Epoxides styrene oxide-type

Table 11.2-4. Enzymatic hydrolysis of styrene oxide-type epoxides, see Fig. 9(b).

Styrene oxide-type epoxides have to be regarded as a special group of substrates, as they possess a benzylic carbon atom, which facilitates the formation of a carbenium ion through resonance stabilization by the adjacent aromatic moiety... 

Styrene oxide-type substrates (Fig. 2) have been hydrolyzed with good enantioselectivity especially by fungal epoxide hydrolases and a recombinant epoxide hydrolase from Agrobacterium radiobacter. However, bacterial epoxide hydrolases, e.g., from No-cardia spp. and related Rhodococcus strains, were not useful for tins substrate pattern. Similarly, enzymes from yeasts and mammals showed only low to moderate selectivity (Table 2). 

Some related reactions are worth mentioning in this context. Addition of allylnickel bromide to styrene oxide to give an alcohol has been reported (example 7, Table IV). Tsutsumi has described the Darzens-type reaction of two molecules of a-bromoketones to give dimethylfurans (example 8, Table IV). This reaction consists of the addition of the ketomethylenic group to the carbonyl group of another molecule, followed by epoxide formation and bromide elimination. A subsequent rearrangement leads to a dialkylfuran. 

Epoxides react with cyanide under basic reaction conditions to yield 3-hydroxypro-pionitriles by nucleophilic attack at the sterically less demanding carbon atom (Scheme4.86). Me3SiCN can also be used as reagent, but trimethylsilyl ethers will be the main products. With some types of epoxide (e.g. styrene oxide [372]) the products readily dehydrate to yield ,/i-unsaturated nitriles [373] (Scheme4.86). 

The same differential behavior can be observed with amine nucleophiles. For example, calcium triflate promotes the aminolysis of propene oxide 84 with benzylamine to give 1-(A -benzyl)amino-2-propanol 85, the result of attack at the less substituted site <03T2435>, and which is also seen in the solventless reaction of epoxides with heterocyclic amines under the catalysis of ytterbium(III) triflate <03SC2989>. Conversely, zinc chloride directs the attack of aniline on styrene oxide 34 at the more substituted carbon center <03TL6026>. A ruthenium catalyst in the presence of tin chloride also results in an SNl-type substitution behavior with aniline derivatives (e.g., 88), but further provides for subsequent cyclization of the intermediate amino alcohol, thus representing an interesting synthesis of 2-substituted indoles (e.g., 89) <03TL2975>. 

Metal mediated epoxidahon is remarkably diverse, with many types of ligand systems being represented. For example, a cytochrome P450 BM-3 mutant (139-3) has been developed using directed evolution, which exhibits high activity towards epoxidation of several non-natural substrates. Thus, exposure of styrene 4 to BM-3 variant 139-3 in phosphate buffer containing methanol and NADPH resulted in the quantitative conversion to styrene oxide 5. For terminal aliphatic alkenes, however, allyhc hydroxylation is the predominant process <04T525>. 

The rearrangement of styrene oxide can also be performed in liquid-phase reactions with the well known catalyst TS-1 [11,25]. The framework-titanium gives the MFI-type zeolite Lewis acidic properties and 100 % conversion and 98 % phenylacetaldehyde selectivity was achieved after 1-2 h at 70 °C in a batch reaction with acetone (100 mL) as solvent, epoxide (50 g), and catalyst (3 g) as feed. 

A detailed study of the reaction of isocyanates with epoxides leading to 2-oxazolidinones has been undertaken. With the exception of styrene oxide, all other epoxides lead to 5-substituted 2-oxazolidinones regardless of the nature of the catalyst type (nucleophilic or electrophilic) (Scheme 107). 

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