Synthesis of condensed tannins. Part 9. The condensation sequence of leucocyanidin with (+)-catechin and with the resultant procyanidins

Molar equivalents of synthetic (2R,3S,4R or S)-leucocyanidin and (+)-catechin condense with exceptional rapidity at pH 5 under ambient conditions to give the all-trans-[4,8]- and [4,6]-bi-[(+)-catechins](procyanidins B3, B6) the all-trans-[4,8:4,8]- and [4,8:4,6]-tri-[(+)-catechins](procyanidin C2, and novel isomer), and the presumed all-trans-[4,8]-linked tetraflavanoid analogue in the proportions 10:1:12:1:3. The facility and sequence of these condensations correlate with the observed absence of leucocyanidins (flavan-3,3′,4,4′,5,7-hexaols) and also the dominance of related condensed tannins in plant extracts.