Precursors to oak lactone. Part 2: Synthesis, separation and cleavage of several β- d-glucopyranosides of 3-methyl-4-hydroxyoctanoic acid

• Published in: Tetrahedron 2004-07, Vol.60 (29), p.6091-6100
• Main Authors: Wilkinson, Kerry L, Elsey, Gordon M, Prager, Rolf H, Tanaka, Takashi, Sefton, Mark A
• Format: Article
• Language: English
• Subjects: Aroma thresholds; Glycosides; Hydrolysis; Oak lactone; Pyrolysis; β-Glucosidase
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2004.05.070

The β- d-glucopyranosides of all four stereoisomers of 3-methyl-4-hydroxyoctanoic acid have been prepared. The (3 S,4 S) and (3 R,4 R) species were prepared from cis-5- n-butyl-4-methyl-4,5-dihydro-2(3 H)-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 (3 S,4 R) and (3 R,4 S) 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 (4 S,5 S) cis-oak lactone and its non-natural (4 R,5 R) 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. A series of glycosides 3 has been prepared. Both the straight glucoside and its galloylglucose derivative have each been shown to produce oak lactone 1 under acidic and pyrolytic conditions.