Schiffbases derivatives

O Donnell, M. J. Cook, G. K. Rusterholz, D. B. Oxygen alkylation of Schiffbase derivatives of amino acids. Synthesis 1991, 989-993. 

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

As a prochiral glycine-derived SchifFbase, not only esters but also amides can be used as suitable substrates for asymmetric alkylation under phase-transfer conditions. 

The catalytic and chiral efficiency of (S,S)-le was also appreciated in the asymmetric synthesis of isoquinoline derivatives, which are important conformationally constrained a-amino acids. Treatment of 2 with a,a -dibromo-o-xylene under liquid-liquid phase-transfer conditions in the presence of (S,S)-le showed complete consumption ofthe starting Schiffbase. Imine hydrolysis and subsequent treatment with an excess amount of NaHCOs facilitated intramolecular ring closure to give 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid tert-butyl ester 38 in 82% yield with 98% ee. A variety of l,2,3,4-tetrahydroisoquinoline-3-carboxylic acid derivatives possessing different aromatic substituents, such as 39 and 40, can be conveniently prepared in a similar manner, with excellent enantioselectivity (Scheme 5.20) [25]. 

Since the stereochemistry of the newly created quaternary carbon center was apparently determined in the second alkylation process, the core of this method should be applicable to the asymmetric alkylation of aldimine Schiffbase 42 derived from the corresponding a-amino adds. Indeed, di-alanine-, phenylalanine- and leucine-derived imines 42 (R1 = Me, CH2Ph, i-Bu) can be alkylated smoothly under similar conditions, affording the desired non-coded amino acid esters 43 with excellent asymmetric induction, as exemplified in Table 5.7 [19]. 

Recently, Maeda and coworkers utilized the (S, S) -le-catalyzed asymmetric alkylation of phenylglycine-derived Schiffbase 42 (R1 = Ph) for the stereoselective synthesis of a 4-hydroxy-2-phenylproline framework [27]. After hydrolysis and transesterification, the resulting (S)-49 was derivatized to its N-tosylate 50. Subsequent treatment of 50 with Br2 in CH2C12 resulted in the formation of y-lactone 51 with high diastereos-electivity this was then treated with NaH in methanol to give essentially pure (2 S,4R)-4-hydroxy-2-phenylproline derivative 52 in 80% yield from 50 (Scheme 5.25). 

Other important Schiffbases include those derived from citronellal, Lyral, Helional, and Canthoxal. One of the more recent and interesting is Meaverte, the product of Tripal and methyl anthranilate. Like most other Schiffbases these products are all strongly yellow in color—a useful way in which to recognize them in a compound. 

While the majority of macrocycles formed by this type of Schiffbase condensation reaction are derived from pyridine containing fragments, considerable attention has also been devoted to the use of other heterocycles, including five membered ones, as the primary macrocyclic precursors. Although these latter ligands are for the most part not completely conjugated, they form an important group of expanded porphyrin-type macrocycles. It is for this reason that they are included in the present review,... 

Y butenolides 22 were obtained in good yields and as essentially single stereoisomers (Table 5.2). In these studies, the tat leucine derived SchifTbase ligand 18a gave rise to the best diastereo and enantioselectivities, and even with the less expensive valine derived Schiffbase ligand, the products were formed with 97% ee. Removal of the N aryl group was readily accomplished with cerium ammonium nitrate followed by acidic hydrolysis of the intermediate aza quinone. 

Pyridoxal phosphate, a derivative of vitamin Be, acts as coenzyme in transamination and decarboxylation reactions. In a transamination reaction the aldehyde group of pyridoxal phosphate first forms a Schiffbase with the amino group of the amino acid, which is then converted to keto acid. Pyridoxal phosphate is thereby converted to pyridoxamine phosphate which transfers the amino group to an other keto acid to form the corresponding amino acid. 

Synthesis of the polymeric Schiffbase complexes 4 (see Eq. 6-3) A mixture of the diglycidyl ether of bisphenol A, a metal complex and tetrabutylammon-ium hydroxide was degassed under vacuum and then cast into a mold and cured by heating in a hot-air oven. The completeness of curing was confirmed by the disappearance of the epoxy group absorption at 917 cm in IR spectra. With a molar ratio of metal complex bisphenol A derivative tetra-butylammonium hydroxide = 1 6 0.2 the curing was carried out at 160 °C for 4 h. 

Shibuguchi T, Mihara H, Kuramochi A, Ohshima T, Shibasaki M (2007) Catalytic Asymmetric Phase-Transfer Michael Reaction and Mannich-Type Reaction of Glycine SchiffBases with Tartrate-Derived Diammonium Salts. Chem Asian J 2 794... 

Good to excellent diastereoselectivities are also obtained with chiral enolates derived from menthol ester 88 [69], Schiffbase of 2-hydroxypinan-3-one 89 [70], bicyclic lactam 90 [71], 2-phenyloxazoline 91 [72], (25,55)-cw-l,3-dioxolan-4-one 92 [73], and imidazolidinone 93 [74, 75] (Fig. 8.4)... 

Schiffbase A class of derivatives of the condensation of aldehydes or ketones with primary amines... 

The addition of cyanide to imine (Strecker-type reaction) can also be catalyzed by chiral Ti complexes. In a pioneering work reported by Snapper and Hoveyda, a series of titanium complexes of tripeptide-based Schiffbase ligands have been discovered by combinatorial chemistry approach for the addition of TMSCN to imine derivatives with >93% conversion of substrates and excellent enantioselectivities (85-97% ee) [227]. This methodology has provided a practical procedure for the... 

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