Showing papers on "Schiff base published in 1994"
TL;DR: Chiral (salen)Mn(III) complexes have been found to be highly selective catalysts for the asymmetric epoxdation of several cyclic and acyclic trisubstituted olefins.
Abstract: Chiral (salen)Mn(III) complexes have been found to be highly selective catalysts for the asymmetric epoxdation of several cyclic and acyclic trisubstituted olefins. These results are interpreted with a transition state model for epoxidation involving a skewed, side-on approach of olefin to a (salen)Mn(oxo) intermediate
225 citations
TL;DR: In this article, the use of Schiff base substrates in amino acid synthesis is briefly reviewed, and a new active catalyst species is proposed for asymmetric PTC alkylation of active methylene compounds.
197 citations
TL;DR: In this paper, the authors determined the direction of the loss or gain of the Schiff base protin in unphotolyzed and in photoexcited D85N, and the D85n/D96N and D85 n/d96A double mutants, in order to understand the intrinsic and the induced connectivities of the retinal Schiff base to the two membrane surfaces.
125 citations
TL;DR: In this article, it was suggested that the isolated compounds can be formulated as metal(IV) derivatives of the dinegative radical ligand Cat-N-SQ, M(Cat-N -SQ) 2, supported by the structural characterization of the titanium and vanadium derivatives.
Abstract: Reactions between 3,5-di-tert-butylcatechol, aqueous ammonia and titanium, vanadium, germanium, and tin salts yield neutral bis- complexes of the resulting tridentate Schiff base biquinone ligand. On the basis of their magnetic, electrochemical, and spectral properties all the isolated compounds can be formulated as metal(IV) derivatives of the dinegative radical ligand Cat-N-SQ, M(Cat-N-SQ) 2 . This suggestion is supported by the structural characterization of the titanium and vanadium derivatives
123 citations
TL;DR: Asymmetric oxidation of sulfides was examined by using (salen)manganese(III) complexes as catalysts and (8S,8′S,1″S,2″S)-complex.
91 citations
TL;DR: The enantioselective addition of trimethylsilyl cyanide to a variety of aldehydes by the aid of a catalyst prepared in situ from titanium tetraisopropoxide [Ti(O- i -Pr) 4 ] and chiral Schiff bases gave the corresponding cyanohydrins in high optical yield (up to 96% e.g. as discussed by the authors ).
89 citations
TL;DR: There are two anomalous results, the inconsistency of the observed hydrogen exchange rates of retinal Schiff base in the two pigments with those predicted from the standard exchange schemes and the enhancement of the rate of hydrogen exchange in the the two proteins over the model Schiffbase in aqueous solution.
83 citations
79 citations
TL;DR: In this article, the reduction and oxidative chemistry of three Ni(L) complexes of formula [Ni(L), where L represents a N2O2 Schiff base pseudomacrocyclic ligand based on salicylaldehyde derivatives and three different diamines, was studied in (CH3)2SO.
79 citations
TL;DR: In this paper, the synthesis of a trialdehyde and its Schiff bases as a new template for molecular imprinting of solid surfaces was reported. But the results were limited to a single step.
