Lithium unsaturated hydrocarbon reduction

Recently, it was found that the triangular skeleton can be easily destroyed in reactions which most likely include an electron transfer step. For example, the reduction of gem-dichloro[3]triangulane 66 with lithium in f-butanol (which is the standard method for the synthesis of cyclopropanes) unexpectedly gave 50% of unsaturated hydrocarbon (equation 45)88. An even more complex mixture of hydrogenolysis products was obtained in the reduction of dichloride 67. ... 

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). 

Reactions involving lithium appear to be particularly good candidates for ultrasonic acceleration. The reduction of some highly unsaturated cyclic hydrocarbons by lithium was not only accelerated but pushed to completion by ultrasonic irradiation(19) ... 

Much more conveniently, even a,)S-unsaturated esters can he transformed into a,)S-unsaturated alcohols by very careful treatment with lithium aluminum hydride [1073], sodium bis(2-methoxyethoxy)aluminum hydride [544] or diiso-butylalane [1151] (Procedure 18, p. 208). An excess of the reducing agent must be avoided. Therefore the inverse technique (addition of the hydride to the ester) is used and the reaction is usually carried out at low temperature. In hydrocarbons as solvents the reduction does not proceed further even at elevated temperatures. Methyl cinnamate was converted to cinnamyl alcohol in 73% yield when an equimolar amount of the ester was added to a suspension of lithium aluminum hydride in benzene and the mixture was heated at 59-60° for 14.5 hours [1073]. Ethyl cinnamate gave 75.5% yield of cinnamyl alcohol on inverse treatment with 1.1 mol of sodium bis(2-methoxy-ethoxy)aluminum hydride at 15-20° for 45 minutes [544]. 

Diels-Alder adduct (2) of allyl alcohol with cyclopentadiene. The unsaturated alcohol was converted into 6-chloromethyl-3-oxatricyclo 3.2.I.O loctane (3). For cyclization of (3) to the tricyclic alcohol (4), lithium in tetruhydrofurane proved superior to sodium in toluene. The alcohol (4) has a compurutively strained cycio-butune ring, but was converted easiiy into the parent hydrocarbon by oxidation (CrOfl-Py) and Wolff-Kiihner reduction. 

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