Allylic oxiranes, ring-opening

Oxirane-ring opening of talopyranose (76) with (30 a) afforded a mixture of thiodisaccharides (77) and (78) in very good yield. Conventional treatment of (77) led to the allyl-2-thio-fucosyl-galactoside (79) (Scheme 23) [32]. 

In the oxirane ring opening reaction (Eq. (2)), it is possible to prepare the ketone 5, the acetonide 10, and the allylic alcohol 11 selectively by controlling the reactivity of the intermediate carbenium ion 12 under some specified reaction conditions. In order to prepare 5 preferentially, a high concentration of perchlorate ion must be employed in CH2CI2, ClCHjCH Cl, or THF. In this system, an incipient carbenium ion 12 would be associated with perchlorate ion and stabilized so that the deprotona-... 

Copper-catalyzed enantioselective allylic alkylation ring-opening reactions of oxiranes (and aziridines) with alkyl metals (e.g., dialkylzincs) has been reviewed by Pineschi <2004NJC657>. 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

Nucleophilic oxirane ring opening with sodium phenyltellurate (Na+ C6H5Te ), followed by telluroxide elimination with base, has been employed as a two-step protocol for isomerizations to allyl alcohols <82TL1177, 83JOM(250)203>. 

Stereoselective transannular cyclizations to hydroazulene systems are also described. These reactions are initiated by ionization of cyclodecenol to form an allylic cation or by oxirane ring opening in cyclodecadiene monoepoxides. 

The next key series of reactions results in the transformation of 625 to 626. The first step is a Sharpless epoxidation of the allylic alcohol. This is followed by a regiospecific oxirane ring opening with potassium benzyloxide to introduce the required oxygen that will eventually... 

Selected examples of ring-opening of oxiranes by chloroalkanes to yield allyl ethers... 

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

The first step is the formation of an alkoxide anion by the initiating alcohol (allyl alcohol is the initiator most commonly used, although other initiators have been suggested). The appropriate oxide(s) is (are) then added to the alcohol initiator. This causes the opening of the oxirane ring in the oxide and propagates the chain growth of the alkylene oxide on the initiator. The last step is the neutralization of the alkoxide anion to terminate the polymerization. 

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