Modifications sodium borohydride reduction

The stereoselective total synthesis of both ( )-corynantheidine (61) (170,171) (alio stereoisomer) and ( )-dihydrocorynantheine (172) (normal stereoisomer) has been elaborated by Szdntay and co-workers. The key intermediate leading to both alkaloids was the alio cyanoacetic ester derivative 315, which was obtained from the previously prepared ketone 312 (173) by the Knoevenagel condensation accompanied by complete epimerization at C-20 and by subsequent stereoselective sodium borohydride reduction. ( )-Corynantheidine was prepared by modification of the cyanoacetate side chain esterification furnished diester 316, which underwent selective lithium aluminum hydride reduction. The resulting sodium enolate of the a-formyl ester was finally methylated to racemic corynantheidine (171). 

Hyperforin is not reduced by sodium borohydride. Reduction with hydride-transfer reagents such as lithium aluminium hydride (LAH), RED-AL, and DIBAL-H, gave varied products in good yields. Its two dicarbonyl systems are amenable to reduction or deoxygenation upon treatment with alane reducing agents and pave the way to new and interesting modifications of the natural product.301... 

Substituted cardenolides or cardenolides with epimeric configurations at several positions of the steroid nucleus have been made by appropriate modifications of known syntheses. In one of these, the steroid (510) was hydroxylated at C(14) by a microbiological method, acetylated, and reduced catalytically to the A/B-c/s-steroid (511). Sodium borohydride reduction gave the 3a-alcohol, which after acetylation and acid hydrolysis was transformed into the 3,14-di-... 

Some of the important chemical modifications are summarized in Scheme 9. Sodium borohydride reduction of compound A resulted in the reduction of not only the aldehyde group but also stereospecific reduction... 

Halogen analogues can also be prepared as, for example, Co(NO)(PF3)2 (MeOPF2) (8.218). Methods of preparation of these complexes include addition (6.234), direct ligand replacement (8.216), sodium borohydride reduction (8.217) or direct modification of the ligand (8.218). 

Trost coined the term chemoselective to describe the process where one of two similar functional groups is made to react, while the other is not affected or affected to a lesser extent [1]. This discrimination usually results from nuanced changes to a reagent s behavior, e.g., by addition of a salt. A typical example is the modification of sodium borohydride reduction by addition of cerium chloride in the Luche process, whereby the double bond of an ot-enone is not saturated during the reaction [85]. Similarly, a new approach to chemoselective reaction of n-pentenyl donors was soon to emerge. 

Industrial Synthetic Improvements. One significant modification of the Stembach process is the result of work by Sumitomo chemists in 1975, in which the optical resolution—reduction sequence is replaced with a more efficient asymmetric conversion of the meso-cyc. 02Lcid (13) to the optically pure i7-lactone (17) (Fig. 3) (25). The cycloacid is reacted with the optically active dihydroxyamine [2964-48-9] (23) to quantitatively yield the chiral imide [85317-83-5] (24). Diastereoselective reduction of the pro-R-carbonyl using sodium borohydride affords the optically pure hydroxyamide [85317-84-6] (25) after recrystaUization. Acid hydrolysis of the amide then yields the desired i7-lactone (17). A similar approach uses chiral alcohols to form diastereomic half-esters stereoselectivity. These are reduced and direedy converted to i7-lactone (26). In both approaches, the desired diastereomeric half-amide or half-ester is formed in excess, thus avoiding the cosdy resolution step required in the Stembach synthesis. 

Carba-sugars of the a-L-altro and P-D-gluco modifications were prepared from 149 by way of 155. 0-Mesylation of 155 with an excess of mesyl chloride and pyridine resulted in formation of the cyclohexenealdehyde 159, accompanied by y -elimination. Reduction of 159 with sodium borohydride gave the cyclohexenemethanol 160, which is the antipode oP 141. 

Nitromethylation of aldehydes has been carried out in a one pot procedure consisting of the Henry reaction, acetylation, and reduction with sodium borohydride, which provides a good method for the preparation of l-nitroalkanes.16b 79 It has been improved by several modifications. The initial condensation reaction is accelerated by use of KF and 18-crown-6 in isopropanol. Acetylation is effected with acetic anhydride at 25 °C and 4-dimethylaminopyridine (DMAP) as a catalyst. These mild conditions are compatible with various functional groups which are often... 

