Stereospecificity 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. 

The synthesis of tetrahydrofurans through cyclization via epoxide ring opening has been well investigated and has been applied to the preparation of a-bisabolol oxides (69IJC1060) and linalool oxides (77H(6)1365) and by Kishi et al. in the synthesis of lasalocid A (195), where the epoxide pathway resulted in a stereospecific construction of six out of the ten chiral centers (Scheme 93) (78JA2933). 

Stereospecific reductive ring opening of bis-epoxides separated by two methylene groups to 2-alkylidene tetrahydrofurans has been brought about by elimination-cyclization of l-iodomethyl-l,5-bis-epoxides with zinc dust (Equation 76) <20030L1931>. 

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. ... 

The benzofuran epoxide ring opening (eq 21) proceeded rapidly, and the cis isomer of the imidazole adduct was formed exclusively. Intramolecular hydrogen bonding between the hydroxyl function and imidazole was proposed for this unexpected stereospecificity. ... 

Neighboring group participation. Epoxides ring-opening mechanisms. Regio-selectivity. Stereospecificity. 

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>. 

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