Sonnerphenolic C (3), which was predicted in a redox product of epirhododendrin (1) isolated from Acer nikoense, was synthesized for the first time via the epimeric separation of benzylidene acetal intermediates as a key step. From a similar synthetic route, 1 was obtained concisely. As a result of their antioxidative evaluation, only 3 revealed potent activity. The redox transformation of 1 into 3 was achieved in the presence of tyrosinase and vitamin C. Moreover, 3 was identified in the decoction of A. nikoense by HPLC analysis with the effective use of synthesized 3. Thus, a novel naturally occurring antioxidant 3 was developed through the sequential flow including redox prediction, chemical synthesis, evaluation of the activity, and identification as the natural product.