Peterson olefination from alpha-silyl aldehydes

A procedure for the stereoselective synthesis of substituted alkenes from alpha-silyl aldehydes, via the Peterson reaction, is described. The protocol for the preparation of alpha-silyl aldehydes is also included. Organometallic addition to the alpha-silyl aldehyde gives erythro-beta-hydroxysilanes in high yields (85-90%), which undergo elimination on treatment with potassium hydride (KH) or boron trifluoride to afford respectively Z- o E-alkenes (87-90%). The method described has been carried out using alpha-silyl aldehydes bearing the tert-butyldiphenylsilyl group. This bulky group increases the stability of the silyl aldehyde, enhances the stereoselectivity of the formation of the beta-hydroxysilanes and favors the stereocontrol of the elimination step, thus providing high yields of stereo-defined alkenes. Here we describe a two-step protocol for the synthesis of Z-1-phenyl-1-hexene from 2-tert-butyldiphenylsilyl-2-phenylethanal and n-butyllithium, followed by elimination of the resulting (1S*,2R*)-1-tert-butyldiphenylsilyl-1-phenylhexan-2-ol with KH. The total time for the synthesis, purification and isolation of the alkene is 2 days.     

Impact of aldehyde content on amphotericin B-dextran imine conjugate toxicity

The biocompatibility of oxidized dextran (40 kDa) was investigated in vitro. The contribution of aldehyde groups to the toxicity of polymer-drug conjugates, such as dextran-amphotericin B (AmB) was evaluated. Oxidized dextran was proved to be toxic against the RAW 264.7 cell line with an IC50 of 3 micromol/mL aldehydes. Modification of aldehyde groups and their reaction with ethanolamine reduced the toxicity at least 15-fold. Accordingly, the antifungal and antileishmanial dextran-AmB imine conjugate, which contains unreacted aldehyde groups, was modified with ethanolamine and compared to dextran-AmB amine and imine conjugates. Modification of the imine conjugate with ethanolamine reduced its toxicity toward the RAW cell line by 100%. The effect on Leishmania major parasites was 5 times higher than that of the dextran-AmB amine conjugate. The dextran-AmB-ethanolamine conjugate was at least 15 times less hemolytic than free AmB. Stability and drug release profiles in buffer solution were investigated. The imine conjugates released free AmB while the amine conjugate did not. It is concluded that aldehyde groups may contribute to cell toxicity. This toxicity is reduced by converting the aldehyde groups into imine conjugates with ethanolamine. The results have direct implications toward the safety of AmB-polysaccharide conjugates used against fungal and leishmanial infections.     

Reversible steps in the reaction of aldehydes with bacterial luciferase intermediates.

Aliphatic aldehydes of different chain lengths were found to differ in their reaction at 22 °C with the B. harveyi luciferase peroxyflavin intermediate. Although similar quantum yields were obtained in the luciferase reaction with the different chain-length aldehydes, the catalytic turnover rates differed. The kinetics of a reaction utilizing two aldehydes of different chain lengths can thus indicate the degree to which the aldehyde reaction is reversible. By such criteria the reactions of octanal and decanal were found to be readily reversible, while that of dodecanal was not. This conclusion was supported both by the effects of long-chain alcohols, which are competitive inhibitors, and by the secondary addition of hydroxylamine, an aldehyde trapping agent. The results are consistent with a model in which there are many intermediates along the reaction path. Since the reactions are monitored by decay of luminescence intensity, it is difficult to determine the position of the rate-determining step. For octanal and decanal the rate-limiting step could be at an early reversible stage of the reaction, but later for dodecanal, subsequent to a less reversible step, but still prior to the final irreversible step which populates the excited state.     

