asymmetric total syntheses of pure (+)-4-[13C]catechin (1) and (−)-4-[13C]epicatechin (2) are described. Labelling was introduced through acylation of a phloroglucinol derivative 4 by 1-[13C]acetic acid, after activation by TFAA. Condensation of the resulting C6−C2 acetophenone building block 6 with a C1−C6 benzaldehyde unit 9 provided the C6−C3−C6 flavonoid skeleton, with benzyl ethers as phenol protecting