Oxidative cleavage olefins, osmium tetroxide

A positional isomer of xylopinine of structure (345), prepared by synthesis, has been subjected to Hofmann degradation to the olefin (346), which on oxidation with osmium tetroxide gives the diol (347). Cleavage of the diol gave the dialdehyde (348), photolysis of which yields a mixture of cis-alpen-igenine (30%) and alpenigenine (339) (1%) (S.B. Prabhakar, et al., J.Chem.Soc., Perkin I 1981, 1273). The 7,8,13,14-tetra-... 

Oxidative cleavage of the olefin is accomplished by the method of ijemieux-Johnson.12 The process begins with dihydroxylation of the double bond using osmium tetroxide (see Chapter 3)T leading to a cis diol and osmium(VI) oxide. The added periodate has two functions first, it reoxidizes the osmium(VI) species to os-mium(VIII), but it also cleaves the glycol oxidatively to an aldehyde. This is the reason for utilizing several equivalents of periodate. The periodate is in turn reduced from the +VH to the +V oxidation state. 

As an oxometal component, osmium tetroxide is the most reliable reagent on the laboratory scale to produce c/s-diols. Ruthenium tetroxide in the presence of NaI04 effects oxidative cleavage of olefins [4], but has been successfully employed for so-called lightning dihydroxylation reactions using a two-phase medium [6]. 

It was reported [39] that osmium tetroxide promoted catalytic oxidative cleavage of ds-stilbene and other olefins. The ds-stilbene catalytic oxidative cleavage gave benzoic acid in 95% yield. The process for the preparation of substituted aromatic and heteroaromatic aldehydes and carboxylic acids by the oxidation of substituted stilbenes has been patented [40]. Stilbenes of various substituents (Ri, R2 = H, Cl-4 alkyl, Cl-4 alkoxy, OH, NO2, CN, COjH, CONH2, SO3H, halogen X = C, N Z = CHO, CO2H ... 

The combination of A -bromoacetamidc, silver acetate, and dry acetic acid has been shown to be superior to Woodward s procedure for the rfy-hydroxylation of olefins. Work up of the reaction mixture is simply effected by hydrolysis of the dioxolenium ion, followed by cleavage of the hydroxyacetate intermediate with lithium aluminium hydride. The use of a co-oxidant, such as sodium chlorate or hydrogen peroxide, allows the addition of catalytic quantities of osmium tetroxide to prepare c/y-diols from olefins. However the reaction is often complicated by further oxidation of the glycol to the a-ketol. The use of tertiary amine A -oxides, particularly A -methylmorpholine A -oxide, prevents this oxidation and gives higher yields of the desired product (Table 6). Another variation on this theme employs... 

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