Sodium borohydride, reductions

The conditions required for the sodium borohydride reduction of 2-amino-5-acylthiazoles are stronger than those used for the reduction of 2-acylamino-5-acylthiazole (476). 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

Nucleophilic addition to carbonyl groups sometimes leads to a mixture of stereoisomeric products The direction of attack is often controlled by stenc factors with the nude ophile approaching the carbonyl group at its less hindered face Sodium borohydride reduction of 7 7 dimethylbicyclo[2 2 IJheptan 2 one illustrates this point... 

Does sodium borohydride reduction of o ribose yield an opti... 

Depending on experimental conditions, sodium borohydride reduction of anthraquinone, in a lower ahphatic alcohol, results in 9,10-dihydroxyanthracene... 

A McMurry coupling of (176, X = O Y = /5H) provides ( )-9,ll-dehydroesterone methyl ether [1670-49-1] (177) in 56% yield. 9,11-Dehydroestrone methyl ether (177) can be converted to estrone methyl ether by stereoselective reduction of the C —double bond with triethyi silane in triduoroacetic acid. In turn, estrone methyl ether can be converted to estradiol methyl ether by sodium borohydride reduction of the C17 ketone (199,200). 

Xyhtol also is obtained by sodium borohydride reduction of D-xylonic acid y-lactone (32) and from glucose by a series of transformations through diacetone glucose (46). 

Lactitol (4-0-p -D-galactopyranosyl-D-glucitol) is obtained by sodium borohydride reduction (99,100) or catalytic hydrogenation (101) of lactose. Potentially large quantities of this sugar alcohol are available from lactose obtained from whey. 

M. M. Cook and co-workers, "Sodium Borohydride Reductions—Novel Approaches to Decolorization and Metals Removal iu Dye Manufacturiug and Textile Effluent Applications," 203rd National Meeting of the American Chemical Society, San Erancisco, Apr. 5—10,1992. 

J. A. Ulman and M. M. Cook, Sodium Borohydride Reductions—"Removal of Color and Metals Erom Paper Dyes," 204th National Meeting of the American Chemical Society, Washiagton, D.C., Aug. 23—28,1992. 

The reduction of 3,5-diphenylisoxazoline with sodium cyanoborohydride produced a mixture of isomeric 3,5-diphenylisoxazolidines. The H and NMR spectra were utilized to distinguish the isomers SOLAIOI). Sodium borohydride reductions likewise reduce isoxazolines to isoxazolidines (equation 56) (80JA4265). 

Sodium Borohydride Reduction of Ethyl 5a.-Cholestan-3-one 2a.-Xanthate ... 

FIGURE 15.2 Mechanism of sodium borohydride reduction of an aldehyde or ketone. 

The carbonyl group of carbohydrates can be reduced to an alcohol function. Typical procedures include catalytic hydrogenation and sodium borohydride reduction. Lithium aluminum hydride is not suitable, because it is not compatible with the solvents (water, alcohols) that are requited to dissolve caibohydrates. The products of caibohydrate reduction aie called alditols. Because these alditols lack a car bonyl group, they aie, of course, incapable of forming cyclic hemiacetals and exist exclusively in noncyclic forms. 

Sodium borohydride reduction of 4-substituted isoquinolinium salts led to vinylogous cyanamides, ureas, and urethanes, as well as the corresponding tetrahydroquinolines (640). Hydrogenation of /8-acylpyridinium salts (641) to vinylogous ureas was exploited in syntheses of alkaloids (642), leading, for instance, to lupinine, epilupinine, and corynantheidine (643, 644). Similarly, syntheses of dasycarpidone and epidasycarpidone were achieved (645) through isomerization of an a,/0-unsaturated 2-acylindole and cyclization of the resultant enamine. 

Condensation between aldehyde 40 and amine 29 followed by sodium borohydride reduction of the resultant imine and cyclisation yielded isoquinoline 41 in good yield. Cyclisation occurred exclusively at the more electron-rich aromatic group. 

Friedel-Crafts alkylation of 8-hydroxycarbostyrils, such as leads to substitution at the C-5 position, namely, In this case an a-haloacyl reagent is employed. Displacement with isopropylamine and careful sodium borohydride reduction (care is... 

This was converted to its imine with methylamine catalyzed by titanium tetrachloride and then sodium borohydride reduction produced 17 as a mixture of diastereomers. This was resolved by column chromatography to give sertraline [5]. Dextrorotatory cis sertraline is substantially more potent than its isomers. 

The final stages of the successful drive towards amphotericin B (1) are presented in Scheme 19. Thus, compound 9 is obtained stereoselectively by sodium borohydride reduction of heptaenone 6a as previously described. The formation of the desired glycosida-tion product 81 could be achieved in dilute hexane solution in the presence of a catalytic amount PPTS. The by-product ortho ester 85 was also obtained in approximately an equimolar amount. Deacetylation of 81 at C-2, followed sequentially by oxidation and reduction leads, stereoselectively, to the desired hydroxy compound 83 via ketone 82. The configuration of each of the two hydroxylbearing stereocenters generated by reduction of carbonyls as shown in Scheme 19 (6—>9 and 82->83) were confirmed by conversion of 83 to amphotericin B derivative 5 and comparison with an... 

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

Sodium borohydride reductions of gold(I) complexes give Au clusters at RT if sodium borohydride in ethanol is dropped slowly into a suspension of the Au(I) complex in the same solvent. The immediate coloring of the reaction mixture (mostly red), even after only a few drops of the borohydride have been added, indicates fast formation of Au clusters. In view of the complicated composition of these compounds the fast formation is surprising. The use of H2 and CO with HjO as reducing agents in the synthesis of gold clusters has been described (see Table 1, Method A, 8.2.2.2). 

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