Osmic acid, esters

A soln. of crotyl bromide (mixture with 3-bromo-l-butene) in abs. ether added slowly to Mg in ether at such a rate as to keep the mixture under reflux, stirring continued 1 hr. at room temp., the ether removed with dry Ng, the residue dissolved in tetrahydrofuran, a soln. of (+)-8-methyl-frans-hydrindan-4-ol-l-one in the same solvent added dropwise, stirred 2 hrs. at room temp., then allowed to stand overnight, the product treated with osmium tetroxide, the resulting osmic acid ester isolated by filtration and refluxed 3.5 hrs. with Na-sulfite in water-ethanol tetrol (Y 88%) dissolved in dioxane, and shaken 75 min. with Pb-tetraacetate under Ng dihydroxyaldehyde (Y 74.5%). H. H. Inhoffen, H. Burkhardt, and G. Quinkert, B. 92, 1564 (1959). 

Cis 2-butene (48a) thus yields the meso 1,2-diol (47), i.e. the overall hydroxylation is stereoselectively SYN, as would be expected from Os—O cleavage in a necessarily cis cyclic ester (46). The disadvantage of this reaction as a preparative method is the expense and toxicity of 0s04. This may, however, be overcome by using it in catalytic quantities only, but in association with H202 which re-oxidises the osmic acid, (HO)2OsOz, formed to 0s04. 

Osmium tetroxide (0s04, sometimes called osmic acid) reacts with alkenes in a concerted step to form a cyclic osmate ester. Oxidizing agents such as hydrogen peroxide (H202) or tertiary amine oxides (R3N+—O-) are used to hydrolyze the osmate ester and reoxidize osmium to osmium tetroxide. The regenerated osmium tetroxide catalyst continues to hydroxylate more molecules of the alkene. 

The role of osmium tetroxide in cis hydroxylation has been established. A cyclic ester of osmic acid 94 is formed. Hydrolysis produces an Os... 

Partial formation of cis-glycols. A soln. of osmic acid in benzene added to 3,20-bis(ethylenedioxy)-6-methyl-5,16-pregnadiene-llj, 21-diol in benzene containing pyridine, stirred 30 min. at room temp., the resulting osmate ester decomposed by addition of Na-sulfite and KHGO3 in water followed by methanol, vigorously stirred for 3 hrs. 3,20-bis (ethylenedioxy)-6-methyl-5-pregnene-... 

Orthophosphoric acid esters, alkylation with - 22, 775 Orthosilicates s. Silicates Osmic acid s. Osmium tetroxide Osmium tetroxide OCv >CC Oxa... s. a. 0-Heterocyclics Oxacyclimmonium salts, 2-hydroxy- 22, 990, 3,5-Oxadiazines, 6-oxo-... 

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