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The general structure of an enamine. An enamine is an unsaturated compound derived by the condensation of an aldehyde or ketone with a secondary amine. [1] [2] Enamines are versatile intermediates. [3] [4] Condensation to give an enamine. [5] The word "enamine" is derived from the affix en-, used as the suffix of alkene, and the root amine.
The first step is the formation of an enamine 2 using N,N-dimethylformamide dimethyl acetal and pyrrolidine. The desired indole 3 is then formed in a second step by reductive cyclisation. The Leimgruber-Batcho indole synthesis. In the above scheme, the reductive cyclisation is effected by Raney nickel and hydrazine.
The Stork enamine synthesis: formation of an enamine from a ketone; addition of the enamine to an alpha, beta-unsaturated aldehyde or ketone; hydrolysis of the enamine back to a ketone; The Stork enamine reaction. The reaction also applies to acyl halides as electrophiles, which results in the formation of 1,3-diketones (Stork acylation). [2]
The likely reaction mechanism for the Kindler modification. The first stage of the reaction is basic imine formation by the ketone group and the amine group of morpholine to give an enamine . This reacts as a nucleophile with electrophilic sulfur, similar to an Stork enamine alkylation reaction.
The Agami mechanism (1984) has an enamine intermediate with two proline units involved in the transition state (based on experimental reaction kinetics) [17] and according to a mechanism by Houk (2001) [18] [19] a single proline unit suffices with a cyclic transition state and with the proline carboxyl group involved in hydrogen bonding.
The enamine is generally generated in situ from catalytic amine (such as pyrrolidine) and a ketone. The enamine then reacts as the dienophile with a 1,2,4-triazine. The initial adduct then expels nitrogen, and the pyridine is rearomatized with loss of the amine. Mechanism Boger Pyridine Synthesis
Below is one published mechanism for the reaction: [5] Mechanism for the Hantzsch Pyrrole Synthesis. The mechanism starts with the amine (1) attacking the β carbon of the β-ketoesters (2), and eventually forming an enamine (3). The enamine then attacks the carbonyl carbon of the α-haloketone (4).
The immediate reaction product is a nucleophilic enamine and the reaction cascades with abstraction of chlorine from a chlorinated quinone. After removal of the amine catalyst the ketone is effectively functionalized with a nucleophile and an electrophile with syn:anti ratio of 8:1 and 97% enantiomeric excess.