Characterization of rat cornea aldehyde dehydrogenase

Aldehyde dehydrogenase has been purified from rat cornea in a single step. The enzyme is a class 3 aldehyde dehydrogenase. Cornea aldehyde dehydrogenase is a 100-kDa dimer composed of 51-kDa subunits, prefers NADP+ as coenzyme, and preferentially oxidizes benzaldehyde-like aromatic aldehydes as well as medium chain length (4-9 carbons) aliphatic aldehydes. The substrate and coenzyme specificity, immunochemical properties, effect of disulfiram, pH profile, and isoelectric point of cornea aldehyde dehydrogenase are identical to those of tumor-associated aldehyde dehydrogenase, the prototype class 3 enzyme. The substrate and coenzyme preferences are consistent with a role for cornea aldehyde dehydrogenase in the oxidation of a variety of aldehydes generated by lipid metabolism, including lipid peroxidation.     

Carnosine is a quencher of 4-hydroxy-nonenal: through what mechanism of reaction?

The aim of this study was to understand the mechanism of action through which carnosine (beta-alanyl-L-histidine) acts as a quencher of cytotoxic alpha,beta-unsaturated aldehydes, using 4-hydroxy-trans-2,3-nonenal (HNE) as a model aldehyde. In phosphate buffer solution (pH 7.4), carnosine was 10 times more active as an HNE quencher than L-histidine and N-acetyl-carnosine while beta-alanine was totally inactive; this indicates that the two constitutive amino acids act synergistically when incorporated as a dipeptide and that the beta-alanyl residue catalyzes the addition reaction of the histidine moiety to HNE. Two reaction products of carnosine were identified, in a pH-dependent equilibrium: (a) the Michael adduct, stabilized as a 5-member cyclic hemi-acetal and (b) an imine macrocyclic derivative. The adduction chemistry of carnosine to HNE thus appears to start with the formation of a reversible alpha,beta-unsaturated imine, followed by ring closure through an intra-molecular Michael addition. The biological role of carnosine as a quencher of alpha,beta-unsaturated aldehydes was verified by detecting carnosine-HNE reaction adducts in oxidized rat skeletal muscle homogenate.     

Solid phase synthesis of peptidyl aldehydes from C-terminal thiazolidinyl peptides

Summary Peptidyl aldehydes are potent transition state analogue inhibitors of cysteine and serine proteinases. The aldehyde function has recently been used for chemoselective peptide ligation. The preparation of peptidyl aldehydes on a solid support requires that the aldehyde be masked during peptide elongation and generated in a final step under mild conditions. We report here the preparation of peptidyl aldehydes by copper salt-mediated neutral hydrolysis of the corresponding C-terminal thiazolidinyl peptides which were elongated on a solid support.     

Asymmetric aldol reaction using boron enolates

The protocol for the preparation of boron enolates and their subsequent reaction with aldehydes is described, providing convenient access to beta-hydroxy ketones in good yields and with high stereoselectivities. The reaction consists of three steps: first, the ketone is rapidly converted to the corresponding boron enolate, by exposure to a chlorodialkylborane and tertiary amine base, which is then reacted in situ with the aldehyde. Finally, oxidative workup of the resultant boron aldolate provides aldol adduct. The reaction procedure requires approximately 28 h to complete over a 2-d period, consisting of 5 h to set up the reaction, whereupon the reaction mixture is left at -20 degrees C overnight (16 h), followed by 7 h for workup and purification.     

Solid phase synthesis of peptidyl aldehydes from C-terminal thiazolidinyl peptides

Peptidyl aldehydes are potent transition stateanalogue inhibitors of cysteine and serineproteinases. The aldehyde function has recently beenused for chemoselective peptide ligation. Thepreparation of peptidyl aldehydes on a solid supportrequires that the aldehyde be masked during peptideelongation and generated in a final step under mildconditions. We report here the preparation of peptidylaldehydes by copper salt-mediated neutral hydrolysisof the corresponding C-terminal thiazolidinyl peptideswhich were elongated on a solid support.     

Copper(I)/TEMPO-catalyzed aerobic oxidation of primary alcohols to aldehydes with ambient air

This protocol describes a practical laboratory-scale method for aerobic oxidation of primary alcohols to aldehydes, using a chemoselective Cu(I)/TEMPO (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyloxyl) catalyst system. The catalyst is prepared in situ from commercially available reagents, and the reactions are performed in a common organic solvent (acetonitrile) with ambient air as the oxidant. Three different reaction conditions and three procedures for the isolation and purification of the aldehyde product are presented. The oxidations of eight different alcohols, described here, include representative examples of each reaction condition and purification method. Reaction times vary from 20 min to 24 h, depending on the alcohol, whereas the purification methods each take about 2 h. The total time necessary for the complete protocol ranges from 3 to 26 h.     

