Lead tetraacetate allylic oxidation

Although lead tetraacetate can attack many polar and nonpolar functions in the steroid molecule, its greatest reactivity is towards vicinal diols. These diols are generally cleaved so rapidly under stoichiometric conditions that other alcohol functions in the molecule need not be protected. Thus lead tetraacetate in acetic acid at room temperature splits the 17a,20-diol group in (9) to yield the 17-ketone (10), the allylic A -3jS-alcohol remaining intact during this oxidation. Since lead tetraacetate is solublein many anhydrous... 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

The rest of the synthesis (Scheme 13) is completely stereospecific and most of the steps are known (20). The bicyclic acid was oxidatively decarboxylated with lead tetraacetate and copper acetate (21). The resulting enone was alkylated with methyllithium giving a single crystalline allylic tertiary alcohol. This compound was cleaved with osmium tetroxide and sodium periodate. Inverse addition of the Wittig reagent effected methylenation in 85% yield. Finally, the acid was reduced with lithium aluminum hydride to grandisol. 

Tetrahydrofuranes. Cekovic, BoSnjak, and Mihailovic have reviewed their work on the oxidative cyclization of aliphatic alcohols to tetrahydrofuranes by oxidation with lead tetraacetate. The reagent should be free of acetic acid. Anhydrous calcium carbonate can be added to neutralize the acetic acid in the oxidant and that formed during the reaction. The reaction involves selective oxygenation of a 8-carbon the actual mechanism is uncertain, but is probably a radical reaction. Direct oxidation of the alcohol is generally of minor importance, but P-fragmentation can be a problem, particularly when a stable benzyl or allyl radical is formed. 

In contrast to lead tetraacetate, simple addition to the double bond does not occur as a side re-action. While allylic rearrangement is common and mixtures of products are frequently obtained, the reaction often proceeds in very high yield and is simple to carry out the alkene is simply heated in an appropriate solvent with mercury(II) acetate until reaction is complete. Mercury(II) acetate has also been us for dehydrogenation, particularly in the steroid field. One interesting example incorporating simultaneous dehydrogenation and allylic oxidative rearrangement is seen in the reaction of abietic acid (37 equation 16). ... 

Acetoxylation of enol thioethers. The reaction of enol thioethers with lead tetraacetate (1,1 equiv.) in THF for 1 hour followed by addition of BFj etherate (or 5 N KOH-ether) results in allylic acetoxylation. The reaction is considered to involve bisacetoxylation of the double bond. Oxidation of sulfur is not observed. Examples ... 

Lead acetate azides, Pb(OAc)4 (N3)n, prepared in situ from lead tetraacetate and azidotrimethylsilane, react with alkenes to yield a variety of products, depending on the structure of the alkene 1,2-diazides. 1,2-acetoxy azides, a-azido ketones, allylic azides, and <5-oxo nitriles (by the oxidative cleavage of cyclohexene rings)97. The diazides and acetoxy azides are formed by preferential syn addition, but the diastereoselectivity (up to 3 1) is far from satisfactory with both acyclic and cyclic alkenes98,99. 

Oxidation of olefins. Kabbe explored briefly the oxidation of cyclohexene, styrene, and o-allylphenol and expressed the view that thallium triacetate is intermediate between lead tetraacetate and mercuric acetate. Anderson and Winstein followed the oxidation of cyclohexene in acetic acid at room temperature (several days) by vapor-phase chromatography and accounted quantitatively for all the five products formed the cij-and rrans-diacetates (1 and 2), the ring-contracted diacetate (3) and aldehyde (4), and the product of allylic oxidation (5). In dry solvent the transdiacetate (2) predominated, and in moist solvent the cis-diacetate (I) predominated. 

Oxidation of a )3,y-unsaturated acid of type (VII) with lead tetraacetate gives either of the allylic acetoxy derivatives (VI11) or (IX), or a mixture. ... 

Oxidation, allylic r-Butyl perbenzoate. f-Butyl chromate. Lead tetraacetate-N-Bromosuc-cinimide. Mercuric acetate. Nickel peroxide. Selenium dioxide. i-amines Cr08-Pyridine. 

Allylic oxidation. A6-Steroids (cholesteryl acetate and diosgenyl acetate) undergo allylic acetoxylation at C7 when oxidized with lead tetraacetate in benzene or glacial acetic acid solution. 7/3-Acetoxy and 7 -acetoxy derivatives are formed in a ratio of 3 2.10... 

ALLYLIC OXIDATION N-Bromosuc-cinimide. Lead tetraacetate. Selenium dioxide. 

As a matter of convenience, the results of oxidation of longifolene with lead tetraacetate, and ruthenium tetraoxide will also be summarized here steric diversion may have little role in forming the products of these reactions. One of the important reactions of lead tetraacetate with olefins is allylic substitution/rearrangement (J02). Since, this pathway is blocked for longifolene, the major product of this reaction is the ring-expanded enol acetate (154) (Chart 17), exactly parallel to what happens with camphene (8J, 99). 

Oxasteroids arising from tandem P-fragmentation and cyclization are products in the photolysis of the hypoiodites generated from 3-hydroxy-A -steroids and lead tetraacetate-iodine, as outlined in Scheme 84. The path of the formation of the oxasteroids is outlined in Scheme 85 one-electron oxidation of the intermediate radicals arising from the cyclization of allylic radicals generates the oxonium ions, which trap acetic acid to give the products. 

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