Sesquiterpenoids. Part XX. X-Ray crystallographic determination of the molecular structure of berlandin, a guaianolide epoxide. Comments on the circular dichroism of sesquiterpenoid α-methylene γ-lactones with αβ-unsaturated ester side chains

Abstract

X-ray analysis of the crystal structure of berlandin (3), a guaianolide epoxide, has resolved stereochemical and structural ambiguities remaining from the earlier chemical and spectroscopic investigation of the sesquiterpenoid; in particular, the acetyl function has been shown to be at C(9) and the C(3), C(4)-epoxide has α-orientation. The c.d. spectra of berlandin and subacaulin have positive Cotton effects, in apparent contradiction of the empirical rule relating the sign of the Cotton effect of the n→π* transition of the C:C·C:O chromophore of an α-methylene γ-lactone to ring-junction stereochemistry. The cycloheptene ring adopts a chair conformation which is slightly skewed, the torsion angle about the double bond C(1)C(10) being 6°. The cyclopentane ring has an envelope (C_s) conformation with C(1) the out-of-plane atom and torsion angles in the range |ω| 1–19°. The α-methylene γ-lactone is trans-fused to the cycloheptene at C(6) and C(7) and has a half-chair (C₂) conformation. Crystals are orthorhombic, a= 6·306(3), b= 37·040(11), c= 8·959(3)Å, space group P2₁2₁2₁, Z= 4. The structure was solved by direct methods and refined by least squares to R 7·9% over 2226 diffractometer reflections.