A natural diphenolic glucoside, sonnerphenolic C: Chemical synthesis, structural revision, redox transformation, and identification as an antioxidant

Abstract

To investigate the redox o-hydroxylation of epirhododendrin (1) with tyrosinase and vitamin C, the possible product, sonnerphenolic C (2), and its epimer, tripodantoside (3), were synthesized for the first time via key steps including glycosylation and epimeric separation. The NMR data of synthetic compounds 2 and 3 were completely consistent with those of natural products 2 and 3, respectively. However, the specific rotation value of synthesized 2 (−15.5, c 1.8 in MeOH) was different from that of the previously reported data (+33.4, c 1.8 in MeOH) of natural product 2. L-Glucoside 5, the enantiomer of 2, was successively prepared through a similar synthetic route using l-glucose as the sugar material. The specific rotation value of 5 was recorded as +18.5 (c 1.8 in MeOH), indicating that the structure of natural product 2 should be revised to that of 5. Additionally, 1 and its epimer rhododendrin (4), were concisely synthesized using the same steps for 2 and 3. The initially proposed redox transformation of 1 to 2 was confirmed by the analysis of the reaction mixture with 1, tyrosinase, and vitamin C. Furthermore, 2 was newly identified in Acer nikoense extract as a natural antioxidant by HPLC and 2,2-diphenyl-1-picrylhydrazyl radical scavenging experiments.