1 Introduction

Flavonoid compounds are widely found in food and so they represent an integral part of the diet.1 More particularly, four classes of compounds are of special interest: flavonols like quercetin (onions), isoflavones like genistein (soya), flavanones like naringenin or hesperetin (Citrus) and finally flavan-3-ols like catechin and their polymerised forms, the proanthocyanidins (wine, tea, chocolate) (Figure 1).

Figure 1: Structures of some of the flavonoids present in the diet

All these compounds are quickly metabolised in the body by methylation, glucuronidation, sulfation1 and/or ring cleavage.2 Thus, in vivo, it is the conjugate forms of flavonoid compounds that exert biological activity and not their native forms. While the general reactions involved in the conjugation of flavonoids are well established, the exact structural identification of the conjugates often remains unclear, mostly due to the lack of appropriate reference compounds. Surprisingly, it is only recently that regioselective methods have been designed for the preparation of flavonoid conjugates. The purpose of this chapter is to review the different chemical methods useful in the preparation of flavonoid conjugates. The survey is restricted to the preparation of methylated, glucuronidated and sulfated conjugates of quercetin, catechin, naringenin and genistein.

2 Strategies for the Preparation of Flavonoid Conjugates