Of keto group

The method of choice for the reduction of keto groups to methylenes remains the WolfT-Kishner reaction (2) or its modification discovered by Huang-Minlon (5). Originally, the hydrazone formation was carried out in a separate step, followed by... 

This derivative is useful for determining the presence and number of keto groups as well as for protecting the ketone from enolization. Some diketones that polymerize readily, such as 2,3-butanedione. should be freshly distilled and the methoxime derivatives should be prepared. 

When we first contemplated thermochemical products available from Glu, a search of the literature revealed no studies expressly directed at hydrogenation to a specific product. Indeed, the major role that Glu plays in hydrogenation reactions is to act as an enantioselectivity enhancer (17,18). Glu (or a number of other optically active amino acids) is added to solutions containing Raney nickel, supported nickel, palladium, or ruthenium catalysts and forms stereoselective complexes on the catalyst surface, leading to enantioselective hydrogenation of keto-groups to optically active alcohols. Under the reaction conditions used, no hydrogenation of Glu takes place. 

By far the most important reducing system for the batchwise application of vat dyes is sodium dithionite (Na2S204) in a solution of sodium hydroxide. Obviously the theoretical concentrations required will depend on the number of keto groups in the dye molecule and on its relative molecular mass and concentration, but the reaction can be represented as in Scheme 12.20 for an anthraquinonoid dye with two keto groups. The effect of air oxidation on alkaline... 

Hydrolysis reactions are illustrated by the deacylation of colchicine (10) (amide hydrolysis), olivomycin A (29), Rifamycin B (46) or thymoxamine (52) (ester hydrolysis). The reduction of pentoxyfylline (32), zearalenone (56) or warfarin (65) are examples of the common reduction of keto groups, generally affording, with a high stereospecificity, one of the alcohol stereoisomers. 

Stereospecific reductases are involved in the reduction of keto groups to secondary alcohols the desired endproducts of a deoxysugar biosynthesis. This has been shown for the biosynthesis of L-rhamnose and CDP-ascarylose (see Sect. 3.1.4). The reduction of keto groups at C-4 have been proposed to be catalyzed by DnrV (daunorubicin 29), EryBIV (erythromycin 14), and StrL (streptomycin 24) [21]. 

The main use of hydrazine in steroid chemistry has been in the reduction of keto-groups to give unsubstituted methylene groups by the Wolff-Kishner process, particularly as modified by Huang-Minion fij. Hydrazones of simple ketones react rapidly at 200° in hydroxylic solvents such as "diethylene glycol containing strong alkali. 

Many experimental procedures have been developed in order to influence the enantioselectivity and the stereochemistry of the products use of organic media, the addition of various compounds to the incubation mixture, or enclosure in a dialysis tube can be helpful. Immobilized BY can be used in water or in organic solvents for the same purpose. Slight modifications of the substrate can obtain the same result and many examples are available. Several other groups can be present in the carbonyl-containing substrate. For instance, the asymmetric reduction of keto groups in compounds containing a cyclopropyl moiety has been achieved (eq 5). ... 

We should also mention that simple nitrogen-containing derivatives of keto groups, such as 3-azo (D-150), 3-azine (D-5), and 3-hydrazone (D-15) derivatives, may also sometimes favorably affect the activity or the dissociation of activities, while others work in the opposite direction, such as 3-dimethylhydrazone (S-115,S-116) or 17-N-pyrrolidinium salt derivatives (E-15). 

The lower reactivity of Raney Ni towards sulfones allows the chemoselective desulfurization of (4) with Raney W-2, but an appreciable amount of reduction of keto groups was observed. Raney Ni W-2 also tolerates sulfonamides in this work an ionic mechanism was suggested in order to explain the substitution reaction observed. Lactones,keto lactones," and unsaturated lactoneswere tolerated during the desulfurization of sulfide or dithioketal" groups, but deactivated (e.g. by acetone) Raney Ni was generally used. In contrast, carbon-halide bonds are cleaved where the desulfurization is stereoselective only because the product is the thermodynamically more stable d5-lactone (equation 2)." e-" f... 

The reduction proved to be chemoselective for aldehyde functions even in the presence of keto groups. The yield in homogeneous reactions varies between 9% and 30%. When aminated polystyrene is used as the basic component, the active anionic species [Rh6(CO)i5H] is immobilized via interaction with the supported ammonium eations and leads to an increase in the yields up to 91-99% [22, 23], while the catalyst can be recovered by simple filtration from the product. 

Yamamura, S., Hirata, Y. Zinc reductions of keto groups to methylene groups. J. Chem. Soc. C1968, 2887-2889. 

Protection of keto groups. Fried and Nutile used the methoxyimino derivative (2) for protection of the A - -diene-3-one system of the steroid (1). Conversion to... 

The Archetypal Poly-p-keto Chain Structural Variations Produced by Reduction of Keto Groups... 

Ivanov and coworkers,in an investigation of the effect of keto groups in the aglycon, foimd that the ease of hydrolysis of the 8-D-glucopyrano-sides studied is in the order 3-acetoxypropanonyI (38)iS>2,3-dihydroxy-propyl (39) > methyl (1). 

The polyketides are a family of natural products containing many important pharmaceutical agents that are synthesized through the multienzyme complex, polyketide synthase, which can display substantial molecular diversity with respect to chain length, monomer incorporated, reduction of keto groups, and stereochemistry at chiral centers (189). This variability, together with the existence of several discrete forms of polyketide synthase, allows the generation of diverse structures like erythromycin, avermectin, and rapamycin. This biochemical diversity has been considerably expanded by the introduction of new sub strate species that were used by the enzymes to produce new or unnatural polyketides (190). 

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