Sesquiterpenoids. Part XVII. X-Ray crystallographic determination of the stereochemistry and conformation of dihydromikanolide, a germacranolide diepoxide

Abstract

The stereochemistry of dihydromikanolide (1,10:2,3-diepoxy-6,8-dihydroxygermacr-4-ene-12, 15-dioic acid di-γ-lactone) has been defined by X-ray analysis of the crystal structure. The conformation of the ten-membered ring is similar to that in pregeijerene and related to one of the less favourable conformations of cyclodecane. The trans double bond of the ring has a torsion angle of 163°. The unsaturated α,γ-fused γ-lactone at C(4)–(6) is almost planar, whereas the β,γ-trans-fused γ-lactone at C(7)–(8) adopts a normal envelope conformation. The crystals are monoclinic, a= 10·451(6), b= 7·122(4), c= 9·233(5)Å, β= 99·77(3)°, Z= 2, space group P2₁. The structure was solved by direct methods from intensity data collected on a diffractometer, and least-squares adjustment of the atomic parameters converged to R 6·2% over 1486 reflections.