Synthesis of triacetonamine N-alkyl derivatives reinvestigated

The N-alkylated 2,2,6,6-tetramethylpiperidin-4-ones 3c-f were prepared from the acetal 6a of triacetonamine (3a) by alkylation followed by hydrolysis of the acetal functionality or alternatively from the corresponding secondary alcohol 2,2,6,6-tetramethylpiperidin-4-ol (7a) by N-alkylation and subsequent oxidation to introduce the ketone unit. Direct alkylation of 3a was only possible by using highly reactive halides such as allyl or benzyl bromide with low yields. Treatment of phorone (5) with primary amines 2c-f with an alkyl group greater than methyl did not lead to the desired heterocycles 3c-f since open-chain addition products 8 and 9 were formed instead. Consequently, the reactions of acetone (1) with benzyl-or n-butylamine (2e,f) in the presence of calcium chloride did not generate the corresponding N-alkylated derivatives of 3a.