Internal Nucleophilic Termination in Acid‐Mediated Polyene Cyclizations. Part 3

Treatment of the unsaturated allenic alcohols (E)‐ 7 , (Z)‐ 7, 10, 13 , and 19 with an excess of FSO₃H in 2‐nitropropane at ?90° to ?30° afforded, in 68–85% yield, diastereoisomer mixtures of racemic tricyclic ethers 14a – d and 20a – d , respectively (Schemes 3 and 5), with high stereoselectivity (see Table and Scheme 6). These stereospecific transformations represent the first reported examples of an acid‐mediated polyene cyclization, in which an alkene is the initiating group and an allenic alcohol serves as the internal terminator. In close analogy to our earlier work, a nonsynchronous process is postulated, whereby the stereochemical course of cyclization is directed by the conformational structure of an intermediate cyclohexyl cation (see Schemes 3 and 6). In addition, the organoleptic properties of 14c and 20c , racemic didehydro and methyl didehydro analogues, respectively, of the known odorant Ambrox^® ((?)‐ 4f ), are briefly discussed.