Experimental and theoretical study of stereochemistry for new pseurotin A3 with an unusual hetero-spirocyclic system

A new pseurotin A3 (3), which was obtained from the fungus Aspergillus terreus was converted into its acetyl derivative (4). Optical rotation (OR), 13C NMR, electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) were measured for (+)-4 and computed for four possible isomers using quantum theory. Absolute configuration of (+)-4 was finally assigned as (5S,8S,9R,10S,11R,12Z) by comparing the predicted OR, ECD and VCD spectra to the experimental data. Therefore, the new compound 3 should have the (5S,8S,9R,10S,11R,12Z) too.