Sodium cyanoborohydride bonds

The reactivity of these tricyclic compounds has been investigated in detail. Reaction of these with sodium cyanoborohydride in acetic acid reduces the imine double bonds to give the tetrahydro-derivatives, for example, 37 gives 39. Reaction of 37 with sodium methoxide results in the ring-opened sulfonate salt 40 re-acidification of this salt gives the corresponding sulfonic acid which cyclizes back to the tricycle 37. Further heating of the sulfonic acid... 

Schiff base interactions between aldehydes and amines typically are not stable enough to form irreversible linkages. These bonds may be reduced with sodium cyanoborohydride or a number of other suitable reductants (Chapter 2, Section 5) to form permanent secondary amine bonds. However, proteins crosslinked by glutaraldehyde without reduction nevertheless show stabilities unexplainable by simple Schiff base formation. The stability of such unreduced glutaraldehyde conjugates has been postulated to be due to the vinyl addition mechanism, which doesn t depend on the creation of Schiff bases. 

Aldehyde-containing macromolecules will react spontaneously with hydrazide compounds to form hydrazone linkages. The hydrazone bond is a form of Schiff base that is more stable than the Schiff base formed from the interaction of an aldehyde and an amine. The hydrazone, however, may be reduced and further stabilized by the same reductants utilized for reductive amination purposes (Chapter 3, Section 4.8). The addition of sodium cyanoborohydride to a hydrazide-aldehyde reaction drives the equilibrium toward formation of a stable covalent complex. Mallia (1992) found that adipic acid dihydrazide derivatization of periodate-oxidized dextran (containing multiple formyl functionalities) proceeds with much greater yield when sodium cyanoborohydride is present. 

React for 2 hours at room temperature. While hydrazone formation does not require the addition of a reductant to create a linkage, including sodium cyanoborohydride in the reaction considerably increases the yield and stability of bonds formed. If the presence... 

Derivatives of hydrazine, especially the hydrazide compounds formed from carboxylate groups, can react specifically with aldehyde or ketone functional groups in target molecules. Reaction with either group creates a hydrazone linkage (Reaction 44)—a type of Schiff base. This bond is relatively stable if it is formed with a ketone, but somewhat labile if the reaction is with an aldehyde group. However, the reaction rate of hydrazine derivatives with aldehydes typically is faster than the rate with ketones. Hydrazone formation with aldehydes, however, results in much more stable bonds than the easily reversible Schiff base interaction of an amine with an aldehyde. To further stabilize the bond between a hydrazide and an aldehyde, the hydrazone may be reacted with sodium cyanoborohydride to reduce the double bond and form a secure covalent linkage. 

Figure 3.14 Carbonyl groups can react with amine nucleophiles to form reversible Schiff base intermediates. In the presence of a suitable reductant, such as sodium cyanoborohydride, the Schiff base is stabilized to a secondary amine bond.

Figure 7.11 Oxidation of glycoproteins with periodate, such as glycosylated antibodies, results in the formation of aldehyde groups that can be used for conjugation to dendrimers containing amine groups. Reductive amination with sodium cyanoborohydride results in coupling via secondary (or tertiary) amine bonds.

To reduce the hydrazone bonds to more stable linkages, cool the cell suspension to 0°C and add an equal volume of 30 mM sodium cyanoborohydride in PBS. Incubate for 40 minutes. Note If the presence of a reducing agent is detrimental to protein activity, eliminate the reduction step. In most cases, the hydrazone linkage is stable enough for fluorescent labeling experiments. 

The hydrazone bond can be reduced to stabilize the linkage by the addition of sodium cyanoborohydride to a final concentration of 50mM. React for 30 minutes at room temperature with mixing. All operations with cyanoborohydride should be done in a fume hood. If the glycoprotein being modified is sensitive to disulfide reduction and potential denaturation, then this step should be avoided. 

Hapten molecules containing aldehyde residues may be crosslinked to carrier molecules by use of reductive animation (Chapter 3, Section 4). At alkaline pH values, the aldehyde groups form intermediate Schiff bases with available amine groups on the carrier. Reduction of the resultant Schiff bases with sodium cyanoborohydride or sodium borohydride creates a stable conjugate held together by secondary amine bonds. 

Santagada and coworkers have disclosed a reductive amination method for the generation of a reduced peptide bond by reaction of a protected amino acid aldehyde with an N-deprotected amino ester using sodium cyanoborohydride as reducing agent [296]. 

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]. 

Reduction of unsaturated ketones to unsaturated alcohols is best carried out Nit v complex hydrides. a,/3-Unsaturated ketones may suifer reduction even at the conjugated double bond [764, 879]. Usually only the carbonyl group is reduced, especially if the inverse technique is applied. Such reductions are accomplished in high yields with lithium aluminum hydride [879, 880, 881, 882], with lithium trimethoxyaluminum hydride [764], with alane [879], with diisobutylalane [883], with lithium butylborohydride [884], with sodium boro-hydride [75/], with sodium cyanoborohydride [780, 885] with 9-borabicyclo [3.3.1]nonane (9-BBN) [764] and with isopropyl alcohol and aluminum isopro-... 

Ketones containing triple bonds in the a,)3-positions are reduced to the corresponding unsaturated alcohols with sodium cyanoborohydride or tetra-butylammonium cyanoborohydride in 64-89% yields [780]. Thus 4-phenyl-3-butyn-2-one gave 4-phenyl-3-butyn-2-ol [780]. If the same ketone was converted to its p-toluenesulfonylhydrazone and this was reduced with bis benzyloxy)borane, 1-phenyl-1,2-butadiene was obtained in 21% yield [786]. 

Hydrazones treated with alkalis decompose to nitrogen and hydrocarbons [845, 923] Woljf-Kizhner reduction) (p. 34), and p-toluenesulfonylhydra-zones are reduced to hydrocarbons by lithium aluminum hydride [812], sodium borohydride [785] or sodium cyanoborohydride [813]. Titanium trichloride hy-drogenolyzes the nitrogen-nitrogen bond in phenylhydrazones and forms amines and ketimines which are hydrolyzed to the parent ketones. Thus 2,4-dinitrophenylhydrazone of cycloheptanone afforded cycloheptanone in 90% yield [202]. 

Successful double-bond reductions are shown in Scheme 36. Formic acid can be used <1970LA(739)32>, although N-formylation may occur <1970M1295>. With the right conditions, sodium cyanoborohydride is also a suitable reagent for dihydrothiazines <1994ACS517> and dihydrothiazin-2-ones <2006SL3259>. 

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