Stereoselectivity of Epoxide Ring Opening

Epoxides, like other ethers, are cleaved by nucleophiles under acidic conditions. For example, cyclopentene oxide produces a mixture of 1,2-cyclopentanediol stereoisomers when treated with water and sulfuric acid. 

How many stereoisomers of 1,2-cyclopentanediol are there What are their stereochemical relationships Are they enantiomers  

The cleavage reaction occurs in three steps O protonation of the epoxide, Sn2 nucleophilic attack on the protonated epoxide, and deprotonation of the ring-opened product. Draw the complete mechanism. How many intermediates are there Which step determines diol stereochemistry  

The product of nucleophilic attack can be anticipated by examining the lowest-unoccupied molecular orbital (LUMO) on protonated cyclopentene oxide. From which direction (top or bottom) would a nucleophile be more likely to approach each epoxide carbon in order to transfer electrons into this orbital Explain. Does one carbon contribute more to the LUMO, or is the orbital evenly spread out over both epoxide carbons Assuming that LUMO shape dictates product stereochemistry, predict which stereoisomers will be obtained, and their approximate relative amounts. Is the anticipated kinetic product also the thermodynamic product (Compare energies of 1,2-cyclopentanediol stereoisomers to tell.) 

The LUMO of protonated cyclopentene oxide reveals the likely site of nucleophilic attack. 

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Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

J. G. Buchanan, Migration of epoxide rings and stereoselective ring opening of acetoxyepoxides, Methods Carbohydr. Chem., 6 (1972) 135-141. 

A relevant extension of the ring opening of epoxides with a titanium salt can be seen in the concomitant scission of the C-C bond of cyclobutane illustrated in Eqs (285) and (286) [648]. Choice of each diastereoisomer enables highly selective construction of the trisubstituted double bond which finds application in terpene synthesis. The tandem cleavage of the C-O and C-C bonds was similarly feasible in the combination of oxetane and cyclobutane as illustrated in the stereoselective synthesis of a homoallyl alcohol (Eq. 287) [648]. 

Ibuka, T., Nakai, K., Akaji, M., Tamamura, H., Fujii, N., Yamamoto, Y. An aza-Payne rearrangement-epoxide ring opening reaction of 2-aziridinemethanols in a one-pot manner a regio- and stereoselective synthetic route to diastereomerically pure N-protected 1,2-amino alcohols. Tetrahedron 1996, 52, 11739-11752. 

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