Enone hydrogenation

The C1-C20 segment (184) The construction started by condensation of ketophosphonate 187 and aldehyde 186 leading to a conjugated enone, hydrogenated to ketone 209 (Scheme 29). The desired configuration at Cg was... 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

With other ring sizes, the photochemistry of cyclic enones may take different courses. For cyclopentenones, the principal products result from hydrogen abstraction processes. Irradiation of cyclopentenone in eyclohexane gives a mixture of 2- and 3-cyclohexyleyclopentanone. These products can be formed by intermolecular hydrogen abstraction, followed by recombination of the resulting radicals ... 

Hydrogenation of 19-nor-A -3-keto steroids also gives 5a- and 5 -product mixtures under the usual conditions but with ruthenium oxide at high pressures only the 5j8-isomer is formed.The presence of a 4-methyl group on a A -3-keto steroid increases the amount of a attack as compared to the parent enone. ... 

Enolizalion of conjugated or /3,y-unsatiirated enones and dienones in O-deiiterated solvents facilitates the introduction of deuterium labels into positions as far as three and five carbon atoms away from a given ketone function. Exchange of the activated hydrogens in androst-4-en-3-one (12) provides a good illustration of the potential of this method. Saturation of the double bond (section V) in the deuterated enone (13) followed by back exchange of the a-deuteriums (section II-B) proves to be an excellent method for the preparation of 6,6-d2-5a-androstan-3-one (15). ... 

There are many examples in the literature reporting the successful exchange of activated hydrogens in various enones (see compounds 19-25). Complications are encountered with 3/3-hydroxy-5oc-spirost-8(9)-en-ll-one since the exchange of the 14a-hydrogen is accompanied by inversion at this tertiary center (22). Another unusual case occurs during the attempted deutera-... 

A large number of examples have been reported in the literature for the deuteration or tritiation of double bonds and for the hydrogenation of enones labeled at an allylic position. The following list of substrates, in addition to those already discussed, provides a representative cross section ... 

Hydrogenation of acetylenes to olefins, 134 Hydrogenation of dienes, 124 Hydrogenation of dienones, 130 Hydrogenation of enones, 130... 

Although at equilibrium the j5,y-tautomer (16a) is preferred, some of the conjugated enone (17) can be obtained by acid-catalyzed equilibration. Hydrogenation of the A-homo-enone (16a) gives a mixture from which A-homo-5a-cholestan-3-one (5b) can be isolated. 

Hydrogenation of enones in MeOH with Pd/C resulted in acetal formation. When ethylene glycol/THF is used as solvent, the related dioxolane is formed in 86% yield. 

We can extend this kind of reasoning even further by imagining that subsequent transformations might be carried out on the aldol products. For example, a saturated ketone might be prepared by catalytic hydrogenation of the enone product. A good example can be found in the industrial preparation of 2-ethyl-... 

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