Schiff-base anion

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]. 

Methyl cyanide complexes of composition U02X2-yMeCN (X = Cl, y = 1 X = N03, y = 2 X = C104, y = 3), (CioH21)4N Np02Cl3-MeCN and U02SB xMeCN, where SB represents a Schiff base anion, have been recorded. In the IR spectra of the last, the frequency of v(CN) is... 

The kinetics of hydrolysis of N-salicylidene-2-aminothiazole (119 = HL) have been studied in aqueous 5% methanol in the presence and absence of Co11, Ni, Cu11 and Zn11.408 The solvent deuterium isotope effect on the rate of spontaneous and hydroxide-catalyzed hydrolysis of the Schiff-base anion (L ) is consistent with intramolecular catalysis by the phenoxide ion. Only... 

A synthesis of a-arylamino acids has been reported using Schiff-base anions 61 derived from amino esters, which are arylated with fluorobenzene complex la to give a-aryl imino esters 62 in 48-76 % yield (R = H, Me, CH2Ph) [49]. Using the same procedure, these authors reported a convenient synthesis of optically pure a-aryl amino acid precursors 64 by enantioselective substitution of fluorobenzene complex la using the Schiff base of L-alanine methyl ester with f 1 R.2R.5 R -2-hydroxy-3-pinanone 63 in the presence of LDA (Scheme 30) [50]. 

Aldol condensations of more complex aldehydes are often sufficiently slow to allow successful alkylation reactions. There are numerous examples of aldehyde enolate methylations in the field of natural product synthesis. As shown in Scheme 29, the methylation of a tricyclic aldehyde, which was employed in the synthesis of ( )-rimuene, provides an illustrative case. As expected for an exocyclic enolate intermediate such as (61), the methyl group was introduced equatorial to the six-membered ring with a high degree of stereoselectivity. a-Alkylated aldehydes may be prepared efficiently by alkylations of enamines, Schiff base anions, hydrazone anions and other methods. A discussion of this methodology is provided in Section 1.1.5. 

Acetylation and ensuing cyclisation to the oxazolone also activates the a-proton (Figure 4.1) and provides the classical route from a natural L-a-amino acid to its dl form through hydrolysis of the derived oxazolone however, the re-protonation of a Schiff-base anion with a chiral acid (e.g. tartaric acid) gives an unequal mixture of d and l enantiomers. 

By a similar strategy of Schiff base anion alkylation a series of 7-substituted cephalosporins were prepared (280-287). In the course of these studies a number of interesting by-products were observed. Deprotection of BOC-D-phenylglycine cephem (288) with trifluoroacetic acid gave rise to cyclized imine 289, whereas an intermolecular condensation resulted in 291 as the major reaction component when 290 was treated with dilute aqueous hydrochloric acid. 

Sulfenylation of C-7 imino carbanions was also exploited by Spitzer and Goodson (1973) in preparing 7a-methylthioamides 354-357, and by Ratcliffe and Christensen (1973) in their synthesis of cefoxitin. The former workers used lithium diisopropylamide in DMF (-78°C) to generate the Schiff base anion. Methylthiolation was performed with excess methoxy-carbonylmethyl disulfide. Liberation of the modified nucleus was achieved with Girard-T reagent in aqueous DMF. This series of methylthioamides was subsequently transformed with chlorine to the corresponding 7a-methoxyamides, presumably via the intermediacy of acylimines. 

Alternatively, various 4-substituted derivatives have been prepared via synthesis of amino acid (68) by reaction of the anion formed from protected glycine and an appropriately substituted Schiff base. 

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

Antidepressant activity is retained when the two carbon bridge in imipramine is replaced by a larger, more complex, function. Nucleophilic aromatic substitution on chloropyridine 31 by means of p-aminobenzophenone (32) gives the bicyclic intermediate 33. Reduction of the nitro group (34), followed by intramolecular Schiff base formation gives the required heterocyclic ring system 35. Alkylation of the anion from 35 with l-dimethylamino-3-chloropropane leads to tampramine 36 [8]. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

When monomeric metaphosphate anion POf (102) is generated form the phos-phonate dianion 170 in the presence of the hindered base 2,2,6,6-tetramethylpiper-idine, it undergoes reaction with added carbonyl compounds147), Thus, it phosphoryl-ates acetophenone to yield the enol phosphate, whereas in the presence of acetophenone and aniline the Schiff base is formed from both compounds, probably by way of the intermediate C6H5—C(CH3) (OPO e) ( NH2C6HS). This reactivity pattern closely resembles that of monomeric methyl metaphosphate 151 (see Sect. 4.4.2). 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

In the presence of Bu OK, (benzotriazole-l-yl)methyl isocyanide (BetMIC) 697 undergoes alkylation on the methylene group to give isocyanide 698. The anion derived from 698, upon its treatment with Bu OK, adds to the electron-deficient double bonds of ajl-unsaturated ketones, esters or nitriles to produce pyrroles 699. A similar reaction of isocyanide 698 with Schiff bases provides imidazoles 700. In both cases, use of unsubstituted isonitriles 697 in the reactions leads to heterocycles 699 and 700 with R1 = H (Scheme 108) <1997H(44)67>. 

---