PREPARATION AND ABSOLUTE-CONFIGURATION OF SOME IRIS ESSENTIAL OIL CONSTITUENTS OF THE 5-METHYLSAFRANIC-ACID SERIES

The beta-dienoate (+)-(5S)-13a (86% ee; meaning of alpha and beta as i n alpha- and beta-irone, resp.) was obtained from (-)-(5S)-9a via acid -catalyzed dehydration of the diastereoisomer mixture of allylic terti ary alcohols (+)-(1S,5S)-15/(+)-(1R,5S)-15 (Scheme 3). Prolonged treat ment gave clean isomerization via a [1,5]-H shift to the alpha-isomer (-)-(R)-16a with only slight racemization (76% ee; Scheme 4). In contr ast, 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 allyl ic secondary alcohol (-)-12, the same ester (+)-13a (Scheme 3), but wi th poor optical purity (13 % ee), due to an initial rapid [1,2]-H shif t. The absolute configuration of(-)-16a-c was confirmed by chemical co rrelation with (-)-trans-19 (Scheme 4). C-13-NMR Assignments and absol ute configurations of the intermediate esters, acids, aldehydes, and a lcohols are presented.