Heterocyclic enamines hydride

Tertiary heterocyclic enamines are reduced with metals in acidic media 142) or electrolytically (237,238) and their salts are reduced with lithium aluminum hydride or sodium borohydride (239,240) to the corresponding saturated amines. 

The apparently simple procedures of partial dehydrogenation of pyrrolidines and partial hydrogenation of pyrroles afford Zl1-pyr-rolines. However, the reaction is complex and is of little preparative value.97-98 A 1-Pyrrolines may be obtained by isomerization of A 3-pyrrolines.100 From the preparative point of view, partial hydrogenation of quaternary pyridine salts in strongly alkaline media to give 1-alkyl-id 2-piperideines is more important.101 Formation of heterocyclic enamines was observed in the reduction of i -methyl-pyrrolidone with lithium aluminum hydride,102 -alkylpiperidones with sodium in ethanol,103,104 and in the electrolytic reduction of N-methylglutarimide.106... 

The preparation of enamines by reduction of aromatic heterocyclic bases and their quaternary salts or of lactams is not the most useful approach (97). The lithium aluminum hydride reduction of N-acyl enamines has been used with both fruitful and unsuccessful results. A series of 3-N-acetyl -d -cholestenes (104) has been prepared by desulfurization of the appropriate thiazolidine (105) (98,99). Lithium aluminum hydride reduction of the... 

This new impurity proved to be derived from the Pd-catalyzed oxidation of DIPA to the enamine via P-hydride elimination. In fact, mixing Pd(OAc)2 with DIPA in DMF-d7 readily formed Pd black along with two species, primary amine and acetone, presumably derived from the enamine through hydrolysis. The resulting enamine or acetone then underwent a coupling reaction with iodoaniline 28. Heterocyclization through the arylpalladium(II) species provided 2-methyl indole 71, as shown in Scheme 4.19. 

Nitrogen heterocycles more electrophilic than benzene are susceptible to attack by hydride ion from a complex metal hydride anion. In protic solvents the intermediate cyclic enamines can undergo further reduction. The proper choice of reducing agent and reaction conditions thus allows the preparation of many partially reduced nitrogen heterocycles unavailable by other routes. These reduction procedures provide a valuable adjunct to catalytic hydrogenation155 for the syntheses of saturated nitrogen heterocycles. 

A number of syntheses have been devised which lead to heterocyclic derivatives of cholestanes. 4-Aza-5a-sitostane (321 R = H, X = H2) and its N-methyl derivative (321 R = Me, X = H2) have been prepared by way of oxidative opening of ring A of 4-sitosten-3-one to give a 3,5-seco-5-oxo-3-oic acid which was cyclized by reaction with ammonium hydroxide or methylamine to give A5-enamine lactams these upon hydrogenation gave the lactams (321 R = H or Me, X = O) respectively. Hydride reduction completed the route to the required 4-aza-steroids.179... 

Claisen rearrangements, in nitrogen heterocyclic systems, 8, 143 Complex metal hydrides, reduction of nitrogen heterocycles with, 5, 45 Covalent hydration, in heteroaromatic compounds, 4, 1, 43 Cyclic enamines and iinines, 6,147 Cyclic hydroxamic acids, 10, 199 Cyclic peroxides, 8, 165... 

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