Diborane epoxides

Diboran reduziert Nitrile schneller als Epoxide und Carbonsaure-ester, aber langsa-mer als Ketone, Olefine und Carbonsauren, so dab selektive Reduktionen moglich sind (s. S. 48). Da aus Nitrilen mit Diboran die stabilen N,N,N-Trialky 1-borazole gebildet... 

On the other hand, the observed syn preference of 59a is consistent with a study of hydroboration of 59a with diborane by Schueler and Rhodes [127], who obtained a mixture of the monoalcohols (symanti = 74 26) upon oxidative work-up. A similar magnitude of. yyn-preference was found (syn anti = 73 27) in the hydroboration with a bulkier borane, 2,3-dimethyl-2-butylborane (thexyl borane) [127]. This lack of effect of the bulk of the reagent in the hydroboration of 59a is consistent with the idea that the n face of 59a is free from steric bias [127], and that the syn preference of 59a found in dihydroxylation and epoxidation is non-sterically determined [128]. 

Diborane in THF reduces epoxides, but the yields are low, and other products are formed by pathways that result from the electrophilic nature of diborane.133 Better yields are obtained when BH4 is included in the reaction system, but the electrophilic nature of diborane is still evident because the dominant product results from addition of the hydride at the more-substituted carbon.134... 

The order of reactivities of various functional groups determined under standard conditions (using externally generated diborane, and tetrahydrofuran as solvent) is acid > alkene > ketone > nitrile > epoxide > ester > acid chloride.33 Acids, aldehydes, ketones, epoxides, nitriles, lactones and azo compounds are reduced rapidly, esters more slowly and chloral, acid chlorides and nitro compounds are inert. Double bonds undergo the hydroboration reaction,25 nitriles and azo compounds are reduced to amines, and the remaining groups to alcohols. Ketones can be reduced selectively in the presence of epoxides. Contrary to the order of reactivities given above, it has been claimed that nitriles are reduced more rapidly than ketones.223... 

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. 

EPOXIDES Alumina. Bisfbenzonltrile)-dichloropalladium(H). Copper(II) sulfate-Pyridine. Diborane. Diphenyl d isu Ifide-Tri-n-bu t y Iphosphine. Organo-cuprates. Sodium cyanoborohydride. Sodium phenyl selenide. Trimethyl-sllyl phenyl sclcnidc. Trimethylsilyl trifluoromethanesulfonate. Triphenyl-phospliine-Thiocyanogen. 

Although diborane (BH3)2 reduces epoxides, it usually gives a mixture of products in addition to the usual alcohols. The reduction of styrene oxide derivatives with diborane has been studied in order to develop a new route to 1,3-diols from epoxides (equation 19).4 a,3-Unsaturated epoxides undergo... 

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. ... 

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 ... 

Reduction of epoxides-1 Styrene oxide and similar aryl epoxides undergo rapid anti-Markownikov opening when reduced with diborane in the presence of boron... 

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. 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

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 readily available chiral epoxide (9), previously shown (see Vol. 4 of these Reports) to have the S-configuration, has been used to effect an asymmetric synthesis of (— )-S-lunacridine (11) (Scheme 1). ° Thus, borohydride-catalysed diborane reduction of the epoxide (9 R = OMe) provided the expected alcohol... 

R)-( + )-Methyloxirane, a useful chiral intermediate, has been employed in the synthesis of a variety of natural products. It is readily prepared in multi-gram quantities from ethyl L-lactate via tosylate 120b [53,54]. Reduction of the ester proceeds quantitatively with diborane over a period of 5 days to afford ( S)-( + )-propane-l,2-diol 2-tosylate (146). Cyclization with KOH gives epoxide 147 with 97% inversion of configuration. 

Regioselective reduction of aryl-substituted epoxides with NaBHj is catalyzed by PdCl2 in the presence of moist alumina (H2O content, 19 wt%) in hexane. The selectivity in reduction of aryl-substituted epoxides was the same as with NaBILj and diborane. In addition, the alumina can be easily recovered and reused without further treatment. 

Oxidative hydroboration 91) of lactone 11.49 afforded a mixture of four lactarorufins (Scheme 20), in which diols 11.30 and 11.33, arising from a attack of diborane, largely predominated (more than 90%). The same type of stereoselectivity was observed for other addition reactions, i.e. epoxidation, osmylation, hydrogenation (775), (704), 43) to 2,9, 3,4- or 6,7-double bond of lactaranolides and marasmanes. Apparently, the tricyclic structures of these substrates provided enough conformational and steric bias to direct approach of reagents from the same side as the bridgehead protons H-2 and H-9. However, when the double bond was located in a different position, exceptions were observed 98). 

The stereoselective reduction of cyclic a, 8-unsaturated epoxides by diborane has recently been extended to acyclic cases (Scheme 16) and shown" to produce (Z)-alkene geometry independent of the substitution pattern, suggesting an intramolecular conjugate reduction (36). 

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. ... 

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