beta-HOMOCYCLOCITRAL → 2-[4-(2,6,6-Trimethyl-1-cyclohexen-1-yl)-2-butenyl]-1, 3-dioxane

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Commercially available 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde is chain-extended by Wittig reaction with commercially available 2-(1,3-dioxan-2-yl)ethyl triphenylphosphonium bromide as follows: To a stirred suspension of the phosphonium salt (20.4 g) in THF (30 ml) is added HMPA (30 ml). The mixture is cooled to -70° under nitrogen and n-butyllithium (17.5 ml of 2.5M solution in hexane) is added dropwise. After 30 minutes the aldehyde (6.65 g in 10 ml THF) is added and the mixture is allowed to warm slowly to room temperature. Water and hexane are added, the organic phase is evaporated and the residue chromatographed with 40% dichloromethane in hexane to provide crude (mostly cis) 2-[4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2-butenyl]-1, 3-dioxane as a colorless oil (5.26 g). This material is hydrogenated in ethanol over a 5% Pd/BaSO4 catalyst for 18 hr under 40 psi hydrogen. Filtration and evaporation provide crude 2-[4 -(2,6,6-trimethyl-1-cyclohexen-1-yl)butyl]-1,3-dioxane as a colorless oil. This is hydrolyzed with acid in refluxing 1:1 dioxane-water to provide 5-(2,6,6-trimethyl-1-cyclohexen-1-yl)pentanal as a colorless oil.

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