Reductions of 2-enals, dehydrogenation of saturated aldehydes and their racemisation

Enoate reductase or clostridia containing this enzyme (Clostridium tyrobutyricum or C. kluyveri) catalyse the reduction of alpha,beta-unsaturated aldehydes (enals). The enantiomeric purity of the saturated aldehydes obtained from alpha-substituted enals is usually rather low and depends heavily on the reaction conditions. The reduction of the corresponding allyl alcohols to the saturated alcohols leads to much higher enantiomeric purities, though the reduction of the enal corresponding to the allyl alcohol to the saturated aldehyde is an intermediary step in the reaction sequence allyl alcohol----saturated alcohol. The explanation seems to be the racemisation of saturated aldehydes caused by enoate reductase. This is illustrated by the reduction of (E)-2-methylcinnamyl aldehyde to (R)-2-methyl-3-phenylpropanal or (R)-2-methyl-3-phenylpropanol under different conditions and measuring the racemisation of the aldehyde as well as the hydrogen-deuterium exchange of 3-phenylpropanal. In contrast to saturated carboxylates saturated aldehydes can be dehydrogenated to alpha,beta-unsaturated aldehydes (enals) by enoate reductase in the presence of electron acceptors such as oxygen or dichlorophenol indophenol. Under these conditions enoate reductase shows in the presence of oxygen a surprisingly high half life (greater than 20 h) as compared to that which is observed when the enzyme was used as a reductase with NADH in the presence of oxygen. In this case the enzyme is inactivated within a few minutes.     

Conversion of a primary amine to a labeled secondary amine by the addition of phenolic group and radioiodination

To preserve the nucleophilicity of amino compounds during conjugative radioiodination, a new method for converting primary amines to phenolic secondary amines was developed. Amino acids were used as model compounds for establishing optimal conditions for the reductive amination. In the first step of the reaction, the aldehyde group of 4-hydroxybenzaldehyde (formylphenol) was reacted reversibly with an amino group to form an imine. The irreversible attachment of formylphenol to the amino group was accomplished by reduction of the imine with sodium cyanoborohydride. The pH optimum for the reaction was 5.0. Higher temperature has favorable effects on the rate and extent of the conjugation. Phenolic derivatives of amino compounds suitable for radioiodination are produced by the reactions described.     

A dual substrate kinetic model for cytochrome P450<Subscript>BM3</Subscript>-F87G catalysis: simultaneous binding of long chain aldehydes and 4-fluorophenol

Objective

To develop a model for binding and catalysis associated with the stimulation of 4-fluorophenol (4-FP) oxidation in the presence of long chain aldehydes by the enzymatic catalyst, cytochrome P450_BM3-F87G.

Results

A variation of the Michaeli–Menten kinetic model was employed to describe interactions at the active site of the enzyme, along with computer aided modeling approaches. In addition to the hydroquinone product arising from de-fluorination of 4-FP, a second product (p-fluorocatechol) was also observed and, like the hydroquinone, its rate of formation increased in the presence of the aldehyde. When only aldehyde was present with the enzyme, BM3-F87G catalyzed its oxidation to the corresponding carboxylic acid; however, this activity was inhibited when 4-FP was added to the reaction. A 3D computer model of the active site containing both aldehyde and 4-FP was generated, guided by these kinetic observations. Finally, partitioning between the two phenolic products was examined with an emphasis on the conditions directing the initial epoxidation at either the 2,3- or 3,4-positions on the substrate. Temperature, reaction time, substrate concentration, and the structure of the aldehyde had no substantial effect on the overall product ratios, however the NADPH coupling efficiency decreased when unsaturated aldehydes were included, or when the temperature of the reaction was reduced.

Conclusions

The unsaturated aldehyde, trans-2-decenal, stimulates BM3-F87G catalyzed oxidation of 4-fluorophenol through a cooperative active site binding mode that doesn’t influence product distributions or coupling efficiencies, while 4-fluorophenol acts as a competitive inhibitor of aldehyde oxidation.

