CONTROLLING THE STEREOCHEMISTRY OF THE RING JUNCTION IN HEXAHYDRODIBENZOFURANS

The 8-hydroxy-1,2,3,4,4a,9b-hexahydrodibenzofuran-3-one formed during the acid-catalyzed rearrangement of 1,4,4a,8a-tetrahydro-1-methoxy-1,4-ethanonaphthalene-5,8-dione is, contrary to published reports, exclusively the cis isomer by X-ray crystallography. The stereochemical outcome of this intramolecular Michael addition results from the cyclid nature of the alpha,beta-unsaturated carbonyl, since, in acyclic systems, the addition product is a trans-2,3-disubstituted 2,3-dihydrobenzofuran. The trans relationship of the 2- and 3-substituents in the acyclic system was confirmed by annulation to the trans-8-hydroxy-1,2,3,4,4a,9b-hexahydrodibenzofuran-3-one and X-ray crystallographic analysis of the 3-cyanohydrin.