ChemInform Abstract: Internal Nucleophilic Termination in Acid‐Mediated Polyene Cyclizations: Synthetic Access to Methyl Homologs of (.+‐.)‐Ambrox and Its Diastereoisomers.

Abstract

Dedicated to Professor Georg Frµter on the occasion of his 65th birthday Treatment of the acyclic tetraenols (E)-and (Z)-2 with an excess of ClSO 3 H in 2-nitropropane at À 808 stereoselectively afforded in 30 and 43% yield, respectively, diastereoisomer mixtures of the racemic, tricyclic ethers 1c,d and 1a,b, together with 20 (Table). Under identical conditions, but with the acyclic pentaenol 10 (1 : 1 diastereoisomer mixture) as substrate, the tricyclic ethers 22a/22b (10 : 1) were isolated in 27% yield. These kinetically controlled stereospecific transformations are thought to proceed via non-concerted pathways (see Schemes 5 and 7), fully consistent with our earlier work. In contrast, another set of reaction conditions (CF 3 CO 2 H, CH 2 Cl 2 , À 158 to À 108) was used for the cyclization of the monocyclic dienols (E)-3 and (Z)-3, which resulted in the non-stereoselective formation of the major products 1c,d and 1a,b, respectively, in 35 -37% yield. Representing novel didehydro analogues of the known ambergris odorant (AE)-Ambrox and its diastereoisomers, the qualitative organoleptic properties of 1a -d and of the 10 : 1 diastereoisomer mixture of the novel tetradehydro analogues 22a/ 22b are briefly described.