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 amts. generated at 90 DegC for 1 h were detd. by isotope diln. assay (IDA). The internal stds. used for IDA, i.e., [13C2]HDMF and [2H3]EHMF, were prepd. in good overall yields in 3 steps: addn. of labeled acetaldehyde or propionaldehyde to tert-butyloxycarbonyl (BOC)-protected and lithiated 3-butyn-2-ol; oxidn. of the BOC-protected diol with permanganate to 1,2-dione; and finally cyclization to the target mols. after removal of the protective groups under acidic conditions. Quant. data confirmed previous findings that HDMF and EHMF are preferentially formed in the presence of glycine and L-alanine, resp. The yields obtained were 2.6-5.1 mg of HDMF and 6.8-10 mg of EHMF per mmol pentose. Formation of both furanones was favored in phosphate-buffered solns. 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. [on SciFinder (R)]