A synthetic quest for tris(imidazolyl) carboxylates and their metal complexes: active site models for quercetin 2,3-dioxygenases and other non-heme redox metalloenzymes

作者：Volkman Jerome; Nicholas Kenneth M^*

来源：Tetrahedron, 2012, 68(16): 3368-3376.

DOI：10.1016/j.tet.2012.02.042

• Tris(imidazolyl)(carboxylate)
• Imidazole synthesis
• Quercetin model
• Lipoxygenase model

Synthetic routes to tris(imidazolyl)carboxylate ligands and representative metal complexes are reported, which provide more accurate active site models for several classes of redox metalloenzymes. In the first route a hydroxypivalate ester is converted in four regioselective steps to an imidazole diester 4: this is then converted to tris(imidazolyl)carboxylic acid ligand 9, featuring regiospecific double addition of an N-protected lithio-imidazole to an imidazole diester 8. Ligand 9 forms a Co(III) complex, (9)(9-H)Co (10), characterized by X-ray diffraction, coordinated via two imidazole units and the carboxylate. A second route was developed, which provides more soluble ligands that are less prone to form oligomeric complexes. It utilizes addition of the Grignard reagent from 4-iodo-2-isopropylimidazole 12 to an imidazole diester 13, producing atropisomeric tris(imidazolyl)carbinol esters 15M. Treatment of 15M with [Cu(CH3CN)(4)]PF6 gives a trinuclear complex 16 having alkoxide bridges between neighboring Cu atoms. A third route avoids the bridging alkoxo groups by reduction of tris(imidazolyl)carbinol 15M to the tris(imidazolyl)methane ester 17, which is hydrolyzed to the tris(imidazolyl)methane carboxylic acid 11a. This ligand reacts with [Cu(CH3CN)(4)]PF6 to produce dinuclear complex [(11a-H)(2)Cu-2](PF6)(2) (18), whose X-ray crystal structure shows each copper in a distorted square pyramidal geometry with coordination to the three imidazoles and two bridging carboxylate oxygens. Complex 18 is geometrically similar to the active site copper in quercetin 2,3-dioxygenase.

• 出版日期2012-4-22