Approach to the Core Structure of Signermycin B

Among the natural tetramic acids with a decalinoyl part, signermycin B is unique because it contains a cis-decalin. In this paper, we demonstrate that the cis-decalin section of signermycin B can be accessed by an anionic oxy-Cope rearrangement. The substrate, a tricyclic dienol was prepared by an intramolecular Diels-Alder reaction of a masked ortho-benzoquinone, generated by oxidation of an α-methoxyphenol in presence of cis-2-hexenol. After a superfluous bromine on the cycloadduct was removed, reaction of the tricyclic ketone with isopropenylmagnesium bromide led to the tricyclic trienol that underwent the oxy-Cope rearrangement to a cis-decalinone. While we could show, that introduction of the 4-ethyl substituent (signermycin B numbering) is possible by enolate alkylation, the 4-epi-isomer was formed.