Colouring matters of australian plants. xxii† ventilagone: Crystal structure and an approach to synthesis

The crystal structure of the title compound, a naturally occurring tricyclic quinone, C₁₆H₁₆O₅, has been found to be (1R*, 3S*)-7, 10-dihydroxy-1, 3, 8-trimethy1-3, 4, 6, 9-tetrahydro-1H-naphtho[2, 3-c]- pyran-6, 9-dione by single-crystal X-ray diffraction methods at 295(1) K and refined by least squares to a residual of 0·033 for 1143 ‘observed’ reflections. Crystals are monoclinic, P2₁, a 8·346(8), b 16·79(2), c 4·911(5) Å, β 101·17(8)°, Z 2. The molecule is shown to have the cis configuration. As an approach to the synthesis of the pigment the model compound 5-chloro-8-methoxy-1, 3- dimethylisochroman has been prepared and the two racemic forms have been separated and characterized.