Tannins and related compounds. CVI. Preparation of aminoalditol derivatives of hydrolyzable tannins having .ALPHA.-and .BETA.-glucopyranose cores, and its application to the structure elucidation of new tannins, reginins A and B and flosin A, isolated from Lagerstroemia flos-reginae Retz.

To avoid tautomerism in the hydrolyzable tannins which lack an acyl group at the glucose C-1 position, a new method for the protection fo the C-1 atom has been developed. That is reaction of the tannins with p-anisidine in the presence of acetic acid, followed by sodium cyanoborohydride reduction, afforded, without notable hydrolysis of ester bonds, the aminoalditol derivatives (1a-7a) in 50-80% yields. The proton and carbon-13 nuclear magnetic resonance (1H-and 13C-NMR) spectra of these derivatives exhibited much simpler signal patterns typical of an open-chain form of glucose, and almost all the singals could be assigned.Application of this method to the structure elucidation of the new tannins, flosin A (22) and reginins A (23) and B (24), isolated from the leaves of Lagerstroemia flos-reginae RETZ. (Lythraceae), established their structures including the orientation of the 4, 6-positioned valoneoyl group. In addition, the structure of a new hydrolyzable tannin, lagerstroemin (21), which was concomitantly isolated from the above species, was elucidated.