Preparation and Absolute Configuration of Some Iris Essential Oil Constituents of the 5‐methylsafranic‐acid series

The β‐dienoate (+)‐(5 S )‐ 13a (86% ee; meaning of α and β as in α‐ and β‐irone, resp.) was obtained from (−)‐(5 S )‐ 9a via acid‐catalyzed dehydration of the diastereoisomer mixture of allylic tertiary alcohols (+)‐(1 S ,5 S )‐ 15 /(+)‐(1 R ,5 S )‐ 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 SnCl 4 ‐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 ). 13 C‐NMR Assignments and absolute configurations of the intermediate esters, acids, aldehydes, and alcohols are presented.