Issue in Honor of Prof. Rainer Beckert ARKIVOC 2012 (iii) 379-390
Synthesis of triacetonamine N-alkyl derivatives reinvestigated Klaus Banert,* Katharina Fink, Manfred Hagedorn, and Frank Richter
Chemnitz University of Technology, Organic Chemistry, Strasse der Nationen 62, 09111 Chemnitz, Germany E-mail:
[email protected]
Dedicated to Professor Rainer Beckert on his 60th birthday
Abstract 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.
Keywords: N-Alkylation, 2-aminothiazoles, 2,2,6,6-tetramethylpiperidin-4-ones, ring closure, steric hindrance, tertiary amines
Introduction
Sterically hindered amines are important compounds because of a variety of applications. Such compounds and their metal salts are particularly useful as bases in synthesis.1 Furthermore, hindered amines play an important role as precursors to persistent nitroxyl radicals, which were used for spin labeling methods.2 Such amines have recently come into industrial use in a variety of gas-treating processes.3
Numerous 2,2,6,6-tetramethylpiperidines of type 4 and free radicals derived from these heterocycles are polymerization inhibitors and thermo-and photostabilizers, known as hindered amine light stabilizers and abbreviated as HALS (Scheme 1).4 Triacetonamine (3a) is the unique starting compound for the synthesis of the desired products 4.5 The piperidine derivative 3a can be prepared from acetone (1) and ammonia (2a) in the presence of an acidic catalyst like calcium
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