Zinc borohydride reduction

Sodium borohydride-Palladium chloride. Sodium borohydride-Rhodium(lII) chloride. Sodium borohydride-Tin(II) chloride. Sodium cyanoborohydride. Sodium 9-cyano-9-hydrido-9-borabicyclo[3.3.1]nonane. Sodium dithionite. Sodium hydride-Sodium t-amyl oxide-Zinc chloride. Sodium trimethoxyborohydride. Tetra-/i-butylammonium borohydride. Tetra-n-butylammonium cyanoborohydride. Tetra-n-butylammonium octahydrotriborate. Tri-n-butyltin hydride. Triethoxy silane. Triisobutylaluminum-Bis(N-methyl-salicyclaldimine)nickel. Zinc borohydride. REDUCTIVE CYCLIZATION Cobaloximc(I). 

Each diastereomer was converted to the enantiomeric alcohols by zinc borohydride reduction, followed by chromatography of the Cls-epimeric mixtures. Saponification of the diolester gave the enantiomeric 9,9-dioxy-9-thia-prostanoids 96. 

Helm, R. F., and Ralph, J. (1993) Stereospecificity for zinc borohydride reduction of a-aryloxy-b-hydroxy ketones. J. Wood Chem. Technol. 13(4), 593-601. 

The wH/f-selective reduction of a-alkylthio ketones is less straightforward. Zinc borohydride has been used successfully in some cases, implying that chelation control involving an alkyl(or aryl)thio group is possible. However, the method is not general, as illustrated by the results in the zinc borohydride reductions of 3-methyl-2-methylthio-l-phenylbutanone and 4-methyl-3-phenyl thio-2-pentanone 81. 

Diastereoselective hydride addition is quite versatile, and it provides facile synthetic access to ( —)-pinidine (661), an alkaloid isolated from several species of Pinus, as well as its unnatural isomer (+ )-pinidine (660b). The unstable aldehyde 655, prepared in four steps from 624 [202], undergoes Grignard addition with 4-pentenylmagnesium bromide followed by Swem oxidation to afford ketone 656 in 90% yield for the two steps. Stereoselective hydride addition with L-Selectride provides the -yn-alcohol 657 (91 9), while zinc borohydride reduction provides almost exclusively the anri-alcohol 658 (>99 1) (Scheme 144). 

Addition of Grignard reagents to threose derivatives, in either cyclic or acyclic form, gave mainly the corresponding xylitol product (Scheme 5). Oxidation of these tetritol derivatives at C-1 followed by zinc borohydride reduction gave mainly lyxitol, whereas L-Selectride favoured the xylitol products these stereoselectivities were discussed. ... 

A rather different phenolic oxidation, that of the oxime (154) with thallium(III) trifluroacetate to give the isoxazole (155), followed by zinc borohydride reduction, has been used to give (156) which is a versatile intermediate for the synthesis of the sponge metabolites ( )-aerothionin (157a), ( )-homoaerothionin (157b), and ( )-aerophobin-1 (158). ... 

Other Borohydrides. Potassium borohydride was formerly used in color reversal development of photographic film and was preferred over sodium borohydride because of its much lower hygroscopicity. Because other borohydrides are made from sodium borohydride, they are correspondingly more expensive. Generally their reducing properties are not sufficiently different to warrant the added cost. Zinc borohydride [17611-70-0] Zn(BH 2> however, has found many appHcations in stereoselective reductions. It is less basic than NaBH, but is not commercially available owing to poor thermal stabihty. It is usually prepared on site in an ether solvent. Zinc borohydride was initially appHed to stereoselective ketone reductions, especially in prostaglandin syntheses (36), and later to aldehydes, acid haHdes, and esters (37). 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