     

Active site residues and mechanism of UDP-glucose dehydrogenase.

UDP-glucose dehydrogenase catalyzes the NAD+-dependent twofold oxidation of UDP-glucose to give UDP-glucuronic acid. A sequestered aldehyde intermediate is produced in the first oxidation step and a covalently bound thioester is produced in the second oxidation step. This work demonstrates that the Streptococcus pyogenes enzyme incorporates a single solvent-derived oxygen atom during catalysis and probably does not generate an imine intermediate. The reaction of UDP-[6",6"-di-2H]-d-glucose is not accompanied by a primary kinetic isotope effect, indicating that hydride transfer is not rate determining in this reaction. Studies with a mutant of the key active site nucleophile, Cys260Ala, show that it is capable of both reducing the aldehyde intermediate, and oxidizing the hydrated form of the aldehyde intermediate but is incapable of oxidizing UDP-glucose to UDP-glucuronic acid. In the latter case, a ternary Cys260Ala/aldehyde intermediate/NADH complex is presumably formed, but it does not proceed to product as both release and hydration of the bound aldehyde occur slowly. A washout experiment demonstrates that the NADH in this ternary complex is not exchangeable with external NADH, indicating that dissociation only occurs after the addition of a nucleophile to the aldehyde carbonyl. Studies on Thr118Ala show that the value of kcat is reduced 160-fold by this mutation, and that the reaction of UDP-D-[6",6"-di-2H]-glucose is now accompanied by a primary kinetic isotope effect. This indicates that the barriers for the hydride transfer steps have been selectively increased and supports a mechanism in which an ordered water molecule (H-bonded to Thr118) serves as the catalytic base in these steps.     

On Schiff's bases and aldehyde-fuchsin: A review from H. Schiff to R.D. Lillie

Summary In histochemistry aldehyde-fuchsin is widely regarded as an azomethine compound, though this hypothesis cannot explain the variety of reaction products. Infrared spectroscopy did not show a C=N bond. It was therefore deemed of interest to review chemical studies of aldehydefuchsin and other Schiff's bases by Schiff and his contemporaries. Schiff regarded reaction products of low molecular aliphatic aldehydes, e.g. acetaldehyde, and aromatic amines as diarylamines; aldehyde-fuchsin was assigned a 23 (dyealdehyde) formula. These reactions were facilitated by alcohol and HCl. Others suggested condensation of two aldehyde molecules which carried a secondary and a tertiary amine respectively. Eibner proved that these compounds were ethylenes, not azomethines, and contained two secondary amines. Condensation of such bases produced ethylenic polymers, the Schultz's bases. Aromatic aldehydes readily yielded azomethines; aliphatic aldehydes formed –C=N– bonds only during prolonged heating. These findings are in agreement with recent chemical data. Clearly, the term Schiff's bases is not synonymous with azomethines, but denotes any reaction product of aldehydes and amines. In 1962, Lillie's histochemical studies confirmed the secondary amine nature of aldehyde aryl amine condensation. Thus, chemical and histochemical studies from Schiff in the 1860's to Lillie in the 1960's indicate that aldehyde-fuchsin is not an azomethine compound, but contains diarylamines and their derivatives.     

Practical Proline-catalyzed asymmetric Mannich reaction of aldehydes with N-Boc-imines

This protocol describes a procedure for the synthesis of alpha, beta-branched-b-amino aldehydes via Proline-catalyzed asymmetric Mannich reaction of aldehydes with N-tert-butoxycarbonyl-imines. The crystalline beta-amino aldehydes are formed in good yields and extremely high levels of diastereo- and enantioselectivities without the need for chromatographic purification and are readily oxidized to the corresponding beta-amino acids. The protocol can be completed in approximately 14 h on small scales or up to 30 h on larger scales.     

