Aldehydes, reduction with cyanoborohydride

Another route to the formation of a hydrazide on a surface is to use an aldehyde-containing particle (such as HEMA/acrolein copolymers) and subsequently modify the aldehydes to form hydrazone linkages with bis-hydrazide compounds, which then can be stabilized by reduction with sodium cyanoborohydride (Chapter 2, Section 5). The resulting derivative contains terminal hydrazides for immobilization of carbonyl ligands (see Figure 14.18). 

Figure 14.21 Aldehyde-particles can be reacted with amine-containing proteins or other molecules to form intermediate Schiff bases, which can be stabilized by reduction with sodium cyanoborohydride.

Thioamides were converted to aldehydes by cautious desulfurization with Raney nickel [1137, 1138] or by treatment with iron and acetic acid [172]. More intensive desulfurization with Raney nickel [1139], electroreduction [172], and reduction with lithium aluminum hydride [1138], with sodium borohydride [1140] or with sodium cyanoborohydride [1140] gave amines in good to excellent yields. 

The use of C-furfuryl nitrones by Merino and co-workers (201) as latent carboxylate functionality (see Section 1.7) has inspired the development of an analogous C-thiazolyl nitrone. Thus, after reaction of this dipole with acrylate esters, the authors take advantage of the thiazolyl-to-formyl synthetic equivalence to reveal aldehyde functionality after reduction with sodium cyanoborohydride. 

Glutaraldehyde is the most popular bis-aldehyde homobifunctional cross-linker in use today. However, a glance at glutaraldehyde s structure is not indicative of the complexity of its possible reaction mechanisms. Reactions with proteins and other amine-containing molecules would be expected to proceed through the formation of Schiff bases. Subsequent reduction with sodium cyanoborohydride or another suitable re-ductant would yield stable secondary amine linkages (Chapter 2, Section 5.3, and Chapter 3, Section 4). This reaction sequence certainly is possible, but other cross-linking reactions also are feasible. 

As with cyanoborohydride, very few functional groups are affected by catechol borane under the tosylhydrazone reduction conditions (25 °C), allowing highly selective conversions in the presence of most moieties, including alkenes and alkynes which are hydroborated at more elevated temperatures (70-100 The only exceptions to this appear to be aldehydes, carboxylates, sulfoxides, amine oxides and anhydrides, which are reduced at rates comparable to tosylhydrazones. ... 

Lhommet and co-workers synthesized (- )-436 by the route shown in Scheme 66 [422). Wittig-Homer reaction between the 2,5-rrans -disubstituted pyrrolidine aldehyde 509, made from (S)-pyroglutamic acid [423), and the protected keto-phosphonate 510 introduced all the skeletal carbon atoms of the target. Simultaneous hydrogenation and //-deprotection of enone 511 gave the aminoketone 512, which spontaneously formed the bicyclic enamine 513 in 98% yield when exposed to trifluoroacetic acid—apparently the first time that such an intermediate has actually been isolated en route to indolizidines. Reduction with sodium cyanoborohydride in acidic medium acid produced a diastereomeric mixture (92 8) of (- )-436 and (+ )-437. The former was isolated in 84% yield after chromatography on silica gel. The overall yield of this 15-step sequence was 8% based on (S)-pyroglutamic acid. 

An important PEG attachment chemistry for reaction with amino groups involves PEG aldehydes. In this chemistry, PEG aldehyde reacts with amino group on the target protein to form a reversible Schiff base linkage. The imine intermediate is then reduced with a suitable reductant such as sodium cyanoborohydride (Eigure 24.3). The most notable feature of this reaction is that, when conducted at low pH, the reaction is specific for the amino terminus of the protein [42,43]. The reason for this selectivity is that, relative to other nucleophilic residues in the molecule, the amino terminus has a lower pKa. Therefore, this reaction scheme can be used to reduce the side reactions that form multipegylated products in other amine selective chemistries. 

Although Schiff base formation can be performed with amine groups, the low stability of the bond in aqueous conditions makes hydrazide a better alternative. Hydrazides can be introduced on the sensor surface via reaction of hydrazine or carbohydrazine to carboxylic groups after activation with EDC/NHS (Fig. 11) [32]. The hydrazide-aldehyde bond forms rapidly and is relatively stable in neutral to alkaline conditions, but disintegrates slowly in acidic buffers. If necessary, the bond can be further stabilized by reduction with sodium cyanoborohydride at pH 4. 

Oxidation of Sephadex G-50 or Enzacryl polyacetal using periodate affords aldehydic pol5nners with which the primary amino groups of the amino acid moieties in aminoacyl-tRNA form Schiff bases, and coupling is rendered irreversible by reduction with sodium cyanoborohydride. Hence, only the amino-acylated tRNA molecules in a mixture are retained, and the tRNA may subsequently be released by washing with ammonium bicarbonate buffer, thus affording a convenient method for the separation of isoacceptor tRNA species. ... 

Other Carbohydrate-based Methods. N-Hydroxy-l,4-dideoxy-l,4-imino-arabinitol, 77, has been prepared in nine steps from ( )-3-0-benzylglyceral-dehyde. A transketolase mediated reaction was used to establish a pentulose (5-0-benzyl-D-xylulose) with correct absolute stereochemistry, and a 1,2-oxazine was the unexpected product of the acid-catalysed reaction of an aldehydic intermediate with triethylorthoformate (Scheme 15). Reduction of this oxazine with sodium cyanoborohydride in acetic acid, did not effect cleavage of the N-O bond, and yielded the iV-hydroxypyrrolidine as a single diastereoisomer. ... 

Sodium cyanoborohydride reduces aldehydes and ketones less rapidly than sodium borohydride, but it reduces iminium ions rapidly. To take advantage of this selectivity, reductive aminations are carried out at mildly acidic pH, where the imines are protonated. Iminium ions are also more reactive than imines toward reduction with hydride. 

Reductive amination of the aldehyde 3 with various amines provides a ready route to the preparation of derivatives. For example, reductive amination of 3 in methanol using 1,10-diaza-18-crown afforded the water soluble crown derivative Similarly, the PEG-carbohydrate conjugates 7 and were obtained by reductive amination with glucosamine and chitosan, respectively. PEG-coated glass beads 9 were obtained from aldehyde sodium cyanoborohydride, and aminopropyl-derivatized, controlled-pore glass. We are presently examining this same reaction for coupling PEG and proteins. Also, as mentioned above, PEG amine was prepared by reductive amination with ammonium acetate. 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

The N-substituted aminoacids required could be prepared by microwave-assisted reductive amination of aminoacid methyl esters with aldehydes, and although in the Westman report soluble NaBH(OAc)3 was used to perform this step, other reports have shown how this transformation can be performed in using polymer-supported borohydrides (such as polymer-supported cyanoborohydride) under microwave irradiation [90]. An additional point of diversity could be inserted by use of a palladium-catalyzed reaction if suitably substituted aldehydes had been used. Again, these transformations might eventually be accomplished using supported palladium catalysts under microwave irradiation, as reported by several groups [91-93]. 

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