Tertiary epoxides acid-catalyzed hydrolysis

Several conclusions are drawn from the product studies of the reactions of propylene oxide and isobutylene oxide in H O.23 In the acid-catalyzed hydrolysis of propylene oxide, cleavage of the secondary C-O bond is favored by a factor of 2-3 over cleavage of the primary C-O bond. In the acid-catalyzed hydrolysis of isobutylene oxide, cleavage of the tertiary C-O bond is highly favored (>99%). These results show that in acid-catalyzed epoxide hydrolysis, cleavage of the C-O bond leading to... 

Simple tertiary carbocations react with water/trifluoroethanol (H20/TFE) solvent with an estimated rate constant of 1012s-1, which is somewhat faster than bulk solvent reorganizes.28 Simple primary and simple secondary carbocations are predicted to be even more reactive with nucleophilic solvent than tertiary carbocations. In water solutions, therefore, the rates at which simple primary and secondary carbocations are predicted to react with solvent would exceed the rate of a bond vibration ( 1013 s-1), and consequently they would not be sufficiently stable to exist as an intermediate. An A-2 mechanism would therefore be enforced on the acid-catalyzed hydrolysis of those epoxides that potentially undergo ring opening to form primary or secondary carbocations. 

Entry 5 is an interesting example that entails both enzymatic and hydrolytic epoxide conversion. In the first step, an enzymatic hydrolysis proceeds with retention of the configuration at the tertiary center. This reaction is selective for the 5-epoxide. The remaining 7 -epoxide is then subjected to acid-catalyzed hydrolysis, which proceeds with inversion at the center of chirality (see p. 186). The combined reactions give an overall product yield of 94%, having 94% e.e. ... 

Compound 17 is reminiscent of a cis-fused decalin, and reactions are therefore expected to occur preferably from the exo (convex) face. Indeed, epoxidation of 17 gave 19 exclusively, and acid catalyzed hydrolysis proceeded in the expected ra/is-diaxial manner. A W-coupling, ascribable to the H5 and H7 protons shown in 20, verified the C7 stereochemistry and, by corollary, the adjacent C6 center. One of the tertiary alcohols of 4 was therefore secure in 20. 

Acetic anhydride has also been used as the acylating agent. The formation of thiiranes from thiocyanatohydrins having a tertiary hydroxy group is best achieved by p-toluenesulfonic acid-catalyzed acetylation.The analogous thiocyanatohydrins with a secondary hydroxyl and a tertiary thiocyanate function give a predominance of epoxide from thiocyanatohydrin acetates since the hydrolysis rate of the secondary acetate grouping becomes competitive with that of the tertiary thiocyanate. 

---