Precursors to oak lactone. Part 2: Synthesis, separation and cleavage of several β-d-glucopyranosides of 3-methyl-4-hydroxyoctanoic acid |
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Authors: | Kerry L Wilkinson Gordon M Elsey Rolf H Prager Mark A Sefton |
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Institution: | a Australian Wine Research Institute, PO Box 197, Glen Osmond, SA 5064, Australia b School of Chemistry, Physics and Earth Sciences, Flinders University, PO Box 2100, Adelaide, SA 5001, Australia c Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852, Japan |
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Abstract: | The β-d-glucopyranosides of all four stereoisomers of 3-methyl-4-hydroxyoctanoic acid have been prepared. The (3S,4S) and (3R,4R) species were prepared from cis-5-n-butyl-4-methyl-4,5-dihydro-2(3H)-furanone (cis-oak lactone) by a process involving ring-opening with base and protection of the carboxyl function as its benzyl ester. The glucose unit was introduced by a modified Koenigs-Knorr procedure. A different strategy was necessary for synthesis of the (3S,4R) and (3R,4S) compounds. This was based on the reductive ring-opening of trans-oak lactone and subsequent protection of the primary alcohol as its t-butyldiphenylsilyl ether. Separation of the individual glucosides was effected by preparative thin layer chromatography. Those corresponding to the nature-identical isomers of oak lactone have been shown to produce oak lactone under both acidic hydrolysis and pyrolysis conditions. The galloyl-β-d-glucoside of the cis-species, obtained as a natural isolate from the wood of Platycarya strobilacea, was also found to produce cis-oak lactone upon both acid hydrolysis and pyrolysis. Both the nature-identical (4S,5S) cis-oak lactone and its non-natural (4R,5R) enantiomer have been prepared from their corresponding glycosides and their aroma thresholds in white wine were determined to be 23 and 82 μg/L (ppb) respectively. The aroma threshold of the nature-identical isomer in a red wine was 46 μg/L. |
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Keywords: | Oak lactone Glycosides Hydrolysis Pyrolysis β-Glucosidase Aroma thresholds |
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