ENAMINE CHEMISTRY-XXV REDUCTION OF ENAMINONONES BY LiAlH4 AND NaBH4 SYNTHESIS OF α,β-UNSATURATED ALDEHYDES

Article abstract of DOI:10.1016/0040-4020(82)80183-X

Cyclohexanone, 2-methyl-cyclohexanone and 4-methyl-cyclohexanone, 1, were transformed into the enaminones 4a-4e by the following two routes: (A): Acylation of the enamines, 2, derived from 1 and secondary amines (pyrrolidine, morpholine and piperidine) by ethyl chloroformate, and (B): Condensation of 1 with diethyl oxalate, giving the β-ketoesters 3, followed by reaction with the secondary amines.Ethyl 2(-1-pyrrolidinyl)-1-cyclopentene-1-carboxylate. 4f, and methyl 3-(1-pyrrolidinyl)-2-buteonate, 4g, were prepared from ethyl 2-oxo-1-cyclopentanecarboxylate and ethyl 3-oxo-butanoate, respectively, by condensation with pyrrolidine.Reduction of 4a by LAH afforded 1-cyclohexen-1-carboxaldehyde, 5a, 1-cyclohexene-1-methanol, 6a, and 1-(1-cyclohexene-1-methyl)pyrrolidine, 7a, in yields depending on the molar ratio of LAH/4a.Reduction of 4f by LAH gave cyclopenten-1-methanol, 6b, 1-(1-cyclopentene-1-methyl)pyrrolidine, 7b, and ethyl-2(1-pyrrolidinyl)-1-cyclopentanecarboxylate, 8b.Compound 4g, when reduced with LAH, yielded methyl 3-(1-pyrrolidinyl) butanoate, 8c (main product) and 1-(2-butenyl)pyrrolidine, 7c (minor).Reduction of 4 by NaBH4 afforded exlusively the saturated β-aminoesters, 8, in high yields.The reductions with LAH and NaBH4 are rationalized in terms of the HSAB principle.