Acyclic Schiff base

Acyclic Schiff base derivatives represent a resourceful class of compartmental ligands which are prepared by self-condensation of appropriate formyl and amine precursors. The condensation reaction is simple and generally leads to the desired product in high yield. Literature data on Lnm mono- and bimetallic complexes, as well as on 4f-d transition metal bimetallic entities with these derivatives are abundant and have been reviewed recently (Vigato and Tamburini, 2004). Extension to multimetallic systems and to complexes with 5f elements... 

Complexes of the acyclic Schiff base ligand are assumed to oligomerize due to coordinative unsaturation [189]. A dinuclear composition was proven for Ln(III) complexes of trivalent oligosilsesquisiloxane ligands of type T7(OH)3 (Fig. 28). Surprisingly, Si-O-Si frame sites can support additional coordination sites [190],... 

In this section complexes of acyclic Schiff base ligands only are considered, macrocyclic Schiff base complexes are included in a later section. 

By analogy with the unsubstituted sugars, it has been suggested by various workers, notably Irvine and Gilmour" and Kuhn and Birkofer, that mutarotation is attributable to an equilibrium involving partial conversion of the compound into its anomer through the intermediate production of the acyclic Schiff base. 

The hydrazones of sugars are capable of existing in various cyclic forms, whose presence is apparent from their nuclear magnetic resonance (NMR) spectra and from the complex mutarotation curves they exhibit110,111117118 (which seldom follow first-order kinetics). The principal structures encountered in saccharide hydrazones are the acyclic, Schiff base-type true hydrazones and the cyclic hydrazino forms, namely glycopyranosyl- and gly-cofuranosylhydrazines. For example, three isomeric forms of D-glucose phenylhydrazone have been isolated.119 The Schiff base derivatives can be... 

Under appropriate experimental conditions, it is much easier to synthesize acyclic systems directly from the reaction solution in high yields with a satisfactory purity. Such condensation reactions are simple and generally do not lead to any formation of by-products (oligomers or polymers) in a considerable amount especially when the precursors bear only one formyl or primary amine group. Several forms of symmetric and asymmetric compartmental acyclic Schiff bases such as [1-1-1] acyclic, [1-1-1] asymmetric end-off, [l- -2] or [2- -l] symmetric end-off, [2- -l] or [H-2] side-off asymmetric, and [3-1-1] or [1-1-3] systems have been classified according to their synthetic procedures, that is, the condensation ratio between the formyl (or keto) and amine precursors. 

Acyclic Schiff base receptors cation binding... 

Condensation of 5,6-diamino-l,3-dimethyl-uracil with D-xylose yielded an acyclic Schiff-base of the sugar which on acetylation yielded the corresponding xylopyranosylamine which in turn was further elaborated to give 7-xylopyranosylxanthine. ... 

In the early laboratory studies, amino amide 15 was treated with pivalaldehyde in refluxing pentane as described in Seebach s original procedure. A crystalline solid was dhectly formed from the reaction mixture and identified as the desired trans imidazolidinone 16 as a single diastereomer in 74% yield. This observation is in contrast with Seebach s case for the corresponding amino acid iV-methyl amide. Seebach reported that the acyclic Schiff base intermediate was actually obtained in this step and then cyclized only when treated with either HCl in MeOH at 0 °C or (PhCO)20 at 130 °C (Scheme 3). Later (Scheme 5), we found that a mixture of transicis imidazolidinones 16/20 and Schiff base 19, produced by treating amino amide 15 with pivalaldehyde in toluene or dichloromethane, was completely converted to the pure trans isomer 16 upon... 

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