Submitted to: Journal of Labelled Compounds and Radiopharmaceuticals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 30, 2004
Publication Date: September 1, 2004
Citation: Schmidt, W.F., Robbins, R.J. 2004. Optimized synthesis of four isotopically labeled (13c-enriched) phenolic acids via a malonic acid condensation. J. Label Compd Radiopharm. 47:797-809.
Interpretive Summary: Interest in naturally occurring plant phenolics stems from their potential protective role against oxidative damage diseases (coronary heart disease, stroke, and cancers) through ingestion of fruits and vegetables. The term 'phenolics' encompasses a large variety of naturally occurring molecules currently classified according to the number of phenol subunits. Polyphenols consist of two or more phenol subunits and encompass compounds such as the flavonoids and tannins. The monomeric subclass consists of the phenolic acids that include two distinguishing constitutive carbon frameworks: the hydroxycinnamic and hydroxybenzoic structures. Beyond the protective antioxidant behavior, other biological activities of phenolic acids have been reported. Caffeic acid, one of the most prominent, naturally occurring cinnamic acids, is known to selectively block the biosynthesis of leukotrienes (components involved in immunoregulation diseases, asthma, and allergic reactions). Recent investigations by Maggi-Capyeron et al. have linked a series of phenolic acids (gallic, caffeic, protocatechuic, p-coumaric, sinapic, ferulic ) with the inhibition of AP-1 transcriptional activity. AP-1 is an activator protein implicated in the processes that control inflammation as well as cell differentiation and proliferation. To better assess the potential health benefits of these phenolic acids, in both their roles in nutrition and as potential pharmaceutical agents, it is necessary to be able to measure these components in plant sources (e.g., food composition data) and in biological fluids (e.g., serum for metabolic investigations). Developing accurate analytical methodologies for their detection and measurement is crucial in this endeavor.
IDMS is an analytical technique of proven accuracy which uses stable (non-radioactive) isotopes. Synthesized isotopically enriched compounds can be used as markers to identify pathways and biochemical transformations such as complex conjugates and metabolites. However, one major factor limiting its use is the lack of commercially available isotopically labeled (13C or D) compounds of the desired chemical structure. Although analytical laboratories can develop these measurement systems, they are often not equipped to carry out lengthy and complicated synthesis reactions. Additionally, the cost of a customized chemical enrichment synthesis from an outside laboratory is often prohibitive. In this communication, we report a simple one step conversion of a series of aldehydes to hydroxycinnamic acids (Table 1). Starting with commercially available 13C labeled and unlabeled aldehydes and 13C labeled and unlabeled malonic acid, we obtained four site selective, isotopically enriched, hydroxycinnamic acids.
Four isotopically (13C) labeled phenolic acids (caffeic [M+3], sinapic [M+2], p-coumaric [M+6] and ferulic [M+6] acids) were synthesized via a simple one step malonic acid condensation with a series of aldehydes. The aldehydes and the malonic acid were variously labeled and unlabeled to vary the enriched sites. 13C and 1H NMR analyses together confirmed the labeled positions. LC/MS confirmed the masses. These acids are intended for use as internal standards for isotope dilution mass spectrometry (IDMS).