Osmium Reagents

The unsubstituted pyrrolo[l,2-f]oxazol-3-one 295 underwent a dihydroxylation with catalytic osmium tetroxide to exclusively give diol 296. This diol resulted from attack of the oxidizing agent from the concave face of the molecule due to the pseudoaxial protons that sterically hinder the /3-face to be attacked by the osmium reagent (Equation 51) <2004AJC669>. 

Sharpless asymmetric dihydroxylation procedure was applied to the synthesis of the side chain of azinomycin A (equation 26)43. Horner-Emmons condensation of phospho-nate 36 with a /J-aziridine substituted acrolein afforded dehydroamino acid diene 37. Treatment of the diene with catalytic amounts of an osmium reagent and dihydroquini-dine (DHQD) p-chlorobenzoate resulted in asymmetric dihydroxylation, producing diol 38. Diol 38 was further converted to the naphthyl ester. 

The fact that only cis product is produced from either 2 or 4 means that both oxygens in the product come from the osmium reagent. This could result from either a concerted, 3 + 2 cycloaddition of OSO4 to the double bond or a sequential addition of one then another oxygen to the same side of the double bond. 

The reaction of alkenes with osmium tetroxide (0s04) is an example of an oxidation reaction (Following fig.). In this case the alkene is not split, but, a 1,2-diol is obtained which is also called a glycol. The reaction involves the formation of a cyclic intermediate where the osmium reagent is attached to one face of the alkene. On treatment with sodium bisulphite, the intermediate is cleaved such that the two oxygen atoms linking the osmium remain... 

The regioselectivity of this new procedure toward terminal olefins is considerably better than that realized with the earlier catalytic oxyami-nation procedures based on chloramine-T.3 However, the catalytic procedure cannot compete with the regiospecificity exhibited by the stoichiometric ferr-alkyl imido osmium reagents.3... 

The oxyamination and diamination procedures described so far suffer from two important limitations. They require a stoichiometric quantity of osmium reagents and it is difficult to remove the tert-alkyl group from the product. Hence, procedures that employ catalytic amounts of osmium tetroxide for the preparation of vicinal hydroxy 4-methylbenzenesulfon-amides, precursors of /1-hydroxy amines, have been developed. 

The use of chiral ligands has allowed the asymmetric oxidation of alkenes with osmium reagents in catalytic amounts [61-66], The cinchona alkaloids have played, and continue to play, a significant role in this useful synthetic methodology. 

MisaetZaneous - Enol chlorides can be converted to the corresponding ketones in fair yields at room temperature by Inclusion of Hg(0Ac)2 in the reaction mixture.A full paper has appeared which gives experimental details for the conversion of olefins to aminoalcohols by means of osmium reagents. 

A tethered aminohydroxylation (TA) reaction extends the utility of carbamate versions of the nitrogen source.29 Combining the nitrogen source with allylic or homoallylic alcohols enabled formation of products not previously accessible. Thus, exposure of 48 to the osmium reagent, in the absence of a chlorinating agent and base, led to cyclized amino alcohol 49. A similar result was observed for homoallylic alcohol derivative 50. 

The intermediate is hydrolyzed with aqueous hydrogen peroxide. Hydrogen peroxide re-oxidizes the osmium reagent back to osmium tetroxide. (Because osmium tetroxide is recycled, only a catalytic amount of this expensive and toxic oxidizing agent is needed.)... 

Because OSO4 is expensive and highly toxic, a commonly nsed modification calls for the nse of only catalytic quantities of the osmium reagent and stoichiometric amounts of another oxidizing agent such as H2O2, which serves to reoxidize reduced osmium. 

Vedejs has suggested that steric interactions between the osmium reagent and the substrate are dominant in dihydroxylations of (E)-alkenes. In the... 

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