Oxime-based linker libraries as a general approach for the rapid generation and screening of multidentate inhibitors

The described oxime-based library protocol provides detailed procedures for the linkage of aminooxy functionality with aldehyde building blocks that result in the generation of libraries of multidentate inhibitors. Synthesis of inhibitors for protein tyrosine phosphatases (PTPs) and antagonists directed against the human tumor susceptibility gene 101 (TSG101) are shown as examples. Three steps are involved: (i) the design and synthesis of aminooxy platforms; (ii) tethering with aldehydes to form oxime-based linkages with sufficient purity; and (iii) direct in vitro biological evaluation of oxime products without purification. Each coupling reaction is (i) performed in capped microtubes at room temperature (20-23 °C); (ii) diluted for inhibitory evaluation; and (iii) screened with targets in microplates to provide IC(50) or K(d) values. The synthesis of the aminooxy platforms takes 3-5 d; tethering with the aldehydes takes 24 h; and inhibition assay of enzymes and protein-protein interactions takes 30 min and 2 h, respectively.     

Fluorescent-labeled crosslinks in collagen: Pyrenebutyrylhydrazine

The aldehydes present in acid-soluble type I collagen react with pyrenebutyrylhydrazine to form various types of complexes under different reaction conditions. These complexes exhibit one or more of three different pyrene fluorescence bands: monomer, excimer, and aggregate fluorescence. Collagen, whose aldehydes have been reduced with NaBH₄, does not react with this fluorescent hydrazine, confirming that the hydrazine reacts specifically with aldehyde groups to form hydrazones. The absence of a reaction with pepsin-treated collagen also shows that the fluorescent labels are primarily in the nonhelical terminal telopeptides. Upon dialysis, the pyrene label bound to a saturated aldehyde in an α-chain is lost; whereas that bound to an unsaturated aldehyde remains on the protein. The pyrene monomer fluorescence in the β-chain of old collagen is stronger than that of young collagen. The formation of the pyrene excimer fluorescence implies the proximity of two pyrene molecules, probably attached to two adjacent aldehydes. Upon changing from acidic to neutral pH, both excimer and aggregate fluorescence bands disappear within a few seconds, revealing a very rapid alteration at the telopeptides.     

Synthesis of peptide macrocycles using unprotected amino aldehydes

This protocol describes a method for synthesizing peptide macrocycles from linear peptide precursors, isocyanides and aziridine aldehydes. The effects of the reaction components on the efficiency of the process are discussed. Macrocyclization is exemplified by the preparation of a nine-membered ring peptide macrocycle. The product is further functionalized by nucleophilic opening of the aziridine ring with a fluorescent thiol. This transformation constitutes a useful late-stage functionalization of a macrocyclic peptide molecule. The experimental section describes the selection of the required starting materials, and the preparation of a representative aziridine-2-carboxaldehyde dimer. The synthesis and isolation of the peptide macrocycle can be accomplished in 6 h, and the ring-opening requires approximately 6-8 h. The aziridine-2-carboxaldehyde reagent is commercially available or can be synthesized from readily available starting materials in approximately 4 d. The strategy described is not limited to the specific peptide, isocyanide, aziridine aldehyde or nucleophile used in the representative synthesis.     

Cytotoxicity, sister-chromatid exchanges and DNA single-strand breaks induced by 4-oxo-4-(3-pyridyl)butanal, a metabolite of a tobacco-specific N-nitrosamine

The tobacco-specific N-nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is metabolized by alpha-carbon hydroxylation to reactive diazohydroxides and aldehydes. The aim of this study was to determine the relative ability of one NNK-derived aldehyde, 4-oxo-4-(3-pyridyl)butanal, to induce cytotoxicity, sister-chromatid exchanges (SCEs) and DNA single-strand breaks (SSBs) in V79 cells. Our data demonstrate that this aldehyde is cytotoxic for V79 cells (IC50 = 0.4 mM) and induces SCEs at concentrations ranging from 0.01 to 0.5 mM. DNA SSBs were observed at concentrations ranging from 0.05 to 1 mM and were repaired within 8 h. When V79 cells were cultured with primary hepatocytes, there was a reduction in the frequency of SCEs induced by the aldehyde. This suggests that hepatocytes can partially deactivate the aldehyde. Our results suggest that this aldehyde is one of the reactive intermediates generated during NNK metabolism.