Halohydrins epoxide ring opening

Scheme 30 Halohydrin ring closure, epoxide ring opening...

Another good strategy for the stereoselective halogenation is via the epoxide ring opening to afford a chiral halohydrin (Scheme 42.14). 

Ring opening of epoxides with nucleophiles other than water (Cl, BrT I, NOz , CN ) can also be catalyzed by halohydrin dehalogenase enzymes (EC 3.8.1.5, also named haloalkane dehalogenase or haloalcohd dehalogenase) (Figure 6.74) [197]. 

Halohydrins Opening of 1,2-epoxide ring by halogen at room temperature to afford l-halo-2-alkanols is catalyzed by this heterocycle (14 examples, 80-99%). Thus, styrene oxide gives a-halomethyl benzyl alcohols. 

The final enzyme discussed in the chapter, haloalcohol dehalogenase, catalyzes the formation of epoxides from halohydrins. However, the reverse reaction, the ring opening of epoxides with nonhalide nucleophiles such as cyanide, azide, and nitrite, has been exploited commercially to generate /3-substituted alcohols that are potential synthons. For this reason, the regioselectivity of the enzymatic ring opening reaction is of... 

As an alternative to the enzymatic hydrolysis of epoxides, nonracemic vicinal diols may be obtained from epoxides via the nucleophilic ring-opening by nitrite catalyzed by halohydrin dehalogenase (a lyase). The corresponding nitrite-monoesters are spontaneously hydrolyzed to yield diols. For the application of this technique see Sect. 2.7.2. 

Scheme 2.238 Regio- and enantioselective ring-opening of epoxides using nonnatural nucleophiles catalysed by halohydrin dehalogenase...

The reaction of epoxides with styrene provides homocin-namyl alcohols in good yield (eq 34). The reaction involves ring-opening of the epoxide to yield the magnesium alkoxide of a vtc-halohydrin, which then undergoes the cobalt-catalyzed styrylation. Use of trimethylsilylmethylmagnesium bromide and cobalt(II) bromide, instead of chlorides, facilitates the reaction of epoxide since the more nucleophilic bromide efficiently attacks the epoxide. Conversion of tosylaziridines affords homocinnamyl amines. 

This is a special case of 10-49 and is frequently used for the preparation of halohydrins. In contrast to the situation with open-chain ethers and with larger rings, many epoxides react with all four hydrohalic acids, although with the reac-... 

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