Ketones partial reduction

The final step can involve introduction of the amino group or of the carbonyl group. o-Nitrobenzyl aldehydes and ketones are useful intermediates which undergo cyclization and aromatization upon reduction. The carbonyl group can also be introduced by oxidation of alcohols or alkenes or by ozonolysis. There are also examples of preparing indoles from o-aminophcnyl-acetonitriles by partial reduction of the cyano group. 

Epoxides are normally hydrogenated in preference to saturated ketones but double bonds are usually reduced under these conditions. It is possible in some cases to selectively cleave an epoxide without saturating double bonds by the use of the deactivated catalysts recommended for the partial reduction of acetylenes (see section IV) or by the addition of silver nitrate to the palladium-catalyzed reaction mixture. " ... 

Aldehydes and ketones are among the most important of ail compounds, both in biochemistry and in the chemical industry. AUdehydes are normally prepared in the laboratory by oxidation of primary alcohols or by partial reduction of esters. Ketones are similarly prepared by oxidation of secondary alcohols or by addition of diorganocopper reagents to acid chlorides. 

The enzymatic reduction of T-configured alkenones represents a valuable alternative to the enzyme-catalyzed reduction of the corresponding alkynone with subsequent partial reduction of the C-C triple bond. Nevertheless, since in our results the Z-configured vinylic alcohols are not accessible in enantiopure form by enzyme-catalyzed reduction of the olefinic ketones, for this class of compounds the two-step process via the propargylic alcohols is a highly valuable new strategy. 

The enantioselective hydrogenation of olefins, ketones and imines still represents an important topic and various highly enantioselective processes based on chiral Rh, Ru or Ir complexes have been reported. However, most of these catalysts failed to give satisfactory results in the asymmetric hydrogenation of aromatic and heteroaromatic compounds and examples of efficient catalysts are rare. This is especially the case for the partial reduction of quinoline derivatives which provide 1,2,3,4-tetrahydroquinolines, important synthetic intermediates in the preparation of pharmaceutical and agrochemical products. Additionally, many alkaloid natural products consist of this stmctural key element. 

Tandem radical addition/cydization reactions have been performed using unsaturated tertiary amines (Scheme 9.11) [14,15]. Radical attack is highly stereoselective anti with respect to the 5-alkoxy substituent of 2-(5f-J)-furanones, which act as the electron-deficient alkenes. However, the configuration of the a position of the nitrogen cannot be controlled. Likewise, tandem addition cyclization reactions occur with aromatic tertiary amines (Scheme 9.12) in this case, acetone (mild oxidant) must be added to prevent the partial reduction of the unsaturated ketone [14]. 

Midland and Graham completed a total synthesis of (-)-pestalotin (81)33 (Scheme 4.3bb). The asymmetric reduction of the ketone 82 gave the propar-gylic alcohol 83 with high enantioselectivity. Partial reduction of the alkyne,... 

CXXII), which gave CXXIII that is, no reduction of the C-16 ketone carbonyl and only partial reduction of the lactam carbonyl. iV-Methyl-isopseudostrychnidine (CXXIV), on the other hand, smoothly gave the C-16 secondary alcohol (112). 

OL-Alkenylcarbinols Reduction of alkynyl chlorohydrins, readily available from reaction of a-chloro ketones with lithium acetylides, with lithium aluminum hydride and sodium methoxidc results in partial reduction of the triple bond as well as a facile rearrangement to provide a-alkenylcarbinols. One useful feature is that only (E)-disubstitutcd alkenes are formed. The addition and reductive rearrangement can be conducted in a single operation. 

The most significant ketone produced by yeast is diacetyl (2,3-butanedione), a vicinal diketone, although malolactic fermentation is a more important source, when it is used in wine production. Having a sensory threshold of 0.2-2.9 mg/L, according to the type of wine, it is characterised by a nutty , toasty or buttery aroma depending on concentration (Martineau et al. 1995). Dry white wines tend to contain lower concentrations (0.1-2.3 mg/L) than red wines (0-7.5 mg/L) (Bartowsky et al. 2002 Martineau et al. 1995). Acetoin, which produces a buttery flavour, is formed by partial reduction of diacetyl, and is itself reduced to 2,3-butanediol. Acetoin is usually present at concentrations (<80 mg/L) much lower than its sensory threshold of 150 mg/L (Romano and Suzzi 1996). 

Nitriles undergo nucleophilic addition to the polar C=N bond in the same way that carbonyl compounds do. The most important reactions of nitriles are their hydrolysis to carboxylic acids, reduction to primary amines, partial reduction to aldehydes, and reaction with organometallic reagents to yield ketones. 

---