Facile preparation of acetals and enol ethers derived from 1-arylpiperidin-4-ones

When primary aromatic amines 6 are heated under reflux with slight excesses each of crude 1,5-dichloropentan-3-one 4 and toluene-4-sulfonic acid monohydrate in dry methanol solution, and an excess of trimethyl orthoformate is then added to the reactants, the corresponding 1-arylpiperidin-4-one dimethyl acetals 9 are obtained in good (74–81%) overall yields. The dimethyl acetals 9 undergo hydrolysis in formic acid–water (9∶1 v/v) at room temperature to give the parent 1-arylpiperidin-4-ones 8 in virtually quantitative yields. When the dimethyl acetals 9 are allowed to react with an excess each of N,N-diisopropylethylamine and boron trifluoride–diethyl ether complex in dichloromethane solution at 0 °C they are converted in good yields into the corresponding enol ethers 10, which are required as reagents in the solid phase synthesis of oligoribonucleotides.