Fungal metabolites. Part V. X-Ray determination of the structure and stereochemistry of new isomers of the ergot alkaloids of the peptide type

Abstract

X-Ray crystal structure analysis of a p-iodobenzoylamino-derivative has established the stereochemistry of the aci-isomers of the ergot alkaloids of the peptide type. The p-iodobenzoyl derivative crystallises as an ethanol solvate with four units of C₂₄H₂₄O₅N₃I, C₂H₅OH in an orthorhombic cell, space group P2₁2₁2₁, with dimensions a= 15·48, b= 22·28, c= 7·52 Å. Phase determination was based initially on the iodine atom and Fourier and least-squares methods were used for the refinement of atomic parameters. The final value of the discrepancy R over the 2292 independent reflexions is 11·2%. The crystal structure reveals an intramolecular OH–benzene hydrogen bond; the proton–benzene distance is ca. 2·1 Å. Both five-membered rings in (VI) adopt the envelope conformation.