Secondary mould metabolites. Part 19. Structure elucidation and absolute configuration of melledonals B and C, novel antibacterial sesquiterpenoids from Armillaria mellea. X-Ray molecular structure of melledonal C

Abstract

The structure, absolute configuration, and preferred conformation, both in the solid state and in solution, of melledonal C (11), a Δ^2,3-protoilludene sesquiterpenoid from Armillaria mellea, have been established by a combination of X-ray and n.m.r. procedures. Ring A of the protoilludene skeleton was found in the crystal in a slightly twisted envelope conformation with C(10) displaced 0.55 Å below the C(9), C(13), C(12), C(11) plane; ring B is a (7β,9α) flattened half-chair and ring C assumes a (4β,6β) highly puckered conformation. A strong intramolecular hydrogen bond between the 4-OH proton and the oxygen atom of 10-OH was evidenced in the solid state as well as in solution ([²H₆]acetone) using the SIMPLE ¹H n.m.r. method (secondary isotope multiplet n.m.r. of partially labelled entities). Contrary to what is observed in solution, in the solid state there is no hydrogen bonding between the 3′-OH proton and the oxygen atom of the adjacent CO₂R group. The structure of melledonals A (9) and B (10), and the absolute configuration at C(13) of melleolides C (6) and D (7) have also been elucidated by n.m.r. and chemical evidence.