Reduction of the carhonyl group to methylene is carried out hy Clemmensen reduction [160, 758], hy Wolff-Kizhner reduction [280, 282], or hy its modifications decomposition of hydrazones with potassium /er/-butoxide in dimethyl sulfoxide at room temperature in yields of 60-90% [845], or hy reduction ofp-toluenesulfonylhydrazones with sodium borohydride (yields 65-80%) [811] (p. 134). 

Reduction of a, -unsaturated ketones to unsaturated hydrocarbon is rather rare, and is almost always accompanied by a shift of the double bond. Such reductions are accomplished in good to high yields by treatment of the p-toluenesulfonylhydrazones of the unsaturated ketones with sodium borohydride [785], borane [786] or catecholborane [559], or by Wolff-Kizhner reduction or its modifications [590]. However, complete reduction to saturated hydrocarbons may also occur during Wolff-Kizhner reduction [597] as well as during Clemmensen reduction [750]. 

One drawback, however, is that the products 5 are unstable during extended storage towards racemization. This can be circumvented by converting the aldehydes 5 in situ into derivatives. Depending on the reaction conditions amino alcohols 6 or oxazolidinones 7 are obtained these also are valuable intermediates. The two types of reductive modification are shown in Schemes 7.5 and 7.6, respectively. Such in situ reductions are performed by treatment with sodium borohydride. 

Peroxide-bleached, chemithermomechanical aspen pulp (BCTMP), prepared by Tembec of Quebec, was used in this study. Prior to chemical modification, the pulp was diluted to 1.5% consistency and stirred for approximately 15 minutes at room temperature with diethylenetriaminepentaacetic acid (0.5% of o.d. pulp) to complex undesirable heavy metal ions. The chelated pulp was washed thoroughly with distilled water and reduced subsequently with sodium borohydride. The method used for reduction was similar to that of Castellan and coworkers (24). The pulp was diluted to 3% consistency and homogenized for 5 minutes with sodium borohydride (40% based on o.d. pulp). The mixture was transferred to glass containers and was shaken frequently over a 6 hour period at room temperature. The reduced pulp was washed with distilled water until a neutral pH was reached. 

The interest in papain increased enormously after the publication by Kimmel and Smith D ] of a modification of the purification procedure of Balls and Lineweaver [5j. This modification permitted the isolation of pure papain from papaya dried latex and was used for many years as the standard method for the production of papain. The crystalline papain of Kimmel and Smith consists of three components active papain, reversibly oxidized papain, and irreversibly oxidized papain. Reversibly oxidized papain can be converted into active papain by reduction of the active-site thiol by low molecular weight thiols [10], sodium borohydride [11], or CN [12]. In active papain, the Cys-25, which is essential far catalytic activity, is present in a reduced form, while in reversibly oxidized papain the Cvs-25 forms a mixed disulfide with cysteine. Drenth et al. reported that in irreversibly oxidized papain the Cys-25 has been oxidized to the sulfuric acid... 

Chemical modifications of the antigens were achieved by oxidation of the monosaccharide residues with periodate and reduction of the carboxyl groups of the uronic acid residues by the carbodiimide and borohydride methods, Fig. (IOC). The periodate oxidation was performed by a procedure described in the literature [37]. The reduction of the uronic acid residues of the antigen was performed by the carbodiimide (CMC) and sodium borohydride method [38], The oxidized and reduced types of antigens no longer reacted with the antibodies, Fig. (IOC). 

Figure 18. Reversible reductive alkylation of amino groups. Amino groups are alkylated first by treatment with (a) glycolaldehyde or (b) acetol in the presence of sodium borohydride. Reversal of the modification is effected by treating the modified amino group with 10-20mM NalOh for 30 min (85).

Three years after Narasaka and Pai s disclosure, Prasad et al. developed a modified procedure to improve syn -diastereoselecti vi ty in the reduction of certain (3-hydroxy ketones6 (Scheme 4.Id). When methoxydiethylborane, in lieu of tributylborane, reacts with p-hydroxy ketones at —70 C in anhydrous methanol, the complex 5BEt2 is formed. Subsequent treatment of the complex with sodium borohydride and quenching the reaction mixture with acetic acid affords yyn-diols in excellent levels of diastereoselectivity regardless of the structure of p-hydroxy ketones. Another practical advantage of Prasad et al. s modification may be an enhanced safety feature, as methoxydiethylborane is generally less hazardous to handle than triethylborane.6... 

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