Epoxide ring openings regiochemistry

Regiochemistry of basic-catalyzed epoxide ring opening has been studied for the case of interaction of methyloxirane with hydroxyl- [7] and formiat-anion [35], and IS,2S-trans—2-methylstyrene oxide (tMSO) with acetate [36]. 

FIGURE 9.77 Effect of pH on regiochemistry of epoxide ring opening. 

There is an important difference in the regiochemistry of ring opening reactions of epoxides depending on the reaction conditions Unsymmetncally substituted epoxides tend to react with anionic nucleophiles at the less hindered carbon of the ring Under conditions of acid catalysis however the more highly substituted carbon is attacked... 

An interesting case of different regiochemistry in the ring opening of an epoxide using a substoichiometric amount (60%) of naphthalene in the lithiation process was observed with epoxides 282. After the naphthalene-promoted lithiation process the most unstable... 

Explain the regiochemistry of the opening of the epoxide ring that occurs in the first step of the process shown in Figure 28.7. 

The regiochemistry of acid-catalyzed ring opening depends on the epoxide s structure, and a mixture of products is usually formed. When both epoxide carbon atoms arc cither primary or secondary attack of the nucleophile occurs primarily at the less highly substituted site. When one of the epoxide carbon atoms is tertiary,however nucleophilic attack occurs primarily at the more highly substituted site. Thus, L,2-epoxypropane jeac t with HCl to give primarily l-chloro-2-propanol, but 2-methyl 1,2-cpoxypropane gives 2-chloro-2-methyl-l-propanol as the major product. 

Table 4 Regiochemistry of Ring Opening of Epoxides with Borohydride Derivatives...

The system of sodium hydride, sodium alkoxide and metal salts developed by Caubere and his coworkers can selectively reduce epoxides to alcohols (Table 6). The regiochemistry of ring opening is con-... 

Substituents that further stabilize a carbocation intermediate lead to reversal of the mode of addition. To summarize, because they are stable compounds, both the stereochemistry and regiochemistry of epoxides can be controlled by the reaction conditions. Ring opening that is dominated by the nucleophilic reagent is determined by steric access. Ring opening that is electrophilic in character introduces the nucleophile at the position that has the largest cationic character. 

Epoxide isomerization. This strong base combination converts epoxides to allylic alcohols. The regiochemistry of the ring opening can be directed by a proximal heteroatom in the substrate. 

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