Aspulvinones, a new class of natural products from Aspergillus terreus. Re-investigation of structures by X-ray crystallographic and spectroscopic analysis

Abstract

The constitutions of unsymmetrically substituted aspulvinone pigments from Aspergillus terreus are re-examined by a combination of X-ray crystallographic and spectroscopic analysis. The crystal structure of aspulvinone (18a) is determined by X-ray diffraction from diffractometer data by direct methods. Crystals are triclinic, space group P, with Z= 2 in a unit cell of dimensions : a= 9.546 ± 0.005, b= 11.019 ± 0.005, c= 13.437 ± 0.006 A, α= 102.94 ± 0.05, β= 113.73 ± 0.05. γ= 91.39 ± 0.15°. The structure was refined by least squares to R 0.054 for 2 106 independent reflections. Comparative spectroscopic data establish constitutions (18b) and (18d) for the structurally related metabolites.The present data, combined with synthetic studies, have led to the revision of structures assigned to all ‘unsymmetrically’ substituted aspulvinones found in A. terreus. The biosynthesis of this new class of natural products is considered.