Bromine reaction with enols

Unfortunately, the hydrazino esters or amides required inconveniently high pressures for their hydrogenolysis (500 psi Schemes 8.25a and b). An improvement involved the direct azidation of the same enolates using arylazide derivatives, which were found to undergo reactions with enolate nucleophiles to provide a C-sulfonyltriazene intermediate which could be decomposed to the a-azido ester (Scheme 8.25c) [110]. Alternatively, azides may be obtained by enolate bromination followed by Sn2 azide displacement note that these techniques are stereochemically complementary. Similar chemistry has also been accomplished using 10-suIfonamidoisobomyl chiral auxiliaries (Scheme 8.25d) [111]. 

A classical reaction leading to 1,4-difunctional compounds is the nucleophilic substitution of the bromine of cf-bromo carbonyl compounds (a -synthons) with enolate type anions (d -synthons). Regio- and stereoselectivities, which can be achieved by an appropiate choice of the enol component, are similar to those described in the previous section. Just one example of a highly functionalized product (W.L. Meyer, 1963) is given. 

Bromination of the enolate anion from the reaction of 3j -acetoxypregna-5,16-dien-20-one (1) with methylmagnesium bromide in the presence of cuprous chloride affords (after treatment with sodium iodide to dehalo-genate any 5,6-dibromide) a mixture of 17a-bromo- and 17)5-bromo-16a-methyl compounds (11) and (12) in a ratio 9 1. The 17a-iodides can be obtained in an analogous reaction. 

Very little is known concerning the simple, monocyclic 3-hydroxy-furans (cf. reference 15). Both the oxo and hydroxy forms of the substituted 3-hydroxyfurans 26 and 27 (R = H, CcHn) have been isolated/ but the individual tautomers slowly undergo interconversion. The enol forms give a positive reaction with ferric chloride, react rapidly with bromine, and form a peroxide with oxygen. From chemical evidence, the benzo derivatives of 3-hydroxyfuran, 28 and 29, appear to exist predominantly in the oxo form, and this is further supported by ultraviolet spectral data. Stefanye and Howard- ... 

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

The proof is based on the fact that the reaction between acetone and bromine is recognised as being unimolecular and not, as would be expected, bimolecular. In the determination of the velocity of reaction, therefore, the (slow) conversion of the ketone into the enol is measured, whilst the addition of the bromine occurs with immeasurable rapidity. 

A detailed examination of OSO4 reactions with A -steroids has been reported." The A-ring conformation of the reactant or derived complex is important in determining the stereoselectivity of these reactions, and the major role of the proximate substituents is to anchor the appropriate conformation favouring a- or /3-attack. Studies on the stereochemistry of electrophilic attack on cholest-5-en-3-one continue." As with bromine chloride," appreciable /3-attack occurs and the 5/3,6j8-epoxide was isolated along with the previously reported 5a,6a-epoxide and the Baeyer-Villiger product, the A-homo-enol lactone (58). Base-catalysed... 

Careful investigation of the bromination reaction of 3 keto steroids, which leads ultimately to 2,4-dibromo derivatives (see, for example, S-2-5-3), revealed that the sequence starts with the formation of a 2a bromo derivative, a futher demonstration of the preferred enolization of the carbonyl group toward the 2 position. 

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