Epoxides substituted, cleavage

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Identify the substitution pattern of the two epoxide carbon atoms—in this case, one carbon is secondary and one is primary. Then recall the guidelines for epoxide cleavages. An epoxide with only primary and secondary carbons usually undergoes cleavage by SN2-like attack of a nucleophile on the less hindered carbon, but an epoxide with a tertiary carbon atom usually undergoes cleavage by backside attack on the more hindered carbon. In this case, an S]sj2 cleavage of the primary C—O epoxide bond will occur. 

The organocopper reagents prepared in this way cleave epoxides in high yield with substitution of the alkyl group at the less hindered site. This cleavage has been used to effect intramolecular cyclization (second example). 

The 1,2-carbonyl transposition takes place through the enJo-epoxide 18 easily prepared through the tosylhydrazone 16, followed by regioselective cleavage to the less substituted double bond (17) with 2 equivalents of methyllithium [4] and epoxidation with MCPBA in chloroform from the more accesible convex face of the decalin system. 

Base-catalysed epoxide cleavage follows an 8 2 reaction in which the attack of the nucleophiles (NuA) takes place at the least substituted carhon of the ring. Therefore, hase-catalysed unsymmetrical epoxide produces 2-suhstituted alcohol. 

Among alkyl-substituted ethylene oxides known to umletrn cleavage on treatment with sodium sulfite are propylene oxide, isobutylene oxide, 1,2-epoxybutane, 1,2-epoxyoctane, and 2,3-cpow-butane.1 75 These reactions with sodium sulfite constitute the bani-, ffrail analytical method developed by Swan1875 for the estimation <[Pg.179]

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