Epoxide opening with hydrogen fluoride

Epoxide Opening with Hydrogen Fluoride in Chloroform-Ethanoi  [c.433]

Fluorohydrin formation by epoxide opening with hydrogen fluoride in chloroform-ethanol, 433  [c.496]

Fluorohydrin formation by epoxide opening with hydrogen fluoride in chloro-form-tetrahydrofuran, 435  [c.496]

The reaction of steroid epoxides with hydrogen fluoride (as well as other halogen acids) yields characteristically only one product, namely, that of diaxial opening. The most satisfactory description of the process is the following protonation of the epoxide ring or complexing with the Lewis acid BF3 increases the polarization of C—O bonds, permitting nucleophilic attack by fluoride ion in concert with breaking of a C—O bond. The stereoelectronic features of this reaction are such that the attacking nucleophile approaches from a peri-planar direction, so that maximum overlap of the participating orbitals is maintained in the transition state. This is most satisfactorily achieved in most cases by attack on the epoxide half chair in such a way that the system passes through a chair-like transition state to the final diaxial product. This type of attack has been termed antiparallel by Vails and Toromanoff, relating the direction of nucleophilic attack  [c.432]

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]  [c.914]

The potentiation of activity due to the I6a-methyl group also proved additive with that which had been observed earlier on incorporation of the 6a-fluoro substituent. Reaction of 16-dehydropregnenolone with methyl Grignard reagent proceeds as with the more complex substrate to give, in this case, 16a-methylpreg-nenolone, 217. Reacetylation followed by epoxidation gives the 5of,6a-epoxide. Opening of the oxirane ring with hydrogen fluoride followed by acetylation gives intermediate, 218. This is then subjected to the side-chain elaboration reaction discussed earlier (transformation of 194 to 198) to give triacetate, 219. Selective saponification at 3 followed by oxidation gives an  [c.199]

See pages that mention the term Epoxide opening with hydrogen fluoride : [c.455]    [c.659]    [c.496]    [c.496]    [c.176]    [c.195]    [c.99]    [c.70]   
Organic reactions in steroid chemistry (1972) -- [ c.0 ]