 3 led to ethyl 4-oxobutanoate 4. In the presence of pyruvate,... 

Biomimetic Synthesis of Solerone. We applied pyruvate decarboxylase [EC 4.1.1.1] (PDC) as key enzyme for the biomimetic synthesis elucidating the formation of solerone 1 figure 1). The thiamine diphosphate depending enzyme from Saccharomyces cerevisiae is responsible for the decarboxylation of pyruvate in the course of alcoholic fermentation. After loss of carbon dioxide from 2-oxoacids the resulting aldehyde is released. Alternatively, the cofactor-bound decarboxylation product can react with a further aldehyde. By the latter acyloin condensation a new carbon-carbon bond will be formed, thus opening a biosynthetic way to a-hydroxy carbonyl compounds 11J2). 

While in the presence of 2-oxoglutaric acid neither decarboxylation nor acyloin condensation had been observed, as expected from previously published results (75), we succeeded in the enzymatic conversion of the mono ethyl ester 3 to ethyl 4-oxobutanoate 4, using both whole yeast cells (Saccharomyces cerevisiae) and purified PDC. The oxo ester 4 served as substrate for a second reaction catalyzed by PDC. Formation of a new carbon-carbon bond was accomplished in the presence of pyruvic acid which acted as donor of a C2-unit. Thus, ethyl 4-hydroxy-5-oxohexanoate 5 was obtained for the first time as the result of an enzymatic acyloin condensation. Finally, traces of acid induced the lactonization of hydroxyester 5, indicating it as direct precursor of solerone 1 (Figure 1). 

Figure 1. Postulated biogenesis of solerone 1 and related sherry constituents.

Figure 2. Stereochemical details of the biogenesis of solerone 1 and solerol 8.

The biogenesis of solerone 1 and related compounds was successfully rationalized by biomimetic model reactions. As key step we established the pyruvate decarboxylase catalyzed acyloin condensation of pyruvic acid with ethyl 4-oxobutanoate 4 or ethyl 2-oxoglutarate 3 with acetaldehyde. The importance of the ethyl ester function in 3 and 4 serving as substrates for the enzymatic formation of a-hydroxy ketones 5 and 6 was demonstrated. The identification of six yet unknown sherry compounds including acyloins 5 and 6, which have been synthesized for the first time, confirmed the relevance of the biosynthetic pathway. Application of MDGC-MS allowed the enantiodifferentiation of a-ketols and related lactones in complex sherry samples and disclosed details of their biogenetic relationship. 

A. HoUnagel, E.-M. Menzel and A. Mosandl. Chiral Aroma Compounds of Sherry II. Direct Enantiomer Separation of Solerone and Solerole. Z. Lebensm. Unters. Forsch., 193. 234-236 (1991). 

The y-lactones (55h, i, j, and k) which carry oxygen-containing substituents in position 4 are found exclusively in alcoholic beverages, except for hydroxylactone (55h) which also occurs in tomatoes 561), Owing to its distinct vinous smell, solerone (55i) is a particularly typical representative of this series 607), The occurrence and the mode of formation of lactones in various wines and in other alcoholic beverages are described in several review articles 400, 547, 609),... 

An important y-lactone in wines is 5-oxohexano-4-lactone known as solerone, which recalls the smell of wine. It occurs as a mixture of (R)- (8-108) and (S)-enantiomers. The reduced form of solerone with a fruity odour, 4,5-dihydroxyhexano-4-lactone, is known under the trivial name of solerole or sherry lactone as it occurs in sherry-type wines, as a mixture of (4R,5R)- (8-109) and (4S,5S)-diastereoisomers. The almost racemic composition of solerone and solerole is attributed to racemisation during wine storage. An important lactone related to phthaUdes is 3fl,4,5,7fl-tetrahydro-3,6-dimethyl-3f/-benzofuran-2-one, also... 

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