1 Introduction

Flavonoids are polyphenols which have a characteristic fifteen-carbon backbone ring structure (C ₆ C ₃ C ₆) consisting of two aromatic rings generally each containing a number of hydroxyl groups (Figure 1). The class of flavonoids called the flavan-3-ols contain varying degrees of hydroxylation on the 5 and 7 positions of the A-ring and 3', 4' and 5' positions on the B-ring (Figure 2). The 3 position on the C-ring is commonly a hydroxyl group or is esterified with gallic acid. Isomers which vary in the spatial orientation at the 3 position are distinguished by using the prefix epi- in the cases where R ₂ = H (Table 1), and the stereochemistry at the 2 position is noted with a (+) or (-). Proanthocyanidins are oligomers of flavan-3-ol monomer units most frequently linked by C C bonds either at 4 ? 6 or 4 ? 8 (Figure 3a). The most common classes of proanthocyanidins are the procyanidins which are chains of catechin, epicatechin, and their gallic acid esters and the prodelphinidins which consist of gallocatechin, epigallocatechin, and their galloylated derivatives as the monomeric units1 (Figure 2; Table 1). Structural variations to proanthocyanidin oligomers may also occur with the formation of a second interflavanoid bond by C O oxidative coupling to form A-type oligomers (Figure 3b). Due to the complexity of this conversion, A-type proanthocyanidins are not as frequently encountered...