Conformational dependent regio- and stereoselectivity in transformations of germacrenes. A theoretical and experimental approach

Conformational calculations on 8-hydroxy-germacrene B 1 with the MNDO method led to a correct prediction of the most stable conformer. Reaction of 1 with diimide resulted in regio-selective reduction which leads to the formation of two racemic mixtures of 4,5-dihydro-8-hydroxy-germacrene B (2+3). The ratio of 2 and 3 could be predicted by conformational analysis The regioselectivity of the reduction may be attributed to differences in sp2-sp3 torsional strain between the endocyclic double bonds. Upon LiAlH4 reduction of 4,5-dihydro-8-oxo-germacrene B 10 conformational induced asymmetric induction results in a highly stereospecific process in which 2 and 3 are formed in a ratio of 9:1.