Eliminations Involving Ammonium Compounds

Protocol 6 involves heating Mannich compound 10 in DMSO, apparently causing elimination of dimethylamine and formation of enone 20 as a reactive intermediate. The corresponding enone has been synthesized in the 9-butyl series,6,22 but comparable yields are obtained in pyridine cyclization reactions involving either the preformed enone or the Mannich HQ salt.7 Prior to addition of 10, ketone 9 is heated with ammonium chloride in DMSO to promote the formation of imine 21. Isomerization of this imine to the enamine tautomer 22, Michael addition of this nucleophile to enone 20, and elimination of water account for the formation of product 11. Like many polycyclic terpyridyl analogues, this product is sparingly soluble in DMSO and is readily isolated by filtration of the reaction mixture. 

It is noteworthy that under certain conditions the mono-Mannich bases are also capable of generating carbocations. Thus on heating compound 182b with methyl iodide, it is converted into the spiro compound 215. The possible mechanism involves a spontaneous elimination of /V-rncthy I piperidine from the intermediate ammonium salts, e.g. 237, and formation of carbocations 211 and 217 as shown in Scheme 44 (cf. Scheme 39)193. Due to the anhydrous media, the immonium salt 238 can be isolated before its hydrolysis. 

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