Enamines cleavage

Keto acids are obtained by acylation of cyclopentanone enamines (see p. I3f.) with acid chlorides and subsequent base-catalyzed mro-aldol cleavage (S. Hdnig, 1960). 

Hunig and Salzwedel (20) report that the acylation of the pyrrolidine enamine of 3-methylcyclohexanone with propionylchloride followed by the hydrolysis and the base cleavage of the resulting dione isomers (71) and (72) and subsequent reduction of the keto groups gave a 3 7 mixture of the carboxylic acids (73 and 74), respectively. Vig et al. (39), however, found o o o o... 

The preparation of long-chain fatty acids has been carried out in this way because cleavage of 115 with strong sodium hydroxide gives the ketoacid (116), which is easily reduced by the Wolf-Kishner method to the saturated acid. A similar sequence of reactions can be carried out starting with the cyclopentanone enamine, and this method allows lengthening the chain... 

The dicarboxylic acid chlorides from sebacic and azelaic acid react with 2 moles of enamine to give the tetraketone (117), which on base cleavage... 

Iminium salts can be made in a number of ways such as protonation of enamines (7), alkylation of aldimines and ketimines (5,9), cleavage of a covalent bond in a... 

Hydrolytic cleavage of vinylogous formamides and ureas has been applied to the generation of enamines (56-58). 

Additions to acyclic enamines (337,343), to 1,1-dipiperidinoethylene (344), and to the analogous ethoxydimethylaminoethylene (344) gave products derived from ring cleavage of an initially formed cyclobutene adduct. 

From the oxidation of enamines with aromatic nitro compounds a-keto-enamines were obtained in modest yields (70J). Photooxygenation led to cleavage of the enamine double bond (706,707). 

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

A novel ring closure was discovered by Stork (6) in which the pyrrolidine enamine of a cycloalkanone reacts with acrolein. The scheme illustrates the sequence in the case of 1-pyrrolidino-l-cyclohexene, and the cyclopentane compound was found to undergo the reaction analogously. The procedure details the preparation of the bicyclo adduct and its cleavage to 4-cyclooctenecarboxylic acid. 

Thiirene dioxides readily react with an entire spectrum of enamines to provide novel acyclic and cyclic systems172. These products result mostly from carbon-carbon or carbon-sulfur bond cleavage in the intermediate fused thiirane dioxide 167 (equation 67). 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

Among other methods for the preparation of alkylated ketones are (1) the Stork enamine reaction (12-18), (2) the acetoacetic ester synthesis (10-104), (3) alkylation of p-keto sulfones or sulfoxides (10-104), (4) acylation of CH3SOCH2 followed by reductive cleavage (10-119), (5) treatment of a-halo ketones with lithium dialkyl-copper reagents (10-94), and (6) treatment of a-halo ketones with trialkylboranes (10-109). 

An enamine provides synthon (21) and the cleavage of (20) to (17) occurs in aqueous base. Attack on the slightly strained five-ring ketone isfaster than on the exocyclic ketone. 

Cyclopropane ring cleavage is also observed in the case of zirconocene 2-alkene and j 2-imine complexes with adjacent cyclopropane rings to give t/3-allyl, /3-azaallyl, and t/ -enamine complexes [29]. 

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