The aromatic furan ring is found in a wide range of aroma chemicals, especially those produced by the Maillard reaction. They include both low-odor materials such as furfural and high-impact materials such as 2-methylfuran-3-thiol. Among the most important are the hydroxyfuranones, especially 4-hydroxy-2,5-dimethylfuran-3(2H)-one (HDMF, 7), which is a material of unusual reactivity. This contrasts with the inert character of its isomer sotolone. Furans are also found as trace components where they can either contribute a desirable character or an off-note depending on context.