Oxidation anti-dihydroxylation

Whereas the yyn-dihydroxylation reaction, which is described above, is performed in the presence of metal oxides KMn04 or OSO4, the anti-dihydroxylation is carried out with... 

The syn dihydroxylation of olefins is usually performed in the presence of metal oxide, for example, KMn04 or OSO4, and with t-BuOOH or HP as terminal oxidants. The anti dihydroxylation can be achieved with peracetic acid and m-ClC6H4C03H in water. However, the atom efficiency of these oxidants is low, and they form equimolar amounts of deoxygenated compounds [36a, bj. 

Oxidation by peroxycarboxylic acids (RC03H) and hydrolysis of epoxides to give anti- dihydroxylation... 

Anti dihydroxylation An oxidation-hydrolysis sequence in which an ami diol forms from an alkenc. 

Hydrogen peroxide in presence of tungsten oxide (WO3) or selenium dioxide (SeO ) react with alkene to give anti-dihydroxylation products (Scheme 73). 

Treatment of oxacyclopropanes with water in the presence of catalytic acid or base leads to ring opening to the corresponding vicinal diols. These reactions follow the mechanisms described in Section 9-9 The nucleophile (water or hydroxide) attacks the side opposite the oxygen in the three-membered ring, so the net result of the oxidation-hydrolysis sequence constitutes an anti dihydroxylation of an alkene. In this way, tram-2-butene gives me50-2,3-butanediol, whereas ds-2-butene furnishes the racemic mixture of the 1R, >R and 2S,3S enantiomers. 

On the other hand, the observed syn preference of 59a is consistent with a study of hydroboration of 59a with diborane by Schueler and Rhodes [127], who obtained a mixture of the monoalcohols (symanti = 74 26) upon oxidative work-up. A similar magnitude of. yyn-preference was found (syn anti = 73 27) in the hydroboration with a bulkier borane, 2,3-dimethyl-2-butylborane (thexyl borane) [127]. This lack of effect of the bulk of the reagent in the hydroboration of 59a is consistent with the idea that the n face of 59a is free from steric bias [127], and that the syn preference of 59a found in dihydroxylation and epoxidation is non-sterically determined [128]. 

Stereoselective dihydroxylation The Os04-catalyzed dihydroxylation of 5-vinyl-4,5-dihydroisoxazoles (1) (obtained by reaction of the nitrile oxide derived from nitroethane with 1,3-dienes) is anti-selective, and the anti-selectivity is markedly enhanced by a cw-substituent on the double bond. 

In addition to being an efficient chiral controller in a number of stereoselective transformations of chiral acrylates, (i.e. the Diels-Alder reaction, the conjugate reduction, the asymmetric dihydroxylation, and the nitrile oxide cycloaddition ) the bomanesultam (11) has been shown to be an exceptionally efficient chiral auxiliary for stereoselective aldol condensations (eqs eq 3 and eq 4). Depending upon the reaction conditions, A -propionylsultam can produce either the syn or anti aldol product with an excellent diastereoselectivity, Furthermore, good diastereoselectiv-ities are also observed for the corresponding acetate aldol reaction (eq 5), ... 

Figure 16.2-53. Chemoenzymatic synthesis of 2-hydroxy-l-indanone. The racemic syn and anti diols were prepared by chemical dihydroxylation of indane. Asymmetric induction was achieved by microbial oxidation (MO) of these diols.

The AD reaction was central in the preparation of (+)-cw-sylvaticin 41,27 a natural product found to have potent anti-tumor activity. The ability of this compound to inhibit ATP production by blockade of the mitochondrial complex I was thought to be the origin of this biological outcome. The AD reaction, in this example, exploited the preference of this reaction for the oxidation of 1,2-frans-alkenes over monosubstituted alkenes. The E,E-isomer of tetradecatetraene 42 could be chemoselectively dihydroxylated at both internal alkenes, while the terminal alkenes remained untouched. Thus, 43 was generated in excellent chemical yield. 

Epoxidation and Dihydroxylation of Alkenes There are several ways to convert alkenes to diols. Some of these methods proceed by syn addition, but others lead to anti addition. An important example of syn addition is osmium tetroxide-catalyzed dihydroxylation. This reaction is best carried out using a catalytic amount of OSO4, under conditions where it is reoxidized by a stoichiometric oxidant. Currently, the most common oxidants are f-butyl hydroperoxide, potassium ferricyanide, or an amine oxide. The two oxygens are added from the same side of the double bond. The key step in the reaction mechanism is a [3 + 2] cycloaddition that ensures the syn addition. 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

Despite the dominance of dihydroxylation reactions employing Os(VlIl) for the production of vicinal diols, alternatives have been utilized in asymmetric syntheses. The best known of these is perhaps Woodward s modification [205] of the Prevost reaction [206]. This classic oxidation is known to generally yield anti-1,2-diacetates from olefinic substrates [207]. Woodward s key modification involves the inclusion of water in the reaction mixture. The intermediate iodo-acetate 303 undergoes hydrolytic cleavage... 

This chapter introduces several stereospecific addition reactions. Some of them occur exclusively via a syn addition (such as hydrogenation or hydroboration-oxidation), while others occur exclusively via an anti addition (such as bromination or halohydrin formation). When drawing the products of a stereoscpecific addition reaction, be careful to avoid drawing a wedge or a dash on a location that is not a chirality center. For example, consider the following syn dihydroxylation. In such a case, it is tempting for students to draw the products as if they have two chirality centers, like this ... 

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