Hydrogenation enamino esters

Numerous studies of the photoreactions of dihydrofurans have been described. On irradiation 2,3-dihydrofurans are known to undergo conversion to acylcyclopropanes by a pathway involving initial carbon-oxygen bond homolysis. The /i-enamino ester (162), for example, affords the cyclopropane (163) on triplet-sensitized irradiation.138 Similar transformations have been observed in 2,2,4-triacyl-2,3-dihydrofurans,139 and a synthesis of a cis-trans mixture of chrysanthemum carboxylic acid has been accomplished in this way.140 The conversion of the 2-thiazolines(164) to the iV-alkenylthio-amides (165) presumably involves an analogous carbon-sulfur bond homolysis, followed by a 1,2-hydrogen shift in the resulting biradical (166).141... 

The spectra of 2-alkoxycarbonyl-3-aminoacrylic esters (73, R1 = H, alkyl, aryl) show that these substances exist in the enamino-ester form with a strong intramolecular hydrogen bond between the amino and COOR2 groups. Conformers 73a (Z, Z, E) and 73b (E,Z,E) predominate in non-polar media conformation 73b is also probably present in polar solvents and is preferred in the solid state109. (The symbols indicate, in the order shown, the alignments of the free C=0, the bonded C=0 and the R1 group with respect to the carbon-carbon double bond.)... 

Heterocyclic / -enamino esters were stereospecifically hydrogenated in the presence of Raney Ni, under thermal control. By this method diastereoisomerically pure pyrrolizi-dines and quinolizidines could be prepared (Scheme 125). 

An alternative approach to the formation of p-enamino esters was through conjugate addition of a primary amine to an alkyne (eq. 51).57-59 This complementary method provided a route from alkyne 205 to P-enamino ester 233, which underwent aza-annulation with acryloyl chloride to give 234. Catalytic hydrogenation efficiently gave the cis substituted tetrahydropyridone species (235). 

Fig. 4.2 Ligands tested in Ru, Rh and Ir complexes for enantioselective hydrogenation of enamino-ester 15, and e.e.s obtained...

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