| Abstract: | The β-dienoate (+)-(5S)- 13a (86% ee; meaning of α and β as in α- and β-irone, resp.) was obtained from (?)-(5S)- 9a via acid-catalyzed dehydration of the diastereoisomer mixture of allylic tertiary alcohols (+)-(1S,5S)- 15 /(+)-(1R,5S)- 15 (Scheme 3). Prolonged treatment gave clean isomerization via a 1,5]-H shift to the α-isomer (?)-(R)- 16a with only slight racemization (76% ee; Scheme 4). In contrast, the SnCl4-catalyzed stereospecific cyclization of (+)-(Z)- 6 to (?)-trans- 8a (Scheme 2), followed by a diastereoselective epoxidation to (+)- 11 gave, via acid-catalyzed dehydration of the intermediate allylic secondary alcohol (?)- 12 , the same ester (+)- 13a (Scheme 3), but with poor optical purity (13% ee), due to an initial rapid 1,2]-H shift. The absolute configuration of (?)- 16a–c was confirmed by chemical correlation with (?)-trans- 19 (Scheme 4). 13C-NMR Assignments and absolute configurations of the intermediate esters, acids, aldehydes, and alcohols are presented. |
