Internal Nucleophilic Termination in Acid‐Mediated Polyene Cyclizations Part 4

Treatment of the unsaturated bicyclic homoallylic alcohols (E)‐ and (Z)‐ 5 and (Z)‐ and (E)‐ 10 and allenic alcohol 16 with an excess of ClSO₃H in 2‐nitropropane or CH₂Cl₂ at − 80° afforded, in moderate yields (ca. 30–70%), diastereoisomer mixtures of racemic tetracyclic ethers 12a – c (Table 1) and 17a,b (Table 2), respectively. These kinetically controlled stereospecific transformations are believed to proceed via concerted or nonconcerted pathways (see Schemes 4 and 6) and the results are fully consistent with our earlier work. Representing novel didehydro bridged analogues of known, olfactively active labdane tricyclic ethers, the organoleptic properties of 12a – c and 17a,b are briefly described, especially those of 12c which, in the context of structure–activity studies, is a racemic didehydro analogue of the known ambergris odorant Ambrox^®.