Piperideines and Pyrrolines

The presence of some enamine, at equilibrium, is demonstrated by the conversion of piperideine into a dimer, indeed, the ability of these two systems to serve as both imines and enamines in such condensations is at the basis of their roles in alkaloid biosynthesis. Formed in nature by the oxidative deamination and decarboxylation of ornithine and lysine, they become incorporated into alkaloid structures by condensation with other precursor units. Hygrine is a simple example in which the pyrroline has condensed with ace-toacetate, or its equivalent. 

Controlled oxidation of A-acyl-piperidines and -pyrrolidines can be used to prepare 2-alkoxy derivatives or the equivalent enamides, which are useful general synthetic intermediates. The former are susceptible to nucleophilic substitution under Lewis-acid catalysis, via Mannich-type intermediates, and the latter can undergo electrophilic substitution at C-3 or addition to the double bond. 

Enol phosphates derived from iV-acyl-piperidin-2-one (or 2-oxoazepane) can be utilized in cross-coupling 

Various oxidants have been used to convert pyrrolidine and piperidine into nitrones (imine oxides), of value for 1,3-dipolar cycloadditions in one method hydrogen peroxide is the oxidant in the presence of a catalyst/ 

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Generally speaking, piperideines and pyrrolines exist predominantly in the imine form and not in the tautomeric enamine form A -alkyl analogues have no alternative but to exist as enamines. These cyclic imines are resistant to hydrolytic fission of the C = N bond, in strong contrast with acyclic imines, but nonetheless they are very susceptible to nucleophilic addition at the azomethine carbon. An example of this is that both piperideine and pyrroline exist as trimers formed by the nucleophilic addition of nitrogen of one molecule to the azomethine carbon of a second molecule, etc. 

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