Total Syntheses of (−)-Methyl Atis-16-en-19-oate, (−)-Methyl Kaur-16-en-19-oate, and (−)-Methyl Trachyloban-19-oate by a Combination of Palladium-Catalyzed Cycloalkenylation and Homoallyl−Homoallyl Radical Rearrangement

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Abstract
Asymmetric total syntheses of (−)-methyl atis-16-en-19-oate (1c), (−)-methyl kaur-16-en-19-oate (2c), and (−)-methyl trachyloban-19-oate (3c) have been achieved by employing a hybrid strategy of palladium-catalyzed cycloalkenylation and homoallyl−homoallyl radical rearrangement. The common synthetic intermediate 5 was prepared from 2-allylcyclohexanone (4) with 98% ee using d'Angelo's asymmetric Michael addition. A series of functional group modifications in 5 via palladium-catalyzed cycloalkenylation led to (+)-14, which had already been prepared by us as racemate. (−)-Methyl atis-16-ene-19-oate (1c) was generated via homoallyl-homoallyl radical rearrangement. On the other hand, Wolff−Kishner reduction of 18 followed by esterification yielded (−)-methyl kaur-16-en-19-oate (2c) together with (−)-methyl trachyloban-19-oate (3c).