The chemistry of fungi. Part 80. The X-ray crystallographic structure of 8aβ-bromo-5aα,5,6,7,8,8a-hexahydro-1,7α-dihydroxy-8α-methoxycarbonylxanthone monohydrate, a rearrangement product of methyl 2α-bromo-2β-(2,6-dimethoxybenzoyl)-7-oxabicyclo[2.2.1]heptane-3β-carboxylate: A novel route to xanthones: The synthesis of pinselin.

Abstract

Treatment of methyl 2α-bromo-2β-(2,6-dimethoxybenzoyl)-7-oxabicyclo[2.2.1] heptane-3β-carboxylate (1; R¹= H, R²= Br) with boron trichloride gives 8aβ-bromo-5aα-5,6,7,8,8a-hexahydro-1,7α-dihydroxy-8α-methoxycarbonylxanthone (4; R = H), as a monohydrate, the crystals of which are triclinic, space group F with 8 molecules in a cell of dimensions a= 8.648(1), b= 17.597(3), c= 20.309(3)Å, α= 94.36(1), β= 91.39(1), γ= 93.41 (1)°. The structure was solved by the heavy atom method and refined by full-matrix least-squares calculations with anisotropic thermal parameters; R= 0.050 for 2 939 reflexions with I > 3σ(I). Dehydrobromination of (4; R = H) gave 5,6,7,8-tetrahydro-1,7α-dihydroxy-8α-methoxycarbonylxanthone (5) which was oxidised to 5,6,7,8-tetrahydro-1-hydroxy-8α-methoxycarbonyl-7-oxoxanthone (6). Bromination/dehydrobromination of (6) gave a monobromo-1,7-dihydroxy-8-methoxycarbonylxanthone. Application of this process to methyl 2α-bromo-2β-(2,6-dimethoxy-4-methylbenzoyl)-7-oxabicyclo[2.2.1] heptane-3β-carboxylate (1; R¹= Me, R²= H) gave 1,7-dihydroxy-8-methoxycarbonyl-3-methylxanthone (pinselin)(3; R¹= R²= Me) in high yield. This process provides a novel route to xanthones of type (3).