Oxirane reactions with halogen acids

The reaction of oxiran with HF to yield CH2FCH2OH was chosen as the model for the ring-opening of epoxides that is caused by halogen acids, and theoretical ab initio calculations for the reaction were performed. For the gas-phase reaction, the preferred mechanism leads to retention of configuration at the carbon atoms of the ring. An alternative pathway via preliminary formation of the conjugate acid of the oxiran was found to fit a borderline A2 mechanism. 

Darzens condensation of an aromatic aldehyde with a-bromo-y-valero-lactone has been used in the preparation of ketides. The acid-catalysed decarboxylation of a-phenylglycidic acids is reported to give abnormal products a non-concerted mechanism involving oxiran ring-opening is invoked. a-Halogenoglycidic esters can be obtained by reaction of tris(di-methylamino)phosphine with trichloroacetic acid derivatives in the presence of carbonyl compounds (Scheme 18) cyclohexanone affords the isomeric halogenated pyruvic acid derivative (51) in quantitative yield. ... 

The hydroxy function may originate from the reduction of an a-halogenated carbonyl compound or a Grignard reaction. The oxirane oxygen is not always incorporated directly from a hydroxy group for instance, in the case of a carboxylic acid ester, it is derived with neighboring-group participation from an ortho-monoester intermediate. 

Nucleophilic Attack at Halogen. Further studies have been reported of the reactions of diols with the triphenylphosphine-carbon tetrachloride reagent. It has now been applied to 1,2-diols (in the presence of potassium carbonate) to form epoxides and to the trans-6 o (84), the nature of the product depending on the relative amounts of phosphine and diol present. The major product of reactions involving equimolar quantities of phosphine and diol is (85). The cyclodehydration product (86) is formed in only poor yield. In the presence of carboxylic acids, the triphenylphosphine-carbon tetrachloride system causes ring-opening of epoxides with the formation of c -enol esters, the reaction presumably proceeding via nucleophilic attack by the oxirane at an acyloxyphos-phonium intermediate. ... 

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