The enzyme catalyses the synthesis of catechin, catechin-4beta-ol (leucocyanidin) and the related flavan-3-ols afzelechin and gallocatechin, which are initiating monomers in the synthesis of plant polymeric proanthocyanidins or condensed tannins. While 2,3-trans-3,4-cis-leucocyanidin is the preferred flavan-3,4-diol substrate, 2,3-trans-3,4-cis-leucodelphinidin and 2,3-trans-3,4-cis-leucopelargonidin can also act as substrates, but more slowly. NADH can replace NADPH but is oxidized more slowly.