Reaction of the lithio-derivative of methoxyallene with hydrazones. Part 2: Formation of 3-pyrrolines and azetidines; synthetic and mechanistic aspects

The reaction of α-lithiomethoxyallene with aromatic hydrazones leads to 3-pyrrolines when run in THF. In the case of SAMP-hydrazones, this reaction occurs with almost complete stereodifferentiation. With other hydrazones (dimethyl, piperidinyl, morpholinyl), the 3-pyrrolines are accompanied by azetidines. However, formation of the latter compounds is reversible, since they are transformed to 3-pyrrolines when the time and temperature of the reaction are increased, but the rate of this transformation depends on the substituents of the terminal nitrogen. A reaction mechanism is proposed which involves intermolecular or intramolecular electron transfers from the lithium amide leading to hydrazinyl radicals. The relative stabilities of these intermediates may then explain the role of the substituents of the terminal nitrogen in the formation of azetidines by a 4-exo-dig radical process.