TAUTOMERISM AND STEREOCHEMISTRY OF DIHYDROXYPERYLENEQUINONES - FORCE-FIELD INVESTIGATIONS

Stereochemistry and tautomerism of cercosporin and several of its part ial structure models were investigated using an MM2 derived force fiel d method. Besides the ''propeller'' type conformer, which was found be fore by X-ray crystallography, the complicated energy hypersurface was shown to contain a novel ''double-butterfly'' conformer of similar st ability. The interconversion barrier between these conformers and thei r enantiomers was found to be unusually high due to buttressing effect s of neighbor substituents. Judged from the calculations, the 4,9-taut omer of cercosporin could also be present in favoring instances beside s the 3,10-tautomer, whereas the 3,9-tautomer is strongly destabilized .