Figure 5
Reaction scheme outlining the conversion of abietic acid (a) upon heat treatment. Subsequent to dehydrogenation, dehydroabietic acid (b) is produced. In the presence of wood from plants containing methanol and further heating, dehydroabietic acid undergoes esterification with methanol to form dehydroabietic acid methyl ester. (c) Keto- and hydroxy- analogues of this methyl ester compound—(d,e), respectively—are formed through oxidation reactions under aerobic conditions. Dehydroabietic acid can also undergo these reactions to form similar oxidation products (f,g). Alternative transformation pathways involve decarboxylation of dehydroabietic acid to produce dehydroabietin (h), or modification of its carboxylic acid group to form dehydroabietane (i). Further exposure to high-temperature heating (at temperatures associated with boiling for pitch) results in the aromatisation of these reaction products to form retene (j), the final and most stable reaction product. These transformation products, along with abietic acid, can be used as indicators of heat-related processes involving plant materials containing abietic acid. The m/z values of these compounds were calculated and retrospectively extracted (either as the native species or as the TMS derivative) from the non-targeted GC-MS data collected in this study.
