Schiff base fragments

Table 6. Polymorphism (SB and SA phases) of LC polymers with Schiff base fragments in side chains 6I5 7S)...

Affinity microparticles (AMPs) were obtained by cross-linking the S-layer lattice on S-layer-carrying cell wall fragments with glutaraldehyde, reducing Schiff bases with sodium borohydride, and immobilizing protein A as an IgG-specific ligand [92]. Thus, AMPs rep-... 

Attention should be paid to the appearance of spurious peaks in the fragmentation patterns of amines determined by GC-MS, when the analytes came into contact with methanol or ethanol as solvents. Thus, for example, Schiff bases may be formed on condensation of a primary amine with traces of formaldehyde or acetaldehyde present in the solvent. Although the peaks of such product may be unresolved in the chromatogram, they may appear as ions with mass increments of +12 or +26 in the mass spectrogram, complicating the identification of the analyte, as was the case with some amphetamine drugs99. 

Binding of pyridoxal phosphate to peptide PP-42 also appears to be selective for lysine 30. As was indicated by NMR spectroscopy and UV/vis experiments, only one of three potential lysine Schiff bases appeared to form. To determine the site or sites of attachment, the aldimine peptide intermediates were reduced, proteolytically cleaved, and the fragments analyzed by mass spectroscopy. This... 

Structural motifs related to Schiff-base ligands have been explored. Three tridentate ligands have been coordinated to the VO (quin) fragment (quin = 8-quinolinato). The octahedral azobenzene complex VO (quin) (2-hydroxy-2 -carboxy-5-methylazobenzene) shown in Fig.22 has a reversible reduction (by CV and controlled potential electrolysis) at ca —0.31 V versus Cp2Fe/CH2Cl2. This potential is 0.58 V more positive than the potential found in the analogous Schiff-base VO (quin) (V-(l-hydroxyethyl)salicylaldimine) complex. The latter complex reduction is also... 

Monocyclic pyrylium salts react with azomethines according to a postulated four-centered mechanism yielding products of exchange between the oxonium atom and the amine fragment of the Schiff base (73ZOR1079 82KGS465). 

Reaction of pyruvic acid with H2NNHC(S)NHNH2 gave the cyclic Schiff bases possessing the 1,2,4,5-tetrazepine fragment (Equation 13) <1996MI67>. 

Schiff-base complexes fragment according to the scheme below 25, 196a) labeling experiments were used to determine which phenyl group was lost. The localization of charge on the iron atoms explains loss of benzene by fission of an N—C bond, which is contrary to the usual /8-fission of amines. 

Hodge7 has advanced several possible routes for the conversion of the enol form of the 1-amino-l-deoxy-2-ketose into melanoidin, and the evidence to support these mechanisms is considerable. Thus, the enol may be converted into the Schiff base of a furaldehyde, or to a reductone by loss of water. It may also be broken down into smaller fragments (for example, hydroxy-2-propanone or pyruvaldehyde), which react further with amino compounds. The enol may also react with an a-amino acid and be converted to an aldehyde by a Strecker degradation. The compounds thus formed from... 

As an example, Schiff base 1, with its bidentate chelation separated by a three-carbon fragment (the correct distance for formation of a six-membered metalla-cycle) from the C-H bond, can undergo palladium-catalyzed C-H bond arylation... 

Satterthwait also has demonstrated that methyl metaphosphate, generated in solution by the Conant-Swan fragmentation, will react with acetophenone in the presence of base to yield the corresponding enol phosphate. When, however, the reaction is carried out in the presence of aniline, the product is the Schiff base. Presumably, the processes take place by the pathways shown by the reactions in Equations 10 and 11. 

Neutral or cationic Grubbs-type complexes bearing an alkylidene fragment and either phosphine, N-heterocyclic carbene (NHC), or Schiff base ligands. 

A novel preparation of the 1,4-diazepine system (174) has been reported through isomerization of the Schiff base (175).186,187 This same type of diazepine (176) was prepared by the reaction of acetylacetone and meso-l,2-diphenylethylenediamine.188 Spectroscopic studies indicated that it was in the form shown. The NMR spectra of these 2,3-dihydro-l,4-diazepines and their cations have been discussed.187 The Schiff base of benzaldehyde and 1,2,3-triaminocyclopropane also isomerizes to give 174 (R = C6H5, R = C6H6CH=N).186 187,189 The mass spectrometric fragmentation of compounds of the type 176 has been reported.180a... 

A different type of trinuclear unit is formed when a cyanometalate fragment is reacted with an octahedral Mn(III) complex in which the equatorial sites are blocked by a tetradentate Schiff base. Despite the presence of two axial labile hgands, a situation that is conductive to the formation of an infinite chain structure, it was shown that this combination of building blocks often leads to trinuclear complexes (Table III) in which two peripheral Mn(III) ions are linked via the central cyanometalate unit. Consequently, in these structures only one axial site at each Mn (III) center is occupied by a bridging CN ligand, while the other axial site remains occupied by a solvent molecule. 

A number of Schiff base complexes containing the dioxo-Mo(VI) fragment have been prepared by various workers (136-141), and two have been structurally characterized (141). We pass briefly over these complexes because their relevance to enzymes has been diminished by recent evidence for facile formation of dinuclear Mo(V) complexes upon reduction (141). 

Figure 20.22. Transaldolase Mechanism. The reaction begins with the formation of a Schifif base between a lysine residue in transaldolase and the ketose substrate. Protonation of the Schiff base leads to release of the aldose product, leaving a three-carbon fragment attached to the lysine residue. This intermediate adds to the aldose substrate to form a new carbon-carbon bond. The reaction cycle is completed by release of the ketose product from the lysine side chain.

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