Abstract: A recent approach in the area of molecular recognition involves template-assisted imprinting' of the surface of rigid polymers such as silica gel. Siloxy-Schiff bases of structurally rigid dialdehydes have been used as templates to introduce two appropriately spaced amino groups for substrate binding on the solid surface. The distance between the two amino groups is defined by the structure of the dialdehydes. Moderate selectivity has been reported in rebinding of the dialdehydes to the modified surface.2 Current objectives in this quest involve the development of multidentate template molecules to expand the repertoire of functional groups available a t the binding sites3 We report herein the synthesis of a trialdehyde and its Schiff bases as a new template for molecular imprinting of solid surfaces. The reaction of cyanuric chloride with 3 equiv of p hydroxybenzaldehyde in benzene gave the desired trialdehyde, TRIPOD (1) (Scheme 11, in a single step. Trialdehyde 1 was then reacted at room temperature with (aminopropy1)triethoxysilane (APTES) or p-(methoxydimethylsily1)aniline (MODSA)4 to afford the corresponding silane-TRIPODS, APTES TRIPOD (2), and MODSA TRIPOD 4 (Scheme 2), respectively. Model compounds PROPYL TRIPOD (3) and ANILINE TRIPOD (5) were also synthesized for future mechanistic studies of Schiff base formation. Acid hydrolysis of all the Schiff bases regenerated the intact TRIPOD. Molecular imprinting was carried out with silaneTRIPOD molecules 2 and 4 on silica gel to yield modified silicas 7 and 8, respectively. Porous silica gel, Fractosil 500, was refluxed in dry toluene for 2 days28 with an appropriate amount of the silane-TRIPODS to get a low substitution level on the silica and ensure site isolation (Scheme 3). Initial experiments with APTES TRIPOD (2) indicated that a partial degradation of 2 took place during the imprinting reaction. Refluxing of the model compound 3 in toluene for 2 days produced p-hydroxybenzaldehyde, suggesting that the degradation was caused by the partial hydrolysis of the Schiff base and subsequent attack on the ether linkage by the amine. On the other hand, MODSA TRIPOD (4) did not decompose, presumably due to the greater strength of the aromatic Schiff
77 citations
TL;DR: Observations constitute direct evidence for the interaction of water with Asp85 at the time when it is protonated by the Schiff base.
Abstract: Fourier-transform infrared spectra were recorded at 170 K before and after irradiating the Asp85-->Asn mutant of bacteriorhodopsin. The difference spectrum exhibits protein bands such as those due to the perturbations of Asp96 and Asp115 and the N-H stretching vibration of tryptophan, characteristic of the L minus all-trans-bacteriorhodopsin spectrum of the wild-type protein. However, some vibrational bands of the peptide backbone and the chromophore are different from L and more characteristic of N of the wild-type protein. Remarkably, the shift observed for the vibrational band due to an internal water molecule upon L formation [Maeda, Sasaki, Shichida, and Yoshizawa (1992) Biochemistry 31, 462-467] is absent. These changes in the spectrum of the mutant could originate from the destruction of a hydrogen-bonding system consisting of Asp85, the water molecule, and the Schiff base, upon replacement of Asp85 with asparagine. These observations constitute direct evidence for the interaction of water with Asp85 at the time when it is protonated by the Schiff base.
TL;DR: A chiral macrocyclic binucleating ligand, S,S-cypim, bearing four and six-coordinate binding sites has been prepared in this paper, where the chiral diamine (1S,2S)-trans-1,2-bis(aminomethyl)cyclopentane induces one topological chirality about the six coordinate site.
Abstract: A chiral macrocyclic binucleating ligand, S,S-cypim, bearing four- and six-coordinate binding sites has been prepared. The macrocyclic framework contains the chiral diamine (1S,2S)-trans-1,2-bis(aminomethyl)cyclopentane, which induces one topological chirality about the six-coordinate site. The macrocyclic closure with the chiral diamine is achieved in good yield by Schiff base condensation of metal dialdehyde complexes. In this way the monometallic complexes [M(S,S-cypim)(H + ) 2 ] 2+ , M=Zn(II), Cu(II), Ni(II), Co(II), and Mn(II), have been prepared and characterized
TL;DR: In this paper, it was shown that cyclic voltammograms of VO(salen) in the presence of trifluoromethanesulfonic acid (CF 3 SO 3 H) or triphenylmethyl tetrafluoroborate (Ph 3 C(BF 4 )) exhibit reversible potentials near 0.5 and 0.8 V (vs Ag/AgCl)
Abstract: Electrochemical confirmation that (N,N'-ethylenebis(salicylideneaminato))oxovanadium(IV), VO(salen), reacts with trifluoromethanesulfonic acid (CF 3 SO 3 H) or triphenylmethyl tetrafluoroborate (Ph 3 C(BF 4 )) to form a deoxygenated complex, V IV (salen) 2+ , and a μ-oxodinuclear complex, [(salen)VOV(salen)]X 2 , (X=CF 3 SO 3 - or BF 4 - ) is presented. Cyclic voltammograms of VO(salen) in the presence of CF 3 SO 3 H or Ph 3 C(BF 4 ) exhibit reversiblewaves with formal potentials near 0.5 and 0.8 V (vs Ag/AgCl)
TL;DR: In this article, the synthesis of Schiff base p-tert-butylcalix[4]arenes 3a-e was described, in which the Schiff base unit bridges two opposite hydroxy groups.
