Schiffbases synthesis

Soloshonok and co-workers have developed a method for the synthesis of a-(perfluoro-alkyl)amines from perfluoroalkyl carbonyl compounds by a transamination involving an azomethine a/omethine (Schiffbase) isomerization. They call this method a biomimetic, base-catalyzed 1,3-proton shift reaction, and have applied it to perfluoroaldehydes,12-15 perfluoroalkyl ketones,12 18 / -(perfluoroalkyl)-/l-oxo esters,15 16 19 24 and - -( perfluoroalkyl)-a-oxo es-ters2 " -26 to synthesize the corresponding a-(perfluoroalkyl)amincs, / -(perfluoroalkyl )-/i-amino acids, and 3 -(perfluoroalkyl)- x-amino acids. 

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]. 

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). 

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

Scheme 5. Strecker synthesis with chiral Schiffbase ligand 12 (Snapper, Hoveyda, and co-workers). TMS = trimethylsilyl.

The synthesis of 2-aminopyrazines (108) unambiguously substituted at the 5-and 6-positions from substituted 2-azabutadienes (107) and ammonia has been described by Lang and Fleury (488). In this way 2-amino-3-cyano(or methoxy-carbonyl)-5-methyl(or ethoxycarbonyl)-6 phenyl(or methyl or 5,6-polymethylene)-pyrazines have been obtained. When the Schiffbase [H2NC(CN)=C(CN)N=C(CN)Ph ... 

TA is also an enzyme of the oxidative pentose phosphate pathway[218). It catalyzes the transfer of the C1-C3 aldol unit from D-sedoheptulose 7-phosphate to D-Gly 3-P, and produces D-Fru 6-P and D-erythrose 4-phosphate (Fig. 14.2-3). TA forms a Schiffbase intermediate and does not require any co-factors. This enzyme is commercially available, and was used in a multi-enzyme synthesis of D-Fru from starch (Fig. 14.2-4) 1233] Here, it accomplished transfer of an aldol moiety from Fru 6-P to d-glyceraldehyde, and formed D-Gly 3-P and D-Fru. 

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. 

Chart 2. Synthesis of polydiphenylacetylene with SchifFbase end groups. 

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