Oximino esters, reduction

Reduction of Hydroxy Esters, Amino Esters, Keto Esters, Oximino Esters and Ester Acids... 

Reduction of oximino esters, i.e. oximes of keto esters, is very useful for the preparation of amino esters. Reductions are very selective since the oximes are easily reduced by catalytic hydrogenation over 10% palladium on carbon in ethanol (yield 78-82%) I094, by aluminum amalgam in ether (yields 52-87%) [750, 70P5], or by zinc dust in acetic acid (yield 77-78%). None of these reagents attacks the ester group. The last mentioned reaction gives an N-acetyl derivative [1096. ... 

Table 28 Reduction of hydroxy esters, amino esters, keto esters, oximino esters and add-esters...

Reduction of a-oximino esters.1 This combination (3.15 2.8) reduces a-ox-imino esters (1 equiv.) to a-amino esters in a buffered solution (pH 7). L-Tartaric acid is a useful buffer it does not induce any enantiomeric selectivity, but affords clean reactions. 

Oximes carrying a second group like a hydroxyl, carbonyl, or car-balkoxyl may form cyclic products, such as pyrazines from a-keto oximes and pyrrolidones from y-oximino esters, upon reduction. ... 

Reductive cyclization occurs when a suitable oximino ester is treated with zinc and acid. 

Borane-trimethylamine has been found to be a superior reagent to BHa-py (3,297) for the reduction of a-oximino-esters to the corresponding AT-hydroxy-a-amino-acid esters. ... 

The pyridine-borane reduction of oximines has been applied to the reduction of a-oximino-esters (51) to (52) (Scheme 32). ... 

Tatsuoka et al. (410) and later Shin et al. (366, 368, 373) obtained a, P-dehydrovaline ester by reduction of the a, P-unsaturated nitro ester with Al-Hg. Only P-disubstituted nitro compounds yield enamines in this reaction, a-oximino esters being obtained from the reduction of P-monosubstituted esters. The analogous reduction of p-methoxy-a-nitro esters to P-methoxy-a-amino esters, which lose methanol on warming to yield enamino esters, is described in ref (370). 

This synthesis of the pyrrole ring system, due to Knorr, consists in the condensation of an a-aminoketone with a 1,3-diketone or the ester of a p-keto-acid, a-Aminoketones are unstable in the free state, readily undergoing self-condensation consequently they must be prepared, by the reduction of an a-nitroso (or oximino) ketone, in the presence of the 1,3-diketone or p-ketoester, to ensure rapid interaction. 

In ethyl 3-keto-2-oximino-3-phenylpropanoate catalytic hydrogenation over palladium on carbon reduced both the keto and oximino group, giving a 74% yield of ethyl ester of -phenylserine (ethyl 2-amino-3-hydroxy-3-phenylpropionate). The reduction is stereospecific and only the erythro dia-stereomer was obtained, probably via a cyclic intermediate 11097]. Similarly, hydrogenation over Raney nickel at 25-30° and 1-3 atm converted ethyl a-oximimoacetoacetate quantitatively to ethyl 2-amino-3-hydroxybutanoate [45]. 

Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191). The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described. a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature. A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described. The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts. The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (X,> 340 nm) of (193) in methanol were identified as A -hydroxy-2-pyridone and (194) from the fission of the C-O bond. Other products were 2-pyridone, (195) and (196) that arise from O-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-O bond heterolysis. 

---