Naturally occurring quinones. Part 27. Sesquiterpenoid quinones and related compounds from Hibiscus elatus: crystal structure of hibiscone C (gmelofuran)

Abstract

The blue-grey or pink-grey heartwood of Hibiscus elatus(Blue Mahoe), which turns to light yellow or buff-grey on exposure to light, contains a group of colourless sesquiterpenoid ketones (hibiscones A–D) and o-naphthoquinones (hibiscoquinones A–D) based on a cadinane skeleton. The structure of the furanodiketone, hibiscone C (gmelofuran)(3) was determined by X-ray crystallography, to which hibiscones A, B, and D were related by spectroscopic and chemical methods.

Hibiscoquinones B and C fade slowly in light but hibiscoquinone A, 8-formyl-7-hydroxy-5-isopropyl-3-methyl-1,2-naphthoquinone, is very photolabile and rearranges quickly in daylight to a colourless lactose. The model compound 8-formyl-7-hydroxy-1,2-naphthoquinone behaves in the same way while 8-formyl-7-hydroxy-1,4-naphthoquinone rearranges much more slowly in daylight to an isomeric lactone.