Base-catalyzed epoxide ring-opening

Base-Catalyzed. Ring Opening of Epoxides. Base-catalyzed ring opening of epoxides provides a route to allylic alcohols.150... 

Epoxides Can undergo base-catalyzed ring opening ... 

If the epoxide is unsymmetrical, the nucleophile attacks primarily at the less substituted carbon atom in base-catalyzed ring opening. 

Base-catalyzed ring opening of epoxides constitutes a route to allylic alcohols. 

Symmetrically substituted epoxides (such as cyclopentene oxide, above) give the same product in both the acid-catalyzed and base-catalyzed ring openings. An unsymmetrical epoxide may produce different products under acid-catalyzed and base-catalyzed conditions, however. 

Epoxides can undergo acid or base-catalyzed ring-opening reactions. If the epoxide used in the reaction is symmetric, the product will be the same under both conditions. But the products will be different, if the epoxide is unsymmetric. 

In a base-catalyzed ring opening of an epoxide, the nucleophilic attack is on the less substituted (less hindered) carbon atom in the epoxide ring. The mechanism is shown below. 

Scheme 9.8 (depicts some base-catalyzed ring-openings of epoxides. 

A MECHANISM FOR THE REACTION ] Acid-Cataiyzed Ring Opening of an Epoxide 525 [ A MECHANISM FOR THE REACTION ] Base-Catalyzed Ring Opening of an Epoxide 526... 

Epoxides can also undergo base-catalyzed ring opening. Such reactions do not occur with other ethers, but they are possible with epoxides (because of ring strain), provided that the attacking nucleophile is also a strong base such as an alkoxide ion or hydroxide ion. 

Acid-Catalyzed Ring Opening of an Epoxide 525 Base-Catalyzed Ring Opening of an Epoxide 526... 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

The conformationally biased cis- and trans-4-t- )vXy derivatives were examined. The stereochemistry of both acid- and base-catalyzed reactions was investigated in 85 15 DMSO-H2O. Under acidic conditions the epoxides give anti ring opening and the reaction is stereospecific. The base-catalyzed reactions involve tranx-diaxial ring opening. The acid-catalyzed reactions occur by preferential opening of the benzylic bond with inversion. ... 

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