Total Synthesis of Mallotusinin.

The total synthesis of mallotusinin, which bears a tetrahydroxydibenzofuranoyl (THDBF) bridge between the 2-oxygen and 4-oxygen of glucose on corilagin with a 3,6-O-(R)-hexahydroxydiphenoyl (HHDP) bridge, is described. The key features of the total synthesis are: 1) improvements of our previously reported method to synthesize corilagin; 2) establishment of the THDBF skeleton via an unusual intramolecular SN Ar reaction of an HHDP analogue, and 3) the application of a two-step bislactonization strategy for a HHDP bridge construction into the 2,4-O-THDBF bridge. Oxidative phenol coupling of 1,2,4-orthoacetyl-3,6-di-(4-O-benzylgalloyl)-α-d-glucopyranose and the orthoester cleavage of the coupling product without the pyranose-furanose ring transformation are key reactions for the improved synthesis of corilagin, which enabled the adequate supply of a corilagin precursor that was required to develop the mallotusinin synthesis. These established methods are expected to help develop the synthesis of other ellagitannins with a bridge between the two oxygens of corilagin.

Non-Enzymatic Oxidation of a Pentagalloylglucose Analogue into Members of the Ellagitannin Family.

The occurrence of more than 1000 structurally diverse ellagitannins has been hypothesized to begin with the oxidation of penta-O-galloyl-ß-d-glucose (ß-PGG) for the coupling of the galloyl groups. However, the non-enzymatic behavior of ß-PGG in the oxidation is unknown. Disclosed herein is which galloyl groups tended to couple and which axial chirality was predominant in the derived hexahydroxydiphenoyl groups when an analogue of ß-PGG was subjected to oxidation. The galloyl groups coupled in the following order: at the 4,6-, 1,6-, 1,2-, 2,3-, and 3,6-positions with respective S-, S-, R-, S-, and R-axial chirality. Among them, the most preferred 4,6-coupling reflected the what was observed for natural ellagitannins. A new finding was that the second best coupling occured at the 1,6-positions. With the detection of a 3,6-coupled product, this work demonstrated that even ellagitannin skeletons with an axial-rich glucose core may be generated non-enzymatically.