A unified strategy for the synthesis of highly oxygenated diaryl ethers featured in ellagitannins
Tsukasa Hirokane1, Yasuaki Hirata1, Takayuki Ishimoto1
1School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda 669-1337, Japan.
Abstract:
Ellagitannins are a family of polyphenols containing more than 1,000 natural products. Nearly 40% of these compounds contain a highly oxygenated diaryl ether that is one of the most critical elements to their structural diversity. Here, we report a unified strategy for the synthesis of highly oxygenated diaryl ethers featured in ellagitannins. The strategy involves oxa-Michael addition of phenols to an orthoquinone building block, with subsequent elimination and reductive aromatization. The design of the building block--a halogenated orthoquinone monoketal of gallal--reduces the usual instability of orthoquinone and controls addition/elimination. Reductive aromatization is achieved with perfect chemoselectivity in the presence of other reducible functional groups. This strategy enables the synthesis of different diaryl ethers. The first total synthesis of a natural ellagitannin bearing a diaryl ethers is performed to demonstrate that the strategy increases the number of synthetically available ellagitannins.
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