Determination of 4-hydroxy-2,5-dimethyl-3(2H)-furanone and 2(or 5)-ethyl-4-hydroxy-5(or 2)-methyl-3(2H)-furanone in pentose sugar-based Maillard model systems by isotope dilution assays

The formation of 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) and 2(or 5)-ethyl-4-hydroxy-5(or 2)-methyl-3(2H)-furanone (EHMF) from pentose sugars was studied in Maillard model systems. The amounts generated at 90 degrees C for 1 h were determined by isotope dilution assay (IDA). The internal standards used for IDA, i.e., [C-13(2)]HDMF and [H-2(3)]EHMF, were prepared in good overall yields in three steps: addition of labeled acetaldehyde or propionaldehyde to tert-butyloxycarbonyl (Boc)-protected and lithiated 3-butyn-2-ol; oxidation of the Boc-protected diol with permanganate to 1,2-dione; and finally cyclization to the target molecules after removal of the protective groups under acidic conditions. Quantitative data confirmed previous findings that HDMF and EHMF are preferentially formed in the presence of glycine and L-alanine, respectively. The yields obtained were 2.6-5.1 mu g of HDMF and 6.8- 10 mu g of EHMF per mmol pentose. Formation of both furanones was favored in phosphate-buffered solutions at pH 7 compared to pH 5, particularly in the presence of an excess of amino acid. These data are well in agreement with the previously proposed formation mechanism of HDMF and EHMF via Strecker-assisted chain elongation of the pentose moiety. However, both furanones were also produced to a lesser extent by sugar fragmentation-condensation reactions.