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

Abstract

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.