Reduction of a,0-unsaturated carbonyls

For a example of nickel-catalyze conjugate reduction of a,(3 unsaturated carbonyl compounds affording enol derivatives, seerBourhis, R. Frainnet, E. Moulines, F. J. Organomet. Chem. 1977, 141, 157-171. 

A further improvement of this reaction was achieved by substituting aluminium isopropoxide by alumina-supported KOH (Scheme 4.15). This method allows for selective 1,2 reduction of a,(3-unsaturated carbonyl compounds and the reduction of acyloins to their respective diols35. 

The conjugate reduction of a,(3-unsaturated carbonyl compounds is a potentially useful way of accessing Sm(III) enolates that has yet to be widely exploited. Cabrera reported a cyclodimerisation sequence of a,(3-unsaturated ketones using Sml2 that involves a diastereoselective aldol cyclisation.163 For example, treatment of chalcone 158 with Sml2 generates cyclopentanol 160 in quantitative yield after chelation-controlled aldol cyclisation of Sm(III) enolate 159 (Scheme 5.104). 

Catecholborane is a versatile reducing reagent, a mild and convenient alternative to the classical (Clemmensen, using acids and Wolff-Kishner, using bases) reduction procedure of tosylhydrazones. Regiospecific isomerization occurs during the reduction of a,(3-unsaturated carbonyl derivatives often leading to unique alkenes (Eqs. 134 and 135) 222-223> and allenes (Eq. 136) 224). 

In 2001, we reasoned that this catalysis strategy might be applicable to the conjugate reduction of a, 3-unsaturated carbonyl compounds if a suitable hydride donor could be identified (Scheme 18). Hantzsch ester 11 seemed to be particularly promising since its reaction with preformed a, 3-unsaturated iminium ions had already been established (Makino et al. 1977 Baba et al. 1980). 

The chemoselective 1,2-reduction of a 3-unsaturated carbonyl compounds has been carried out with metal hydride or by hydrogenation. However, chemoselective 1,4-reduction of a (B-unsaturated carbonyl compounds is challenging. Recently, a(B-unsaturated carbonyl compounds 1.23, 1.25 and 1.27 were selectively reduced to the corresponding saturated carbonyl compounds 1.24, 1.26 and 1.28, respectively, by cobalt octacarbonyl and water [Co2(CO)8-H20 system]... 

Brewster et al. have investigated reductions of a,3-unsaturated carbonyls with a 1 3 mixture of LiAlH4/AlCb. The reduction is closely similar to the results mentioned above, with an allyl cation intermediate trapped by hydride attack to afford an alkenic product. As seen in equations (17) and (18),... 

Birch-style reduction of a, 3-unsaturated carbonyl compounds is described in Chapter 26. 

Reduction of a,[3-unsaturated carbonyl compounds [105] A solution of the carbonyl compound 36 (1.0 mmol) in methanol (20 mL) was added to the resin (0.5 g, 1.25 mmol). After completion of the reaction, the resin was fil-... 

D. Conjugate Reduction of a,(3-Unsaturated Carbonyl Compounds and Nitriles... 

Conjugate reduction of a, 3-unsaturated carbonyl compounds and subsequent carbon-carbon bond formation provides a versatile domino process of carbocyde and heterocyle synthesis [20]. These reactions proceed by the formation of enolate 49 through conjugate reduction of a,P-unsaturated carbonyl compound 48 with a reducing agent (M-H) followed by trapping of the enolate with an electrophile 50 to give 51 (Scheme 9.11). 

Finally, the use of other hydrogen donors, including various formate salts, are convenient reducing reagents for the Pd-catalyzed reduction of a,/3-unsaturated carbonyl compounds. 

The reduction of a,3-unsaturated carbonyls with lithium in ammonia is a versatile reaction of great utility. The advantage of this method is that regiospecific enolates are obtained that are sometimes not accessible by other routes. This technique finds important applications in steroid-like systems (equation 15). °... 

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