Total Synthesis of Spiromamakone A and Structure Revision of Spiropreussione A

Spiromamakone A is a racemic natural product having a naphthyl acetal group on a spiro­[4,4]­nonadiene skeleton. Its total synthesis was achieved by double oxa-Michael addition of 1,8-dihydroxy­naphthalene to 2-(1-bromoalkylidene)-4-isopropoxy-4-cyclopentene-1,3-dione, which was prepared by palladium­(II)-catalyzed ring expansion of 4-(1-alkynyl)-4-hydroxy-3-isopropoxy-2-cyclobuten-1-one, and a subsequent intramolecular aldol reaction. The synthesis using optically active intermediates enabled identification of the racemization step of spiromamakone A and revealed that spiromamakone A and spiropreussione A are identical; the latter had been reported as a constitutional isomer of the other.