Sodium borohydride aldimines

The reaction of butyllithium with 1-naphthaldehyde cyclohexylimine in the presence of (/C )-l,2-diphenylethane-1,2-diol dimethyl ether in toluene at —78 °C, followed by treatment with acetate buffer, gave 2-butyl-1,2-dihydronaphthalene-l-carbaldehyde, which was then reduced with sodium borohydride in methanol to afford (1 R,2.S)-2-butyl-1 -hydroxymcthyl-1,2-dihydronaphthalene in 80% overall yield with 91 % ee83. Similarly, the enantioselective conjugate addition of organolithium reagents to several a,/J-unsaturated aldimines took place in the presence of C2-symmetric chiral diethers, such as (/, / )-1,2-butanediol dimethyl ether and (/, / )- ,2-diphenylethane-1,2-diol dimethyl ether. 

The reactions of simple imine complexes are mainly restricted to hydrogenation or reduction by hydrides or by addition of nucleophiles such as alcohols. For example, complexes of macrocycles (18) undergo catalytic reduction to generate two chiral centres.66 Sodium borohydride has been used to reduce the imine complexes shown in equation (3).67 This technique enables convenient removal of the metal and the consequent synthesis of the reduced free macrocycles. Complexes of the aldimine (20) undergo nucleophilic addition of alcohols.49... 

Aldehydes and ketones react reversibly with the amino groups of proteins to form aldimines or ketimines (Schiff bases),16 The SchifF base thus formed may be reduced with sodium borohydride, to afford alkylamines, which are stable. This reductive amination technique has been widely used for the alkylation of the 6-amino group of lysine.77... 

Also reported in Scheme 24 are the PhSeBr promoted cyclization reactions of alkenyl aldimines described by De Kimpe [93,94]. As indicated by the reactions of 158 and 161, both, the 5-exo-trig and the 5-endo-trig cyclizations, can take place. The initially formed iminium salts 159 and 162 are converted into the corresponding pyrrolidines 160 and 163 by reduction with sodium borohydride. Compound 163 was obtained as an almost equimolecular mixture of two diastereomers. 

Conversion of alditols to aldoses without the need to protect all hydroxy groups has been achieved by monotosylation of one primary hydroxy group, displacement with azide ion and photolysis in methanol to yield the aldimine,which was then hydrolyzed to the aldose. The procedure was illustrated using 3 4-0 isopro ylideno-D-mannitol to produce D-mannose. The synthesis of D-[U- Cjgalactose from methyl <-D-[n- Cjglucopyranoside via aqueous bromine oxidation to the 4.-uloside, reduction by sodium borohydride and hydrolysis has been described, along with the isolation of D-glucuronic acid and methyl o( D-mannopyranoside as by-products. 

Chitosan is a multi-nucleophilic polymer due to the presence of the NH2 and OH functional groups. The initial sites where substitution occurs are the more nucleophilic amino groups. However, the experimental conditions and protection of the NH2 groups reduces the intermolecular hydrogen bonding and creates space for water molecules to fill in and solvate the hydrophilic groups of the polymer backbone (Sashiwa and Shigemasa 1999). A -alkylated derivatives can be obtained by the treatment of chitosan with aldehydes or ketones via formation of Schiff base intermediates, aldimines (from reactions with aldehydes), or ketimines (from reactions with ketones) followed by reduction of the imine with sodium borohydride. 

Cross-linking is initiated by the formation of allysine and/or hydroxyallysine from specific Lys residues in the telopeptides of the interstitial collagens (Fig. 6). This step requires the action of the specific enzyme, lysyl oxidase. These aldehydes may then form initial bifunctional cross-links, termed reducible cross-links as they can be reduced by sodium borohydride. These are either aldol condensation products or aldimine or ketoamine links. The aldimine and ketoamine involve specific Lys or Hyl residues either from the telopeptides or from the helical domain of the collagen. The sites of these specific residues have been determined (Fig. 6) (50). 

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

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