Pr)4, " borohydride-exchange resin,and formic acid. When the last is used, the process is called the Wallach reaction. Conjugated aldehydes are converted to alkenyl-amines with the amine/silica gel followed by reduction with zinc borohydride.In the particular case where primary or secondary amines are reductively methylated with formaldehyde and formic acid, the method is called the Esch-weiler-Clarke procedure. It is possible to use ammonium (or amine) salts of formic acid, " or formamides, as a substitute for the Wallach conditions. This method is called the Leuckart reaction,and in this case the products obtained are often the N-formyl derivatives of the amines instead of the free amines. Primary and secondary amines can be iV-ethylated (e.g., ArNHR ArNREt) by treatment with NaBH4 in acetic acid. Aldehydes react with aniline in the presence of Mont-morillonite KIO clay and microwaves to give the amine. Formaldehyde with formic acid converts secondary amines to the N-methyl derivative with microwave irradiation. 

Zinc borohydride has been found to effect very efficient reductive amination in the presence of silica. The amine and carbonyl compound are mixed with silica and the powder is then treated with a solution of Zn(BH4)2. Excellent yields are also obtained for unsaturated aldehydes and ketones.96... 

Dissolution of a zinc-ruthenium alloy in hydrochloric acid leaves an explosive residue of finely divided ruthenium [1], More probably this is the hydride, which may decompose on slight stimulus, the evolved hydrogen probably igniting because of the catalytic activity of the metal. Ruthenium prepared from its compounds by borohydride reduction is especially dangerous in this respect [2],... 

TABLE 3-3. Asymmetric Acylation Using (2A,5A)-24 and Subsequent Stereoselective Reduction with Zinc Borohydride in THF at —78°C... 

The high-yield reductive methylation of numerous alkyl and arylamines and of dialkyl-and alkyl-arylamines with paraformaldehyde in the presence of zinc chloride and zinc borohydride has been reported (equation 52)151. 

Brimble and coworkers176 studied the asymmetric Diels-Alder reactions of cyclopentadiene with chiral naphthoquinones 272 bearing different chiral auxiliaries. The highest endo and facial selectivities were obtained using zinc dichloride as the Lewis acid catalyst and (—)-pantolactone as the chiral auxiliary. Thus, the reaction between cyclopentadiene and 272 afforded a 98 2 mixture of 273 and 274 (equation 76). The chiral auxiliary was removed easily by lithium borohydride reduction. 

The double bond in indole and its homologs and derivatives is reduced easily and selectively by catalytic hydrogenation over platinum oxide in ethanol and fluoroboric acid [456], by sodium borohydride [457], by sodium cyanoborohydride [457], by borane [458,459], by sodium in ammonia [460], by lithium [461] and by zinc [462]. Reduction with sodium borohydride in acetic acid can result in alkylation on nitrogen giving JV-ethylindoline [457]. 

The analogue in which carbon replaces oxygen in the enol ring should of course avoid the stability problem. The synthesis of this compound initially follows a scheme similar to that pioneered by the Corey group. Thus, acylation of the ester (7-2) with the anion from trimethyl phosphonate yields the activated phosphonate (7-3). Reaction of the yhde from that intermediate with the lactone (7-4) leads to a compound (7-5) that incorporates the lower side chain of natural prostaglandins. This is then taken on to lactone (7-6) by sequential reduction by means of zinc borohydride, removal of the biphenyl ester by saponification, and protection of the hydroxyl groups as tetrahydropyranyl ethers. 

Reduction of l,10-phenanthroline-2-aldehyde to 1,10-phenanthroline-2-carbinol is efficiently accomplished by a dihydronicotinamide derivative in acetonitrile solution catalyzed by zinc ions. This was the first example of the reduction of an aldehyde by a NADH analog in a nonenzymic system. It also supports the catalytic function of the metal ion in the enzymic system.359 l,10-Phenanthroline-2-carbinol, obtained by sodium borohydride reduction of 2-carbomethoxy-1,10-phenanthroline, is phosphorylated by adenosine triphosphate in the presence of zinc ions.360... 

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