Reduction with diborane

A solution of this product in cyclohexane is chromatographed on alumina elution with benzene and benzene-25 % ether yields 2.29 g of material which on recrystallization from cyclohexane gives pure hydroxy ketone, mp 151-152°. 

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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... 

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. 

The stereochemistry of reductions with diborane has been little studied. 

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... 

Stereochemistry of reduction, 34 Stereochemistry of reductions with diborane, 90... 

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],... 

OH/100 C under optimal conditions of the reduction with diborane. These conditions were the hydrolysis with NaOH and the oxidation of C = C bonds in the presence of H2O2. 

The reaction of the 9-hydroxy-ergoline derivative (74), prepared by reduction with diborane and anti-Markovnikov hydration of lysergic acid, with phosphorus oxychloride-pyridine results49 in expansion of ring C by displacement of the equa-49 L. Bernardi, C. Elli, and A. Temperilli, J.C.S. Chem. Comm., 1976, 570. 

The high-dilution method (for technical details see for example Ref. 21) has been used not only to form simple cryptands, but also in the syntheses of more complex systems — cylindrical macrotricyclic molecules24), lateral macrobicycles 25), spherical macrotetracyclic systems 26), and speleands 27). Nevertheless, the important drawback of the method is the last step, e.g. reduction with diborane. That means only compounds which do not interfere with this reagent can be formed in this manner. Also, even a small contamination of the solvents and reactants with water has a significant influence on the yield. 

The resulting acetate 116 is formulated as the tawis-isomer, too. The carboxylic group was then converted by selective reduction with diborane to the alcohol 117. The alcohol function was converted to the nitrile 119 via the tosylate 118 and displacement of the tosylate group with sodium cyanide. Methanolysis led to the methylester 120 because the acetate moiety was not cleaved under these conditions (MeOH/HCl). Hydrolysis with base yielded the deprotected lactone acid alcohol 121, which was purified by converting it into the methylester 122. 

II, 2.4]-triazin-3(2/y)-one (151, R = Me, R = H) (70JHCI23I). Similarly, 150 (R = Me) reacts with a-cyanobenzyl p-toluene sulfonate lPhCH(CN)OTS] or a-cyanobenzyl bromide PhCH(CN)Br] to afford 151 (R = Me, R = Ph) (7IJHC62I). Structure 151 was proved by its reduction with diborane to 152, which was also obtained by an independent synthesis involving the reaction of 153 with thionyl chloride followed by base-catalyzed cyclization (70JHCI231 71JHC621). To ascertain whether the methyl group at the I-position in 150 (R = Me) influences the direction of cyclization, the thione 150 (R = H) was allowed to condense with a-cyanobenzyl-p-toluene sulfonate and ethyl a-bromophenylacetate. In both cases 151 (R = H, R = Ph) was obtained. Compound 150, however, on reaction with 1,2-dibromoethane, furnished another bicyclic system (154). The difference in the behavior of 150 toward dibromoethane and other reagents such as ethyl bromoacetate and a-cyanobenzyl p-toluene... 

Optically active amines. Esterification of an oxime, for example 2-hydroximino-1-phenylpropane (1), with a chiral carboxylic acid chloride, S-( + )-2-phenylbutyric acid chloride (2), gives an oximoester (3), which on reduction with diborane gives a mixture of optically active amines (4a, 4b) in which the S-enantiomcr (4a) predominates slightly. (The optical yield is about 6%.)... 

PtO2). ° The reaction is carried out under hydrogen atmosphere, at ordinary or elevated pressure, at room temperature, in EtOH. In each case, it is necessary to adjust the conditions to obtain a full hydrogenation. Reduction with diborane in THF has been less frequently used. Starting from... 

An alternative synthetic approach to amine alkylation, reductive amination, has also been used in solution-phase combinatorial chemistry (Table 3.4). Parallel syntheses implying alkylation of amines via acylation and subsequent reduction with diborane has also been reported [29,86]. 

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. 

Perloza functionalization is performed by treatment with acrylonitrile followed by reduction with diborane to yield aminopropylcellulose. Substitution levels are typically 0.3-1.2 mmol/g. Bromoacetyl linker [168],... 

A new application of the Schmidt rearrangement is to give unnatural aminoacids <91BMC125>. Treatment of the optically active cyclic -ketoester (266) with hydrazoic acid gives the optically active azepinone (267) which upon selective reduction with diborane furnishes the novel cyclic amino acid ester (268) (Scheme 36). 

Review. Pelter= has reviewed reduction with diborane and some reagents hased on diborane. 

Other possible intermediates are 3-benzyl-4-hydroxycoumarins. Starting from the parent compound (41) the reaction to 3-benzyl-4-chromone (42) is achieved in moderate yield by reduction with diborane, followed by oxidation with sodium bichromate in aqueous sulfuric acid 42). 

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