Lithium borohydride epoxides

Epoxides Cyanotrimethylsilane. Dilithium tetrabromonicklelate. Lithium borohydride. Organoaluminum reagents. Sodium hydrogen telluride. 

The reverse regiocontrol, giving 1,2-diols, is observed with DIBAL-H (diisobutylaluminum hydride). The remarkable effect of titanium tetraisopropoxide as an additive to lithium borohydride has also been reported. In this reaction benzene is a better solvent than THF, probably because a Ti complex using both oxygens in epoxy alcohols is formed in benzene before the hydride attack. Other metal hydrides used include sodium hydrogen telluride (NaHTe) and an ate complex derived from DIBAL-H and butyllithium, both of which reduce epoxides to alcohols, although they have been tested with only a small number of examples. In the former case the reaction may proceed via a 2-hydroxyalkyltellurol intermediate. 

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 ... 

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... 

The reductive ring opening of 330a with sodium cyanoborohydride/titanium tetrachloride in acetonitrile occurs with no ester reduction whatsoever to provide 421 in 83% yield. Subsequent conversion to the tosylate followed by reduction with lithium borohydride/lithium triethylborohydride affords in 61% yield the crystalline diol 422. Lithium aluminum hydride or sodium borohydride reduction of the tosylate of 421 fails to produce clean reductions to 422. Epoxide ring closure of 422 is achieved with two equivalents of sodium hydroxide in methanol to fiimish in 93% yield (2 S, 3i )-2-benzyloxy-3,4-epoxybutan-l-ol (423) [140] (Scheme 94). 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

M-Octylbromide and p-bromotoluene are completely inert toward 9-BBN. Although the reaction of epoxides with 9-BBN is quite sluggish and requires 3-8 days for completion, the introduction of a catalytic (7.5 mol%) of lithium borohydride converts 1,2-butylene oxide to 98% of 2-butanol (Eq. 25.6). 

The azidohydrins obtained by azide ion opening of epoxides, except for those possessing a tertiary hydroxy group, can be readily converted to azido mesylates on treatment with pyridine/methanesulfonyl chloride. Reduction and subsequent aziridine formation results upon reaction with hydrazine/ Raney nickel, lithium aluminum hydride, or sodium borohydride/cobalt(II)... 

The importance of reactions with complex, metal hydrides in carbohydrate chemistry is well documented by a vast number of publications that deal mainly with reduction of carbonyl groups, N- and O-acyl functions, lactones, azides, and epoxides, as well as with reactions of sulfonic esters. With rare exceptions, lithium aluminum hydride and lithium, sodium, or potassium borohydride are the... 

Sodium borohydride is a much milder reducing agent than lithium aluminium hydride and like the latter is used for the reduction of carbonyl compounds like aldehydes and ketones. However, under normal conditions it does not readily reduce epoxides, esters, lactones, acids, nitriles or nitro groups. 

In 23-cpoxybutyric acid sodium borohydride opened the epoxide ring without affecting the carboxyl. Varying ratios of 2- and 3-hydroxybutyric acid were obtained depending on the reaction conditions. Sodium borohydride in alkaline solution gave 18% of a- and 82% of -hydroxybutyric acid while in the presence of lithium bromide the two isomers were obtained in 60 40 percentage ratio [1000]. 

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