Hydroxy-de-hydrogenation

Oxidation of Aldehydes to Carboxylic Acids Hydroxylation or Hydroxy-de-hydrogenation... 

Asymmetric hydrogenation. Hydrogenation of the a-substituted /8-kcto ester 2 with a Ru(ll) catalyst coupled with (R)-l provides the desired /8-hydroxy ester 3 in 90% ec but with only moderate. vyrr-selectivity (74% de). Hydrogenation with high cnantio-and diastereosclectivity can be achieved using a BINAP derivative in which the phenyl... 

Mandelic acid. This preparation is an example of the synthesis of an a-hydroxy acid by the cyanohydrin method. To avoid the use of the very volatile and extremely poisonous hquid hydrogen cyanide, the cyanohydrin (mandelonitrile) is prepared by treatment of the so um bisulphite addition compound of benzaldehj de (not isolated) with sodium cyanide ... 

Both reactions were carried out under two-phase conditions with the help of an additional organic solvent (such as iPrOH). The catalyst could be reused with the same activity and enantioselectivity after decantation of the hydrogenation products. A more recent example, again by de Souza and Dupont, has been reported. They made a detailed study of the asymmetric hydrogenation of a-acetamidocin-namic acid and the kinetic resolution of methyl ( )-3-hydroxy-2-methylenebu-tanoate with chiral Rh(I) and Ru(II) complexes in [BMIM][BF4] and [BMIM][PFg] [55]. The authors described the remarkable effects of the molecular hydrogen concentration in the ionic catalyst layer on the conversion and enantioselectivity of these reactions. The solubility of hydrogen in [BMIM][BF4] was found to be almost four times higher than in [BMIM][PFg]. 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

Interestingly, one-electron oxidants partly mimic the effects of OH radicals in their oxidizing reactions with the thymine moiety of nucleosides and DNA. In fact, the main reaction of OH radicals with 1 is addition at C-5 that yields reducing radicals in about 60% yield [34, 38]. The yield of OH radical addition at C-6 is 35% for thymidine (1) whereas the yield of hydrogen abstraction on the methyl group that leads to the formation of 5-methyl-(2 -de-oxyuridylyl) radical (9) is a minor process (5%). Thus, the two major differences in terms of product analysis between the oxidation of dThd by one-electron oxidants and that by the OH radical are the distribution of thymidine 5-hydroxy-6-hydroperoxide diastereomers and the overall percentage of methyl oxidation products. 

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