Alkenes into halohydrins

Many examples of reactions that may involve electrogenerated chlorine and bromine can be found in the literature. Thus, alkenes may be converted into halohydrins [157-160] ethylene chlorohydrin can be formed [157] in 90% yield at a graphite anode, but subse-... 

Recall from Section 9.8 that alkenes can be converted into halohydrins when treated with a halogen in the presence of water (see Mechanism 9.5). 

Electrophilic addition of bromodimethylsulphonium bromide to alkenes gives, after dehydrobromination, variable yields of vinylsulphonium salts. Hydrolysis of the vinyl thioether derivatives gives a formal conversion of an alkene into a carbonyl compound with some degree of control over regioselect-ivity. Halohydrins (derived from alkenes) can be converted into ketones by dehydrohalogenation with potassium carbonate, catalysed by palladous acetate. ... 

So the last step of our synthesis will likely be conversion of the alkene into the halohydrin. 

Ketones from halohydrins. Palladium acetate complexed with a triarylphos-phine, particularly tri-o-tolylphosphine, converts halohydrins into ketones in the presence of K2C03. Yields are about 70-85% for substrates in which the halogen is secondary or tertiary, but less than 50% when the halogen is primary because of epoxide formation. The reaction is useful for conversion of alkenes to ketones in those instances in which halohydrins are formed regioselectively. 

Alkenes can be converted into epoxides by treatment with peroxy acids or via halohydrin formation and subsequent epoxidation. Both procedures are stereospecific. 

Oxirans. - The synthesis of l,2-anhydro-3,4-di-0-benzyl-6-deoxy-a-D-glucopyranose and its conformational analysis have been reported. A range of epoxides have been prepared by base treatment of bromohydrins, which were made by reaction of hydrogen bromide with aldonolactones. A one-pot conversion of vicinal diols into epoxides employs halohydrin ester intermediates generated from cyclic orthoacetates and either acetyl bromide or trimethylsilyl chloride. Levoglucosenone has been transformed into l,6 3,4-dianhydro-p-D-talopyranose by way of a trn/w-iodo-acetoxylation of the alkene moiety... 

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