Transition–metal mediated asymmetric synthesis. Part 6. Organometallic approaches to the enantiomer synthesis of tridachione: a role for planar chirality in synthesis design

Abstract

Two series of complexes which contain planar chirality have been examined as intermediates for the synthesis of tridachione. Alkylation of tricarbonyl(η⁵-2-methoxy-3,5-dimethylcyclohexadienyl)iron(1+) hexafluorophosphate(1–) with trimethylsilyl cyanide and reaction of the nitrile product with ethyl-lithium afforded an acyl-substituted intermediate for the synthesis of tridachione. Complexation of 1-methoxy-2,4-dimethylbenzene by Mn(CO)₃⁺ gave a complex that was alkylated by vinyl Grignard reagents at the correct position to introduce the 1-methylpropenyl group in tridachione. Methylmagnesium bromide gave predominantly an unexpected regioisomer. These products were converted into Mn(CO)₂NO⁺ dienyl complexes. The influence of the use of planar chirality on the design of synthetic strategies is discussed.