Epoxides conversion into halohydrins

Alkaline hydrogen peroxide epoxidation / 10 Conversion of an oxirane into its stereoisomer / 14 Epoxide formation via halohydrins / 15 Spectra / 18... 

The conversion of halohydrins into epoxides by the action of base is simply an adaptation of the Williamson synthesis (Sec. 17.5) a cyclic compound is obtained because both alcohol and halide happen to be part of the same molecule. In the presence of hydroxide ion a small proportion of the alcohol exists as alkoxide this alkoxide displaces halide ion from another portion of the same molecule to yield the cyclic ether. 

Conversion of olefins into epoxides has been achieved in a great ifnbfiT of cases through halohydrin intermediates generated by " kiition of hypoholous acids across the olefiiuc double bond (Eq, 127). 

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. 

Since halohydrins are nearly always prepared from alkencs by addition of halogen and water to the carbon-carbon double bond (Sec. 6.14), this method amounts to the conversion of an alkenc into an epoxide. 

Steroid trans-halohydrins are similarly converted into the corresponding olefinic products. Cornforth employed the combination zinc dust-AcOH-Nal-NaOAc for conversion of epoxides to olefins via the intermediate iodohydrins. 

Mixtures of halohydrins with (dihaloethyl)phosphonic derivatives are obtained by the appropriate additions to ethenylphosphonic derivatives. Under phase-transfer conditions in the presence of tetrabutylammonium hydroxide, the direct formation of (epoxyethyl)-phosphonic acid derivatives has been observed. However, several earlier reports describe the conversion of isomeric halohydrins (Scheme 31) into the corresponding epoxides ... 

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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