Crystal and molecular structure of eupaformonin, a cytotoxic germacranolide from Eupatorium formosanum HAY

Abstract

Single-crystal X-ray analysis has established the structure and relative stereochemistry of the title compound (I). The absolute configuration follows from the sign of the Cotton effect of then n→π* transition of the α-methylene γ-lactone CC–CO chromophore. Crystals are orthorhombic, space group P2₁2₁2₁, a= 14.28(2), b= 11.52(2), c= 9.68(2)Å, Z= 4. The structure was solved by direct phase-determining methods and atomic parameters refined by full-matrix least-squares calculations to R 0.093 over 1 437 reflections from photographic data. The Δ¹⁽¹⁰⁾trans, Δ⁴cis-cyclodecadiene ring is in a chair, boat conformation with the C(4) and C(10) methyl groups oriented anti. The anomalous relationship between the sign of the measured Cotton effect of the CC–CO chromophore n→π* transition for Δ¹⁽¹⁰⁾trans, Δ⁴cis germacranolide C(6), C(7)-trans-fused α-methylene γ-lactones and the position and stereochemistry of the lactone ring fusion is discussed with respect to the preferred cyclodecadiene ring conformation.