Epoxides stereospecific 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). 

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. 

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. ... 

Approximately half of the papers on Ru porphyrins appearing during the last 5 years or so deal with olefin epoxidation. Epoxides are found in naturally occurring compounds and are important intermediates in organic synthesis in the case of asymmetric epoxidation, stereospecific ring-opening of the epoxide generates the chiral alcohols . 

Epoxide Opening. Epoxides undergo stereospecific opening reactions when treated with a combination of trimethylaluminum, EtsN, and TBDMS triflate in CH2CI2 at —50 °C to give the corresponding alkylation-silylation products in excellent yield (eq 34). TMS triflate and TES triflate are equally effective and have been used in combination with other trialkylaluminum reagents. ... 

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. ... 

An even sweeter chlorodeoxysucrose was later synthesized by the reaction of sucralose with triphenylphosphine and diethyl azodicarboxylate to give the 3, 4 -lyxo epoxide, followed by stereospecific opening of the epoxide with chloride to... 

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

Additional side-chain manipulations of 12 eventually furnished isolaurepinnacin 3. Similarly, Murai et al. s synthesis of (+)-obtusenyne 1 has demonstrated that a combination of Et2NH2Cl and Ti(0/-Pr)4 effected the stereospecific opening of epoxide 13 to afford chlorohydrin 14 with moderate regioselectivity (4 1), a pivotal intermediate used for the completion of the total synthesis of the target natural product." ... 

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