Selective Synthesis of Either Complex Aromatic Alkenes or (1,4-Cyclohexadienyl)alkenes by Tandem Arylation-Multistep Reduction of α,β,γ,δ-Unsaturated Ketones

Article abstract of DOI:10.1021/jo00334a006

(1,4-Cyclohexadienyl)alkenes are conveniently prepared in excellent isolated yields from α,β,γ,δ-unsaturated ketones by tandem alkylation-multistep reductions.By this one-pot procedure, complex diene benzyl alkoxides, generated in situ by arylation of α,β,γ,δ-unsaturated ketones with organolithium reagents, are reduced by a multistep reduction process in lithium-ammonia-ethanol to the corresponding (1,4-cyclohexadienyl)alkenes.Examples include the synthesis of 2-methyl-6-(1,4-cyclohexadien-1-yl)-2-heptene (7a), (+/-)-β-curcumene (7b), 2-methyl-6-(4-methoxy-1,4-cyclohe xadien-1-yl)-2-heptene (7c), 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(1,4-cyclohexadien-1-yl)butane (9a), 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(4-methyl-1,4-cyclohexadien-1-yl)butane (9b), and 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(4-methoxy-1,4-cyclohexadien-1-yl)butane (9c) using phenyllithium, p-tolyllithium, and p-methoxyphenyllithium with 6-methyl-3,5-heptadien-2-one (1) and β-ionone (2), respectively.In contrast, use of the corresponding Grignard reagents prepared by the Rieke procedure results in the selective synthesis of the corresponding aromatic alkene.Examples include the synthesis of 2-methyl-6-phenyl-2-heptene (8a), (+/-)-α-curcumene (8b), 2-methyl-6-(p-methoxyphenyl)-2-heptene (8c), 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-phenylbutane (10a), 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(4-tolyl)butane (10b), and 1-(2,6,6-trimethyl-1-cyclohexen-1-yl)-3-(p-methoxyphenyl)butane (10c) using phenylmagnesium bromide, p-tolylmagnesium bromide, and p-methoxyphenylmagnesium bromide with 6-methyl-3,5-heptadien-2-one (1) and β-ionone (2), respectively.The latter series of results suggest that some species, present in the Grignard sequence, is preventing the 1,4-reduction (Birch reduction) of the aromatic ring.