Stereospecific reactions epoxide ring opening

Entry 8 involves a migration initiated by epoxide ring opening. This reaction involves migration of a vinyl substituent. Entry 9 is a stereospecific migration of the aryl group. The DiBAlH both promotes the rearrangement and reduces the product aldehyde. 

Epoxides are useful synthetic intermediates, and the conversion of an alkene to an epoxide is often part of a more extensive molecular transformation.85 In many instances, advantage is taken of the high reactivity of the epoxide ring to introduce additional functionality. Because epoxide ring opening is usually stereospecific, such reactions can be used to establish stereochemical relationships between adjacent substituents. Such two- or three-step operations can accomplish specific oxidative transformations of an alkene that may not easily be accomplished in a single step. Scheme 12.13 provides a preview of the type of reactivity to be discussed. 

Epoxides also react stereospecifically because the ring-opening reaction is an Sn2 reaction. A single diastereoisomer of epoxide gives a single diastereo-isomer of product. 

Dihydroxyiation of epoxides can be carried out with syn stereospecificity using OSO4 as the active oxidant. The reaction occurs by a cycioaddition mechanism. Epoxidation is aiso a stereospecific syn addition. Ring opening of epoxides by hydroiysis aiso ieads to diois. This is usuaiiy an anf/addition with inversion of configuration at the site of nucieophiiic attack, ieading to overaii anf/dihydroxyiation. 

Epoxide Ring Opening. One-pot alkylation-O-silylation reactions of epoxides take place in excellent yields when the epoxides are treated with trialkylaluminum-TMSOTf and EtsN (1.5 equiv of each one) in methylene chloride at —50 °C (eq 92). Other trialkylsilyl triflates also undergo this reaction, which is stereospecific awft -compounds are obtained from fran -epoxides while cis-epoxides yield 5yn-adducts. ... 

Finally, it is important to mention that there are other related publications in which porphyrin macrocycles are not directly used as dipolarophiles but are transformed into new derivatives that can react with carbonyl ylides via ACE (alkene cyclobutene epoxide) reactions. This idea arose in 1997, when Russell and co-workers found that fused ester-activated cyclobutene epoxides 86 can be ring-opened to give carbonyl ylides 87, and that these can be trapped stereospecifically by ring-strained alicyclic dipolarophiles, such as 2,5-norbomadiene, to form hetero-bridged norbomanes 88 in good yields, through ACE transformations (Scheme 31) <97CC1023>. 

Now the epoxide is opened with HBr to give the only possible trans diaxial product (Chapter 18). The role of the bridge in fixing the conformation of the ring is more important in this stereospecific reaction because the bromide ion is forced to attack from the top face. The alcohol is protected as a silyl ether. 

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