A Silyl Enol Ether Variation of the Robinson Annulation

Article abstract of DOI:10.1021/jo00222a014

The Lewis acid catalyzed reaction of regioselectively generated silyl enol ethers with vinyl ketones has been explored as an alternative to the Robinson annulation sequence.Alkylation of the trimethylsilyl enol ethers of cyclohexanone, 2-methylcyclohexanone, and 2,3-dimethylcyclohexanone with ethyl vinyl ketone, 3-penten-2-one, and methyl vinyl ketone ethylene ketal gave a series of 1,5-diketones.Cyclization of the diketones affords 2-octalones in fair to good overall yield.This procedure has been used to prepare in good yield 5,10-dimethyl-Δ¹⁽⁹⁾-2-octalone, an important intermediate for sesquiterpene synthesis, with a cis/trans ratio of 3 to 1.Alkylation of the trimethylsilyl enol ether of isobutyraldehyde with 3-penten-2-one, methyl vinyl ketone, and its ethylene ketal followed by cyclization affords 4,4,5-trimethyl- and 4,4-dimethyl-2-cyclohexen-1-one.This method has been employed in the synthesis of 5-(hydroxymethyl)-2,4,4-trimethyl-2-cyclohexen-1-one, a potential synthon for ring A of the taxane diterpenes.