Biomimetic Construction of a syn-2,7-Dimethyloxepane Ring via 7-Endo Cyclization
Takeo Sakai1, Sae Mizuno1, Akitaka Sone1
1Faculty of Pharmacy, Meijo University, 150 Yagotoyama, Tempaku-ku, Nagoya 468-8503, Japan.
Abstract:
syn-2,7-Dimethyloxepane is a unique structure observed in natural ladder-shaped polycyclic ethers (LSPs), such as Caribbean ciguatoxins, gymnocin-B, and brevisulcenal-F that exhibit potent biological activities. Thus, the successful construction of this seven-membered ring is desirable, but its ring strain and the 1,3-repulsion between its two methyl groups makes this process difficult. Herein, we prepared syn-2,7-dimethyloxepanes via 7-endo cyclizations of vinyl epoxides that break Baldwin's rules. Such a biomimetic approach to syn-2,7-dimethyloxepanes has not yet been reported; however, we achieved this challenging cyclization with the aid of a cis-olefin tether and an unsubstituted vinyl group. The NO-ring fragment of gymnocin-B was also prepared from one of these 7-endo cyclized products, demonstrating the potential application of this strategy in constructing bioactive LSPs.
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