Abstract: The article describes the synthesis of Schiff base p-tert-butylcalix[4]arenes 3a-e in which the Schiff base unit bridges two opposite hydroxy groups of p-tert-butylcalix[4]arene (1). The synthesis of 3a-e have been achieved by refluxing p-tert-butylcalix[4] arene 1,3-dicarbaldehyde 2 with appropriate diamino compounds in acetonitrile-methanol. Yields range from 14 to 96%. The complexing properties of 3a-e toward alkali and alkaline earth cations, transition and heavy metals, and lanthanides are reported
TL;DR: In this paper, an optically active 2-pyridinylmethylamino alcohol with dendrimeric structure was synthesized from 2-Pyridinecarboxaldehyde.
Abstract: Enantioselective Catalysis, 90[1]. - Optically Active Nitrogen Ligands with Dendrimeric Structure
2-Pyridinecarboxaldehyde (1) reacts with (1S,2S)-2-amino-1-phenyl-1,3-propanediol (2) to afford a mixture of the Schiff base 3 and the oxazolidines 3a/3a′, 3b/3b′, which was reduced with NaBH4 to yield the optically active 2-pyridinylmethylamino alcohol 4. Similarly, 8 was synthesized from 2,6-pyridinedicarboxaldehyde (7). By acylation of the hydroxy and amino groups, compound 4 was expanded to the corresponding ester amides 5 and 6. Boc and cbz protection of the amino group of 2 produced 10 and 19, respectively. The hydroxy groups of 10 were esterified with 4-(chloromethyl)-benzoyl chloride (11) and 3,5-bis(chloromethyl)benzoyl chloride (12) to give 13 and 14. Compound 19 was converted to the diester 21 by treatment with 3,5-dimethylbenzoyl chloride (20). Substitution of the chloro substituent in 13 and 14 by (1R,2S)-ephedrine and (1S,2S)-2-(benzylamino)-1-phenyl-1,3-propanediol, respectively, lead to the tertiary amines 15a-17a. After removal of the N-protection, the primary amino groups of 17b-22 were treated with the aldehydes 1, 7, 27, and 35 to give the corresponding aldimine chelate ligands 23-26, 28-34, and 36. Starting with L-N-boc-aspartic acid (37) the tripeptide 39 was formed with two equivalents of L-aspartic acid dimethyl ester hydrochloride (38). After removal of the boc group followed by condensation with salicylaldehyde, imine 40 was generated. (S)-2-amino-1,1,4,4-tetraphenyl-1,4-butanediol (41), derived from L-aspartic acid, was treated with the aldehydes 27 and 35. The resulting products 42 and 43 in solution formed mixtures of the diastereomeric oxazolidines 42a and 43a as well as the oxazinanes 42b and 43b. The ligands have been tested in the CuI-catalyzed cyclopropanation of styrene with ethyl diazoacetate.
TL;DR: It is concluded that the geometry of the proton donor and acceptor in the Schiff base-D85 pair, mediated by bound water, is a determinant of theProton transfer equilibrium.
