Ketones selective reduction

Selective reduction of ketones. Selective reduction of a ketone in the presence of an aldehyde is possible by selective reaction of the aldehyde with -bulylamine (4 A molecular sieves), reduction of the ketone with lithium tri-(-butoxyaluminum hydride, and, finally, cleavage ol the aidimine to the aldehyde with aqueous HCI. The three-step sequence can be conducted in one pot. The selectivity is high. The method is not applicable to conjugated ketones because of conjugate reduction. 

Reduction of Ketones.—Selective reduction of the 20-oxo-group in 17a,21-dihydroxy-pregn-4-ene-3,20-dione (Reichstein s S) with sodium borohydride in methanol at 0 °C gave the 20fi- and 20a-ols in ratio 2 1, contrary to an earlier report that the 20jS-ol is formed almost exclusively. 

The preparation of disubstituted epoxides is similarly accomplished by the addition of nucleophiles to more highly substituted ketones. Selective reduction of keto sulfides with either L-Selectride or zinc... 

Rhodium-alumina acetic acid Sec. alcohols from ketones Selective reduction... 

Hydrocarbons from ketones Selective reduction s. 13,112 Phthalides from o-dicarboxylic acid anhydrides s. 11,123... 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

The aromatic primary and secondary stibines are readily oxidized by air, but they are considerably more stable than their aHphatic counterparts. Diphenylstibine is a powerful reducing agent, reacting with many acids to Hberate hydrogen (79). It has also been used for the selective reduction of aldehydes and ketones to the corresponding alcohols (80). At low temperatures, diphenylstibine undergoes an addition reaction with ketene (81) ... 

Reductive amination ol aldehydes or ketones by cyanoborohydride (or tnacetoxyborohydride) anion Selective reduction of carbonyls to alcohol, oximes to N alkylhydroxylarmnes, enamines to amines... 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

The chemistry of indium metal is the subject of current investigation, especially since the reactions induced by it can be performed in aqueous solution.15 The selective reductions of ethyl 4-nitrobenzoate (entry 1), 2-nitrobenzyl alcohol (entry 2), l-bromo-4-nitrobenzene (entry 3), 4-nitrocinnamyl alcohol (entry 4), 4-nitrobenzonitrile (entry 5), 4-nitrobenzamide (entry 6), 4-nitroanisole (entry 7), and 2-nitrofluorenone (entry 8) with indium metal in the presence of ammonium chloride using aqueous ethanol were performed and the corresponding amines were produced in good yield. These results indicate a useful selectivity in the reduction procedure. For example, ester, nitrile, bromo, amide, benzylic ketone, benzylic alcohol, aromatic ether, and unsaturated bonds remained unaffected during this transformation. Many of the previous methods produce a mixture of compounds. Other metals like zinc, tin, and iron usually require acid-catalysts for the activation process, with resultant problems of waste disposal. 

Carnell et al. discovered that whole cells of Cunninghamella echinulata NRRL1384 were able to deracemize racemic N-(l-hydroxy-l-phenylethyl)benzamide (24) to produce the (R) enantiomer (Figure 5.17) [30]. The deracemization involves fast, highly (S)-selective oxidation, followed by slower, partially (R)-selective reduction of the ketone (25). Optimization by removing competing extracellular amidase/prote-ase activity resulted in 82% yield and 92% ee. 

With this reaction available, a simple synthesis of unsymmctrical dibenzyl ketones (21) can be planned. The carbonyl group can be derived from a nitro group (22) by TlClg-catalysed hydrolysis (p T 183 ). Reversing the selective reduction gives (23) and a,6-dlsconnection separates this Into aldehyde (24) and nitro compound (25), available by reduction of (20). 

Reduction of carbon-carbon double bond Microalgae easily reduce carbon-carbon double bonds in enone. Usually, the reduction of carbonyl group and carbon-carbon double bond proceeds concomitantly to afford the mixture of corresponding saturated ketone, saturated alcohol, and unsaturated alcohol because a whole cell of microalgae has two types of reductases to reduce carbonyl and olefinic groups. The use of isolated reductase, which reduces carbon-carbon double bond chemoselectively, can produce saturated ketones selectively. 

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