Ketones reductive removal

Formation of an 11-methoxybenzyl ether occurs in satisfactory yield during the preparation of a 17,21 -benzylidene derivative [e.ff., (26)]. The usefulness of this ether and of related mixed acetals in removing the inhibitory effect of the 11 j5-hydroxyl group on the 20-ketone reduction has been reported. ... 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

Reductive removal of oxygen from functional groups such as ketones and aldehydes,... 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Evidently, these or closely related intermediates are accessible and reactive, since the synthesis was successfully achieved as outlined in Scheme 13.28. In addition to the key cationic cyclization in Step D, interesting transformations were carried out in Step E, where a bridgehead tertiary alcohol was reductively removed, and in Step F, where a methylene group, which was eventually reintroduced, had to be removed. The endocyclic double bond, which is strained because of its bridgehead location, was isomerized to the exocyclic position and then cleaved with Ru04/I04. The enolate of the ketone was then used to introduce the C(12) methyl group in Steps F-3 and F-4. 

Vicenzi, J.T., Zmijewski, M.J., Reinhard, M.R. et al. (1997) Large-scale stereoselective enzymatic ketone reduction with in-situ product removal via polymeric adsorbent resins. Enzyme and Microbial Technology, 20, 494-499. 

Queen substance (140) was synthesized from the same telomer 137 (127). The PdCl2-catalyzed oxidation of the terminal double bond produced the methyl ketone. Reduction of the internal double bond was followed by partial hydrolysis and the displacement of the carboxyl group with phenylselenyl group, which was removed to produce queen substance (140) ... 

Reductive removal of electronegative o-substituents from ketones, acids and derivatives... 

The reaction involves formation of an imine through reaction of ammonia with the ketone, followed by reduction of this imine (see Section 7.7.1). As we noted earlier (see Section 15.1.1), nicotinamide coenzymes may also participate in imine reductions as well as aldehyde/ketone reductions, further emphasizing the imine-carbonyl analogy (see Section 7.7.1). The reverse reaction, removal of ammonia from glutamate, is also of importance in amino acid catabolism. 

Ketosulfoxides, which are intermediates in the alkylation of ester by Li-CH2 SOCH3, are cleaved to ketones (Eq. (234) ) 5 21K Analogously, the acetoxy group has been reductively removed from a-acetoxylated nitriles 522 ... 

A 1,2-carbonyl transposition sequence based on the Woodward dithioacetalisa-tion reaction was used to relocate a carbonyl group in 95.1 [Scheme 2.95] to the adjacent position in 95 6.190 A total of 6 steps were required in which the key step was the dithioacetalisation of the a-hydroxymethylene derivative 95 2. After reduction of the carbonyl group, the dithiane was hydrolysed to the a-acetoxy ketone whence dissolving metal reduction removed the acetate function. 

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