Ketones dihydroxylation

Since Hassner s initial report in 1975,7 oxidation of an enol silyl ether with peracid has been a reliable method for the preparation of a-siloxy and ot-hydroxy ketones. However, the submitters have found that, if the enol silyl ether possesses certain structural features, the reaction, with more than two equivalents of the oxidant, affords oc.a -dihydroxylated ketones (i.e., introduction of two oxygen atoms in a single-step) instead of the expected monohydroxylated compounds.8... 

The completion of the synthesis of the polyol glycoside subunit 7 requires construction of the fully substituted stereocenter at C-10 and a stereocontrolled dihydroxylation of the C3-C4 geminally-disub-stituted olefin (see Scheme 10). The action of methyllithium on Af-methoxy-Af-methylamide 50) furnishes a methyl ketone which is subsequently converted into intermediate 10 through oxidative removal of the /j-methoxybenzyl protecting group with DDQ. Intermediate 10 is produced in an overall yield of 83 % from 50) , and is a suitable substrate for an a-chelation-controlled carbonyl addition reaction.18 When intermediate 10 is exposed to three equivalents of... 

The iyn-preference of 105b and 105c is similar to those observed in the reduction of the related ketones, 34 and in the epoxidation and dihydroxylation of the related olefins 71 [104]. Although the trajectories of the attacking reagents are considered to be different in these reactions [83-87, 170, 171], all three types of reactions favor iyn-addition, which excludes a predominant role of divergent trajectories in these dibenzobicyclic systems. 

The oxidation of enol ethers and their derivatives is a useful method for the synthesis of a-hydroxy-ketones or their derivatives, which are versatile building blocks for organic synthesis. Since enol ethers and esters are types of olefin, some asymmetric epoxidation and dihydroxylation reactions have been applied to their oxidation. 

To avoid the retro-Diels-Alder reaction, 56 was dihydroxylated prior to the introduction of the bromine atom (57). Removal of the acetonide group followed by cleavage of the diol afforded a bis-hemiacetal. Selective reduction of the less-hindered hemiacetal group gave 58. The remaining hemiacetal was protected, and the ketone was converted to an enol triflate, thus concluding the synthesis of the electrophilic coupling component 51. 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

However, oxidation processes like epoxidation or dihydroxylation reactions are important transformations in solid support chemistry, because they allow the synthesis of ketones [226], aldehydes [227, 228] and even sulfoxides and sulfones [229]. 

The first observation of the c/x-dihydroxylation reaction with RuO was made by Sharpless et al. in 1976, who noted that E and Z-cyclododecene were oxidised by stoich. RuO /EtOAc/-78 C to the threo and erythro diols [299]. Later RuCyaq. Na(IO )/EtOAc-CH3CN/0 C was used and reaction conditions optimised for many alkenes [300] a useful paper with good practical examples discusses the scope and limitations of the procedure (Table 3.2) [301]. Later oxidations were done with stoich. RuOyaq. acetone/-70 C [302] the same reagent converted A, and A steroids to cw-diols, ketones or acids [303], while RuO /aq. Na(10 )/acetone gave diones and acids [304]. 

Abstract This chapter covers one of the most important areas of Ru-catalysed oxidative chemistry. First, alkene oxidations are covered in which the double bond is not cleaved (3.1) epoxidation, cis-dihydroxylation, ketohydroxylation and miscellaneous non-cleavage reactions follow. The second section (3.2) concerns reactions in which C=C bond cleavage does occur (oxidation of alkenes to aldehydes, ketones or carboxylic acids), followed by a short survey of other alkene cleavage oxidations. Section 3.3 covers arene oxidations, and finally, in section 3.4, the corresponding topics for aUcyne oxidations are considered, most being cleavage reactions. 

In the ensuing discussion we consider first of all those oxidations which do not involve cleavage of the C=C bond (epoxidation, Section 3.1, c/x-dihydroxylation 3.1.2 and ketohydroxylation 3.1.3, other non-cleavage oxidations 3.1.3.4). Cleavage reactions follow in 3.2 with formation of aldehydes or ketones (3.2.1) and acids (3.2.2). 

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