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Knorr, Rudolf and Neuner, Brigitte (2018): Regioselective Dialkylations of N-(tert-Butyl)iminocyclopentane via Deprotonating One-Pot Procedures. In: Helvetica Chimica Acta, Vol. 101, No. 6, e1800037

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The title compound (1) was chosen as a model for the /-regioselectivity of deprotonation and subsequent alkylation adjacent to the C=N bond. With the bulky base lithium N,N-diisopropylamide (LDA) as a catalyst, the one-pot deprotonation steps can be performed through titration with methyllithium, using gas-volumetric observation of the liberated methane. In the first step with ensuing methylation by iodomethane, the primary product is born at -40 degrees C in its metastable (Z) configuration (kinetic control) and may be either isolated or converted insitu at 30 degrees C into its thermodynamically favored (E)-isomer via cis to trans stereoinversion at the N-atom. Being slow enough on the laboratory time scale, this stereoinversion process can serve to control the regioselectivity of the second deprotonation/alkylation sequence as follows. The ,-products are formed from the intermediate (Z)-imine, whereas ,-products result from the intermediate (E)-imine;in either case, syn deprotonation (cis to Bu-t at nitrogen) by LDA is apparently disfavored by the Bu-t group, so that anti deprotonation becomes obligatory. If a third one-pot deprotonation step is too slow with LDA, it may be performed with the stronger base butyllithium/HMPA which, however, reacts regio-unselectively. Regioselective one-pot, LDA-catalyzed deprotonation with alkylation by oxiranes (alone, or alternatingly with iodomethane) opens a short access to spiro-[2.4]heptan-4-ones.

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