Abstract: Bacteriorhodopsin contains all-trans-retinal linked via a protonated Schiff base to K216. The proton transport in this pump is initiated by all-trans to 13-cis photoisomerization of the retinal and the ensuing transfer of the Schiff base proton to D85. Changed geometrical relationship of the Schiff base and D85 after the photoisomerization is a possible reason for the proton transfer. We introduced small volume/shape changes with site-specific mutagenesis of residues V49 and A53 that contact the side chain of K216, in order to force the Schiff base into somewhat different positions relative to D85. Earlier [Zimanyi, L., Varo, G., Chang, M., Ni, B., Needleman, R., & Lanyi, J. K. (1992) Biochemistry 31, 8535-8543] we had described the kinetics of absorbance changes in the microsecond to millisecond time range after photoexcitation with the scheme L M1 M2 + H+ (where the first equilibrium is the internal proton transfer and the second is proton release on the extracellular surface). Testing it at various pH values with mutants, where selected rate constants are changed, now confirms the validity of this scheme. The kinetics of the M state thus allowed examination of the transient equilibrium that develops in the L M1 reaction and represents the redistribution of the proton between the Schiff base and D85. From the structure of the protein, the V49A and V49M residue replacements were both predicted to cause decreased alignment of the Schiff base and D85, and indeed we found that they both changed the equilibrium toward the protonated Schiff base. In contrast, the residue replacements A53V and A53G were predicted to move the Schiff base in opposite directions, away from and closer to alignment with D85, respectively. The former indeed changed the equilibrium toward the protonated Schiff base and the latter toward the deprotonated Schiff base. In addition, the hydroxyl stretch band of a bound water in the L state was affected by all mutations that disfavor proton transfer to D85. We conclude that the geometry of the proton donor and acceptor in the Schiff base-D85 pair, mediated by bound water, is a determinant of the proton transfer equilibrium.
TL;DR: The intrinsic fluorescence of the pyridoxal 5'-phosphate (PLP) enzyme O-acetylserine sulfhydrylase-A was studied in order to gain insight into the structural basis for binding of substrates and products and for catalysis.
Abstract: The intrinsic fluorescence of the pyridoxal 5'-phosphate (PLP) enzyme O-acetylserine sulfhydrylase-A (OASS-A) was studied in order to gain insight into the structural basis for binding of substrates and products and for catalysis. Excitation of OASS-A with 298-nm light gives an emission spectrum with two maxima, 337 and 498 nm. OASS-A has two tryptophan residues, and the 337-nm maximum indicates that at least one of these is exposed somewhat to aqueous solvent. The 498-nm emission observed is due to fluorescence of the PLP Schiff base. Some of this long-wavelength fluorescence is likely due to direct excitation by incident radiation. However, the concomitant quenching of 340-nm emission and the enhancement of 498-nm emission observed upon reconstitution of apoenzyme with PLP support the conclusion that some of the long-wavelength emission is due to singlet-singlet transfer from at least one tryptophan residue to the PLP Schiff base. Enhancement of 498-nm fluorescence by either of the products, acetate or cysteine, of the enzymatic reaction without a quenching of 337-nm fluorescence is consistent with triplet-singlet transfer from one or both of the tryptophan residues to the PLP Schiff base. This would require a rigid environment for the tryptophan donor when the product is bound. However, a conformational change which affected principally the environment of the PLP Schiff base, resulting in a longer lifetime of its excited singlet state, would also increase the intensity of the 498-nm emission. Enhancement of OASS-A long-wavelength fluorescence by each product requires the unprotonated form of a different group on enzyme. Enhancement by acetate binding requires the unprotonated form of an enzyme group with a pK of 7 and is insensitive to substitution on the methyl group. L-Cysteine binding enhances 498-nm fluorescence when a group with a pK of 8 is unprotonated, and substitution at the thiol or the methylene bridge does not affect the enhancement elicited. Binding of L-cysteine to free enzyme (E) likely results in the formation of the external Schiff base accompanied by a conformational change giving fluorescence enhancement. The carboxylate moiety of acetate likely binds to the alpha-carboxylate subsite for amino acid reactants such as L-cysteine, resulting in a conformational change in the internal Schiff base and giving rise to the observed fluorescence enhancement. Data are interpreted in terms of the mechanism of OASS-A.
