Schiff bases cleavage

A cost-efficient synthesis of foHc acid via Schiff base formation is feasible only if 6-formylpterin (23) is readily available. This compound is prepared by the reaction of 2-bromomalondialdehyde dimethylacetal [59453-00-8] (25) with trianainopyrimidinone (10), followed by acetylation and cleavage of the acetal to give compound (23) in 51% overall yield (38). 

The electronic effects (8+ on carbon and S on nitrogen) that favor the hydration of heteroaromatic molecules and of Schiff bases to give Dimroth bases are the same as those that would favor the ringopening of the hydrated heteroaromatic molecules and cleavage of the C—bond in Dimroth compounds. 

Heating the mesoionic l-amino-2-thioxo-l,2,4-triazolo[l,5-c]quinazo-lines 59 with aromatic aldehydes and ethanolic hydrochloric acid resulted in the formation of Schiff bases and simultaneous pyrimidine ring cleavage... 

The reaction of 2,2 -dithiodibenzaldehyde with Ni11 complexes containing coordinated primary amines results in Schiff base condensation and cleavage of the disulfide bond to form a chelating... 

An unexpected cleavage has also been described for the coordinated Schiff base of 2-aminobenzenethiol in alkaline solution. Instead of neutral complexes of the Schiff bases, anionic 1 2 Tc(V) oxo complexes are formed the donor groups are now the thiol and generated amino groups [206]. 

The NMR spectra have shown the formation of Schiff base as an intermediate product in the synthesis of the fully N-deacetylated oligomers from chitosan.32 The mechanism of the Schiff base reaction leading to chain cleavage and formation of 5-hydroxymethyl-2-furfural has been proposed. 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

Certain Schiff bases, i.e. 122, were synthesized as model compounds for Latia luciferin. This compound exhibits strong blue chemiluminescence ( max 385 nm) on oxidation with oxygen in DMSO/potassium t.-butylate, the main products being acetone and 2-formamido pyridine 124. The mechanism suggested by Me Capra and Wrigglesworth includes the concerted bond cleavage of a dioxetane derivative 123. 

Potassium enolates derived from the chiral Schiff bases obtained by reaction of racemic a-amino esters with 2-hydroxypinan-3-one undergo diastereoselective protonation, as evidenced by release of optically active a-amino esters on subsequent cleavage of the imine (Scheme 5). ... 

In the catalysis of the lyase from C. perfringens, the participation of lysine residues forming intennediary Schiff bases between enzyme and substrate molecules, and of histidine residues, has been demonstrated with the aid of photooxidation, reagents for histidine modification, and borohydride reduction in the presence of substrate.408-418 Thus, according to Frazi and coworkers,414 the lyase belongs to the class I lyases (aldolases). The catalytic mechanism proposed is outlined in Scheme 3. Evidence has been educed for the existence of a similar mechanism of cleavage of sialic acid by the lyase enriched from pig kidney.411... 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

The hydrolysis of certain Schiff bases is catalyzed by divalent metal ions such as cupric or nickelous ion. Spectrophotometric evidence indicates the formation of a metal ion complex of the substrate with subsequent facile cleavage of the complex. 

It is postulated that the facile cleavage of the complex is due to the polarization of the carbon-nitrogen double bond. At pH 7, where these investigations were carried out, the spontaneous decomposition of the Schiff base is very slow, while the metal ion-catalyzed reaction has a half life of a few minutes. Since the hydrolysis of Schiff bases is catalyzed by hydrogen ion, the metal ion catalyst can be postulated to be a superacid catalyst present in neutral solution (17, 18). 

The peculiar metal ion specificity of the ATP cleavage reaction may perhaps be explained by reference to some studies on the metal complexes of Schiff bases, which have provided clues to many aspects of biological metal catalysis. It was shown that metal ions will split the carbon-nitrogen double bond in thiophenalde-hyde-ethylenediamine (18, 21) as a consequence of the electronic-drift-to-metal... 

The enzyme is inactivated by borohydride in the presence of substrate, and acid hydrolysis of the inactivated enzyme yielded e-N-isopropyllysine. Decarboxylation occurs from a Schiff base by a mechanism analogous to that of the aldol cleavage shown in Eq. 13-36.236 Mechanistically related is 4-oxalocrotonate decarboxylase.2363... 

Below the structures of the adducts in Eq. 14-20 are those of a 2-oxo acid and a (3-ketol with arrows indicating the electron flow in decarboxylation and in the aldol cleavage. The similarities to the thiamin-dependent cleavage reaction are especially striking if one remembers that in some aldolases and decarboxylases the substrate carbonyl group is first converted to an N-proto-nated Schiff base before the bond cleavage. 

Side chain cleavage (Group c). In a third type of reaction the side chain of the Schiff base of Fig. 14-5 undergoes aldol cleavage. Conversely, a side chain can be added by (3 condensation. The best known enzyme of this group is serine hydroxymethyltransferase, which converts serine to glycine and formaldehyde.211-21313 The latter is not released in a free form but is transferred by the same enzyme specifically to tetrahydrofolic acid (Eq. 14-30), with which it forms a cyclic adduct. 

A (3 replacement reaction catalyzed by the PLP-dependent tryptophan synthase converts indoleglycerol phosphate and serine to tryptophan. Tryptophan synthase from E. coli consists of two subunits associated as an a2P2 tetramer (Fig. 25-3). The a subunit catalyzes the cleavage (essentially a reverse aldol) of indoleglycerol phosphate to glyceraldehyde 3-phosphate and free indole (Fig. 25-2, step s).67 The P subunit contains PLP. It presumably generates, from serine, the Schiff base of aminoacrylate, as indicated in Fig. 25-2 (step f). The enzyme catalyzes the addition of the free indole to the Schiff base to form tryptophan. The indole must diffuse for a distance of 2.5 ran... 

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