Influence of conformational factors on acid‐catalyzed cyclizations of germacranolides: Molecular structure of the cyclization products of gallicin and 8α‐hydroxygallicin (shonachalin a)

Acid‐catalyzed cyclization of the natural germacranolide gallicin (8) yielded, among other products, the 1,4‐epoxyeudes‐manolide 9, which has a trans‐fused decaline system. Under the same conditions the closely related germacranolide shonachalin A (8α‐hydroxygallicin) (3) cyclized to the eudesmanolide 4 with a cis fused decaline system. The structures of these cyclization products were secured by means of chemical correlations and X‐ray diffraction analyses.