TL;DR: The 474 nm photoproduct of the mutant pigment, which contained a protonated Schiff base, activated transducin and was maximal at the pH optimum of trans Ducin activation by the mutant R* and was abolished in the presence of GTP gamma S.
Abstract: Rhodopsin, a G protein-coupled seven-transmembrane helix receptor, contains an 11-cis-retinal chromophore covalently linked to opsin apoprotein by a protonated Schiff base. Photoisomerization of the chromophore followed by Schiff base deprotonation forms metarhodopsin II (MII, lambda max = 380 nm), the active state (R*) that catalyzes guanine nucleotide exchange in transducin, the G protein of the photoreceptor cell. Schiff base deprotonation is required for R* formation. The Schiff base positive charge in rhodopsin is stabilized by a carboxylic acid counterion, Glu113. The position of the carboxylate counterion was moved by one helix turn to position 117 by site-specific mutagenesis. Photolysis of the mutant pigment E113A/A117E (lambda max = 491 nm) resulted in a mixture of two photoproducts: (1) an MII-like form with an unprotonated Schiff base (lambda max = 382 nm) favored at alkaline pH; and (2) a photoproduct with a protonated Schiff base (lambda max = 474 nm), spectroscopically similar to metarhodopsin I, favored at acidic pH. Here, we have studied the interactions between the mutant E113A/A117E photoproducts and transducin in detail. Transducin slowed down thermal conversion of the 474 nm form to the 382 nm form by stabilizing the 474 nm photoproduct. This effect was maximal at the pH optimum of transducin activation by the mutant R* and was abolished in the presence of GTP gamma S. In addition, the amount of the 474 nm species correlated with transducin activation rates during the thermal conversion of the photoproduct mixture. Thus, the 474 nm photoproduct of the mutant pigment, which contained a protonated Schiff base, activated transducin.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this paper, the preparation, properties, and applications of threep-amino-substituted Schiff base chromophores to derivatize primary NH2 groups for the exciton chirality method are described.
Abstract: The C D exciton chirality method is a versatile and unambiguous method for the microscale determination of absolute configurations and conformations of organic molecules in solution. I t is particularly powerful when two different chromophores are involved since the coupled C D covers a wide spectral range and becomes a fingerprint for that compound. This paper describes the preparation, properties, and applications of threep-amino-substituted Schiff base chromophores to derivatize primary NH2 groups for the exciton chirality method; the Schiff bases are formed in high yield under mild conditions and yield chromophores which also couple with 0-acyl chromophores. Moreover, protonation of these Schiff bases yields cyanine-type chromophores exhibiting drastically red-shifted (ca. 100 nm) and 2-3-fold intensified C D couplets. The aldehydes p-(dimethylamino)benzaldehyde, julolidinecarbaldehyde, and p-(dimethy1amino)cinnamaldehyde form Schiff bases with aminocyclohexane to yield Chromophores-I, -11, and -111, respectively. These chromophores have the following A, , (e values) for the neutral and protonated species, respectively: Chrom-I, in MeCN 305 nm (24 300), in MeCN/TFA 395 nm (51 700); Chrom-11, in MeCN 331 nm (21 400), in MeCN/TFA 240 nm (48 300); Chrom-111, in MeOH 361 nm (37 000), in MeOH/TFA 460 nm (64 500). NH2 groups are converted into Schiff bases without protection of OH groups; in cases where N,O coupling was required, the OH was converted into the (4-methoxyphenyl)-2,4-pentadienoate (Chrom-IV), in MeOH 333 nm (40 000). All neutral Schiff bases and protonated Schiff bases of (1 R,2R)-tran~-cyclohexanediamine gave exciton-split C D curves with intense amplitudes. However, in the case of protonated Chrom-111, inversion of the CD sign occurred in 45 min a t room temperature in methanol and hence the CD should be measured soon after protonation; N M R and MM2 calculations show that the imminium bond in one of the protonated Schiff base chromophore undergoes an E 2 isomerization. Derivatives with Chrom-I and -11 do not isomerize, and amplitudes of the exciton couplet remain unchanged. Furthermore, the original substrate may be recovered quantitatively when derivatized with Chrom-I. Intense exciton coupling is still observed between chromophores with absorption maxima as far apart as 134 nm.
