Treatment of the unsaturated allenic alcohols (E)-7, (Z)-7, 10, 13, and 19 with an excess of FSO3H in 2-nitropropane at -90degrees to -30degrees 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(R) ((-)-4f), are briefly discussed.

AbstractTreatment 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.

Treatment of ten monocyclic dienols 8-11 with an excess of fluorosulfonic acid in 2-nitropropane at -90-degrees afforded diastereoisomeric mixtures of racemic tricyclic ethers 12-14 in 81-91 % yield (see Tables 1 and 2). These transformations represent further examples of biomimetic acid-mediated cyclisations in which an OH group serves as the internal nucleophilic terminator. A non-synchronous process is postulated, and the examples described strongly re-inforce our working mechanistic hypothesis, whereby the stereochemical course of cyclisation is directed by the orientation of the side chain vicinal to the intermediate cyclohexyl cation (see Schemes 4 and 5). It is also demonstrated that the efficiency of this process is independent of the nature of the OH group, which may be primary, secondary, or tertiary. In addition, the organoleptic properties of 12 14, Me homologs of known odorants such as Ambrox(R) ((-)-3a) and its diastereoisomers, are briefly discussed.

Treatment of 10 structurally related trienols and dienols 5-8 with an excess of fluorosulfonic acid in 2-nitropropane at -90-degrees-C afforded, in 74-87% yield, diastereoisomeric mixtures of the odoriferous norlabdane oxides 9-15 ((-)-9 (Ambrox) is a naturally occurring ambergris odorant). These transformations represent examples of efficient biomimetic acid-mediated cyclizations in which the hydroxyl group serves as the internal nucleophilic terminator. The stereochemical outcome of these kinetically controlled processes has been analysed in detail, and mechanistic hypotheses consistent with the results have been proposed. For the four acyclic trienols 5, the major reaction pathway can be rationalized by a totally synchronous process involving three internal anti additions via chair or skew-boat conformations of the nascent cyclohexane rings. An alternative explanation postulates a non-synchronous process in which ring closure to an intermediate cyclohexyl cation is followed by rapid cyclization, directed by a strong kinetic preference for equatorial C-C and C-O ond formation. In contrast, for the monocyclic dienols 6-8 only a nonsynchronous process, involving prior protonation of the cyclohexenyl bond, is fully consistent with the results. In the nonsynchronous processes, the orientation of the side chain vicinal to the cyclohexyl cation directs the stereochemical course of the cyclization. For the acyclic trienols, this factor is predetermined by the configuration of the C(7) = C(8) bond, whereas, for the acyclic trienols, this factor is predetermined by the configuration of the C(7) = C(8) bond, whereas, for the monocyclic dienols, this orientation is determined by the stereoselective axial protonation of the cyclohexenyl bond in 6, or by the distribution of cyclohexene and cyclohexane conformers in 7 and 8, respectively. In the cases studied, it is clear that conformational inversion of the six-membered ring is slower than cyclization and thus ensures that an equatorial side chain leads to a trans A/B ring junction in the cyclization product, whereas an axial side chain affords a cis A/B ring junction.