Reduction by diborane

Reduction by diborane of the sodium salt of 3/3-t-butyloxy-5,7-seco-B-norcholestan-5-on-7-oic acid gave the 5j3H lactone (339) which could be further reduced to the corresponding 5a,7-diol. Conversion into the 5a,7-dimesylate was followed by reaction with sodium sulphide to give 6-thiacholestanol butyl ether... 

ATP, in metabolic processes 541 Azide rearrangement 632 Azides, reaction with peraminoethyl-enes 725-727 reduction, by diborane 68... 

Metal hydride reductions occur by nucleophilic attack at the carbonyl carbon atom of acyl derivatives. Reduction of carboxylic acids with hydride reagents occurs slowly, but reduction by diborane occurs rapidly. Based on the structure of BH3, the active reagent in diborane reductions, suggest the structure of the first intermediate formed in the reaction. 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Reductions with diborane have been investigated by Brown and his co-workers. Diborane is an inflammable toxic gas which is available commercially or may be generated by the reaction of sodium borohydride and... 

Contrary to Brown s results, a later paper claims that chloral, esters and acid chlorides are readily reduced by diborane in tetrahydrofuran it is suggested that diborane can complex with tetrahydrofuran before effecting reduction. 

Thakar and Subba Rao showed that reductions with diborane give the same result regardless of whether diborane is generated externally or produced internally by the action of NaBH4 and boron trifluoride or aluminum chloride. They found that alicyclic or dialkyl ketones are not reduced beyond the alcohol stage even under drastic conditions however, diaryl ketones are hydrogenolyzed under normal conditions, while aryl ketones or a,i5-unsatu-rated ketones are hydrogenolyzed, in part, under drastic conditions. 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Chemoselective demethylation of 1247 by treatment with boron tribromide led to a 4-hydroxy compound, which, after reduction with diborane, was reacted with triflic anhydride (Tf20) to afford the benzyl derivative 1248. Palladium(0)-catalyzed... 

Amides require vigorous reaction conditions for reduction by LiAlH4 so that little selectivity can be achieved with this reagent. Diborane, however, permits the reduction of amides in the presence of ester and nitro groups. 

Prepn of the 3,5-dinitro by diborane reduction of H03C.CH2.C8Ha-3I5-(N02)2 is given in... 

Octahydrotnborate( The octahydrotriborate(l-) anion [12429-74-2], [B3H8 commonly referred to as the triborohydride ion, is produced by the reduction of diborane with sodium amalgam. Large quantities can be prepared more conveniently by the reaction of sodium... 

Jackson and Zurqiyah129 carried out a study similar to Kupfer s. In their work the reaction mixture was always kept alkaline to avoid the possibility161 of reduction of the double bond by diborane f ormed when solutions of NaBH4 are acidified. The compounds studied were 3-methylcyclohexenone, carvenone and cholestenone, and the solvents used were 2-propanol, diglyme,triglymeand pyridine. Since borohydride does not reduce allylic alcohols, this precludes the possibility that the saturated alcohols are formed from the allylic alcohols. 

Direct reduction of a peptide bond with diborane 59 or a thioamide bond with several reductive procedures 60 is an alternative route for the production of a reduced peptide bond in a peptide. In some cases the reductive amination does not give satisfactory results. As described earlier, preparation of Boc-Pher )[CH2N]Pro-OH by reductive amination yields two diastereomers (Scheme ll). 57 In this case treatment of Boc-Phe-Pro-OBzl by diborane yielded the reduced pseudodipeptide Boc-Pher )[CH2N]Pro-OBzl without epimerization (Scheme 12). However, in some cases diborane is not entirely selective for the amide bond and can reduce ester functions when they are present. Another procedure is to prepare endothiopeptides directly from protected dipeptides 61-66 followed by their selective reduction. 60 ... 

Alternatively, the remaining sulfate ester of 70 may serve as a leaving group for a second nucleophilic displacement reaction. When this displacement is by an intramolecular nucleophile, a new ring is formed, as was first shown in the synthesis of a cyclopropane with malonate as the nucleophile [68] and of aziridines with amines as the nucleophiles [76]. The concept is further illustrated in the double displacement on (/J,/ )-stilbenediol cyclic sulfate (72) by benzamidine (73) to produce the chiral imidazoline 74 [79]. Conversion of the imidazoline (74) to (.V,.S )-stilbenediaminc 75 demonstrates an alternative route to optically active 1,2-diamines. Acylation of 75 with chloroacetyl chloride forms a bisamide, which, after reduction with diborane, is cyclized to the enantiomerically pure trans-2,3-diphenyl- 1,4-diazabicy-clo[2.2.2]octane (76) [81],... 

The use of oxazaborolidines as asymmetric reduction catalysts257 and the enantioselectivity of diphcnyloxazaborolidinc reduction of ketones have been reviewed.258 Large-scale practical enantioselective reduction of prochiral ketones has been reviewed with particular emphasis on the Itsuno-Corey oxazaborolidinc and Brown s 5-chlorodiisopinocampheylborane (Ipc2BCl) as reagents.259 Brown himself has also reviewed the use of Ipc2BCl.260 Indolinoalkylboranes in the form of dimers have been confirmed by 11B NMR as the products of the reduction of trifluoroacetylindoles by diborane.261... 

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