Diborane reaction with epoxides

Reduction of epoxides. The reaction of diborane alone with epoxides is complicated. Thus 1,2-butylene oxide requires 48 hrs. and gives a mixture of butanols (96% 2-butanol and 4% 1-butanol) in only 48% yield. The reaction with trisub-stituted epoxides is even more complicated and only trace amounts of simple alcohols are formed. Brown and Yoon1 found that the presence of trace amounts of sodium or lithium borohydride greatly enhances the rate of reaction and modifies the course to give predominantly anti-Markownikov opening of the epoxide ring. Thus 1-methylcyclohexene oxide is reduced mainly to m-2-methylcydohexanol ... 

Selective reductions. Brown et al.2 conducted an extensive study of reductions with diborane in THF. Most aldehydes and ketones are readily reduced unusually high stereoselectivity was realized in the case of norcamphor, which was reduced to 98% endo-norbornanol and 2% exo-norbornanol. p-Benzoquinone is reduced to hydroquinone at a moderate rate, but reduction of anthraquinone is sluggish. Carboxylic acids are reduced very rapidly indeed this group can be reduced selectively in the presence of many other substituents. Acid chlorides react much more slowly than carboxylic acids. Esters and ketones are reduced relatively slowly. Reactions with epoxides are relatively slow and complex. 

The configuration of the produced diol is influenced by the relative stabilities of the benzyl- or alkylcarbonium ions formed during the reaction. Similar reactions were investigated earlier. The rate and stereochemistry of the diborane reaction is altered by a small quantity of LiCl. Wide-ranging research has been performed with regard to the mechanism and stereochemistry of the diborane reduction in connection with cyclic and aliphatic a, 3-unsaturated and allylic epoxides, on diterpene models, " and by study of the reduction of epoxy-methylenecyclohexane and 2,3-epoxy-3-methylcyclohexanone. ... 

Oxymercuration followed by borohydride reduction of 6-methylenebicyclo-[3,l,l]heptane (166) gave the alcohols shown in Scheme 8. Similar reaction with 5-methylenebicyclo[2,l,l]hexane gave entirely monocyclic products. Stereochemical aspects are discussed for the formation and reduction of epoxides of 2-methylene- and 3-methylene-bicyclo[3,l,l]heptanes, the hydrobora-tion-oxidation of 3-methylenebicyclo[3,l,l]heptanes, and reduction by lithium aluminium hydride and diborane of bicyclo[3,l,l]heptanones. ... 

Oxirane cleavage. Isobe and co-workers effected the stereoselective reduction of 1 to 2 by sequential treatment of the epoxide with sodium cyanoborohydride in anhydrous HMPT and diborane in THF. The authors suggest that this reaction involves internal hydride transfer in the hypothetical intermediate 3. Reduction in DME or THF results in products with the opposite configuration at C7. 

In the 8-methoxy series, the (—)-epoxide 109 was treated with diborane-lithium borohydride, and the product (110) of this reaction was cleaved selectively to the 2-quinolone 112 methylation then afforded lunacridine... 

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