TL;DR: In this article, the macrocyclic tetra Schiff base ligand was derived from condensation of two molecules of 2,6-diformyl-4-methylphenol.
Abstract: Reduction with NaBH 4 of the macrocyclic tetra Schiff base ligand formed by condensation of two molecules of 2,6-diformyl-4-methylphenol with two molecules of 2,6-bis(aminomethyl)-4-methylphenol affords the corresponding tetraamino, tetraphenolic macrocyclic ligand L r H 4 . Determination of the structure of the hydrochloride salt with the remarkable solvation composition L r H 4 .4HCl.H 2 O.CH 3 OH.C 2 H 5 OH, reveals a dome-shaped L r H 8 4+ cation with a water molecule and two of the chloride ions H-bonded inside the domed macrocyclic cavity
TL;DR: In this article, disalicylidene-1,2-phenylenediamine (H{sub 2}dsp) complexes with the formula M[Ln(dsp)-sub 2] (M = Li, Na, K, Cs) were prepared.
Abstract: Novel rare earth (III) disalicylidene-1,2-phenylenediamine(H{sub 2}dsp) complexes with the formula M[Ln(dsp){sub 2}] (M = Li, Na, K, Cs) were prepared. UV-visible absorption and NMR were used to study the disproportionation equilibrium for these complexes in DMSO. Structures for these complexes are suggested.
TL;DR: Two vanadium complexes of the Schiff base ligand, o-N-salicylidene amino-methylphenol (abbreviated as Salamp), have been synthesized and characterized as mentioned in this paper.
TL;DR: Nickel complexes with Schiff bases 2-(2-pyridylmethyleneamino) phenol (PMAP) and 2-quinolylmethylene amino (QMAP) were prepared and the composition of the crystalline complexes was determined by elemental analysis, solubility, UV-Vis, IR and mass spectrometry as mentioned in this paper.
TL;DR: Optically pure D-(+)-N-methylfucosamine has been synthesized from a TBDMS-protected methyl L-serinate benzophenone Schiff base (O'Donnell's Schiffbase, 1) in seven steps in 13% overall yield.
Abstract: Optically pure D-(+)-N-methylfucosamine (8) has been synthesized from a TBDMS-protected methyl L-serinate benzophenone Schiff base (O'Donnell's Schiffbase, 1) in seven steps in 13% overall yield. Efficient construction of the requisite amino triol acetate 3a with the proper stereochemical configuration is accomplished in two steps with a chelation-controlled reduction-alkylation reaction using |Bu 2 AlH.A-iBu 3 /E-LiCH=CHCH 3 , followed by oxidation with OsO 4 . Conversion of the Schiff base to the N-benzhydryl protecting group and methylation (Eschweiler-Clark) is accomplished in one pot with NaCNBH 3 in the presence of H 2 C=0. Deprotection and oxydation of the primary alcohol, followed by deacetylation with KCN, provides the desired product 8
TL;DR: In this paper, some semisynthetic penicillins 6-substituted with benzo-condensed heterocyclic derivatives were prepared using the reaction of carbon suboxide with Schiff bases and disubstitution of benzoic acids.