Regioselectivity of epoxide opening

A limitation of epoxide deoxygenation with functionalized substrates became apparent in the second case where a mixture of olefins was isolated. Thus, the regioselectivity of epoxide opening and elimination of the titanium oxygen species was too low for practical use. 

Unsymmetrical epoxides (39) can form two isomers, (40) and (41), on reaction with nitrate anion and so raise the issue of regioselectivity. Under acidic conditions terminal epoxides are found to predominantly yield the primary nitrate ester (41) although this is not clear cut and propylene oxide is reported to yield an ill defined mixture of isomers. A comprehensive study on the regioselectivity of epoxide opening with nitrate anion under acidic conditions was conducted on glycidol. ... 

The regioselectivity of epoxide-opening reactions using alkynylaluminum reagents for prostaglandin synthesis has been studied by Matthews and Eickhoff [75], With two different cyclopentane oxide derivatives, they pointed out that simple substitution of an aluminum ate complex for the usual trialkylaluminum can sometimes be useful in achieving the desired regioselectivity, as observed in Sch. 46. 

An interesting regioselectivity of epoxide opening occurs in the reaction of sugar 75 with HF pyridine (O Scheme 41) [66]. Excellent selectivity of fluoride ion substitution at C-5 is obtained giving compound 76 after acetylation. 

We used the sEH model to investigate the regioselectivity of epoxide opening for two substrates, MSO and... 

Scheme 8 The regioselectivity of epoxide opening is determined by competing eiectronic and steric factors, interactions with a hydrogen bond- or proton-donor can enhance the eiectronic preference for the more substituted carbon (Nu = nucieophiie).

The reaction mechanisms and regioselectivities of sEH, LEH, and HheC were analyzed in detail. The computational results support the proposed mechanisms and are able to explain the experimentally observed regioselectivities of these enzymes. The quantum chemical approach also allowed us to identify and quantify the importance of individual functional groups for the reaction mechanism and the regioselectivity of epoxide opening. 

The regioselectivity of epoxide opening mediated by CeCl, is opposite to that by TiCl4. 

We explained in Chapter 15 that Sn2 reactions adjacent to carbonyl groups are very fast. The regioselectivity of the ring opening of a cyclic sulfate, like that of an epoxide, is directed by the competition between relative rates of two nucleophilic substitution reactions. Benzylic and carbonyl-substituted positions usually open faster. There is more discussion of the regioselectivity of epoxide opening on p. 351. 

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