Four A-Type Procyanidin Trimers are Main Flavanol Components of Cinnamomum burmanii

Cinnamon spice is derived from bark of several Cassia and Cinnamomum genera. Depending on the origin, there are distinctive differences between composition of the water-soluble procyanidins, which are mainly consisting of A-type procyanidins, attributed with anti-diabetic activity. Trimers are dominant group, while tetramers are at ca. 20% of the trimer content and other oligomeric DPs contributing smaller pools. A Normal and Reverse Phase HPLC analysis of the bark of C. burmannii revealed a clear pattern of four trimers. The presence of cinnamtannin B-1 (1) and D-1 (2) have been previously documented. Two other trimers were isolated by the use of Centrifugal Partition Chromatography (Kromaton FCPC) and preparative HPLC and identified for the first time as lindetannin (3) and aesculitannin B (4). All contain one A-type interflavan bond between top unit being epicatechin and the middle unit. Typically, some steric hindrance exists in these structures preventing free rotation around B-type bond producing different sets of signals detected by NMR, which complicates interpretation of the NMR spectra. The ratios of rotational isomers is different in each of them and equal to 1:1.4, 1:2.5, 1:9, and 1:7.5, respectively. These differences are explained using DFT optimization for different conformers showing two minima with energy barrier calculated from ¹H NMR temperature spectra as well as ¹³CNMR shielding constants and TD-DFT calculations.