Secondary alcohols, ring-opening

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). 

Li et al.2S9 Nishikawa et al. described how an organomanganese compound could ring-open an allylic tetrahydropyran to give allylic secondary alcohols by a different mechanism.292... 

Starting with a copper catalyzed epoxide ring opening, using phenyl magnesium chloride, a secondary alcohol was generated, which in turn was transformed into an electrophile by tosylation (Scheme 29). The tosylate 117... 

The Tafel rearrangement only occurs in acid medium. Simultaneous reduction of both carbonyl groups leads to interaction and formation of a cyclopropane. Acid catalysed cyclopropane ring opening follows to yield an a-diketone 28 which undergoes the electrochemical Clemmensen reduction step to the hydrocarbon. Side products include the two monoketones derived by partial deoxygenation of the a-diketone and the secondary alcohols from reduction of these raonoketones. Separate experiments show that the a-diketone 28 can be reduced to the hydrocarbon. 

Kspecially interesting is the fact that the epoxide ring is activated by intramolecular chelation" <37). Pnor to this discovery, 18 standard reagents had tailed in opening the epoxide.7 The reducing agent attacks activated epoxide 37 selectively from the sterically more accessible side and produces a tertiary alcohol Selective protection of the secondary alcohol leads to compound 16... 

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