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Issue 12, 2013
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Serendipitous synthetic entrée to tetradehydro analogues of cobalamins

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Tetradehydrocorrin macrocycles occupy a saturation level halfway between that of octadehydrocorrins and corrins (e.g., cyanocobalamin). While octadehydrocorrin chemistry is relatively well understood, few studies have been done on more reduced macrocycles owing to synthetic limitations. The reaction of a 2,3-dihydro-1,3,3-trimethyldipyrrin (Western half) bearing a 7-(4-iodophenyl) group and a 1-(dimethoxymethyl)-9-bromo-2,3-dihydro-3,3-dimethyldipyrrin (Eastern half) bearing a 7-p-tolyl group in the presence of Ga(OTf)3 afforded the corresponding B,D-tetradehydrocorrin (1) in 24% yield. Use of a Western half bearing a 7-(4-ethylphenyl) group afforded the B,D-tetradehydrocorrin (2) in 29% yield. Each tetradehydrocorrin bears a 1-methyl group at the A–D ring junction (and hence is enantiomeric), a geminal dimethyl group in each pyrroline ring, and distinct substituents at the 7- and 17-positions; characterization included 1H and 13C NMR spectroscopies, absorption spectroscopy, mass spectrometry, and X-ray crystallography (2 only). Treatment of 1 or 2 with CoCl2 afforded the cobalt chelate as a mixture of stereoisomers containing an apical methoxide or hydroxide ligand. The cobalt-tetradehydrocorrin was characterized by absorption spectroscopy, mass spectrometry, infrared spectroscopy and electrochemistry. The cobalt chelate was smoothly titrated with THF, pyridine, and KCN or Bu4NCN. While the assignment of the oxidation state of the cobalt remains provisional, the use of natural abundance versus isotopically substituted solvents or titrants (CD3OD, K13CN) enabled clear identification of the apical ligands. The facile access to stable, synthetically tailored B,D-tetradehydrocorrins should open the door to fundamental studies of simple surrogates of cyanocobalamin.

Graphical abstract: Serendipitous synthetic entrée to tetradehydro analogues of cobalamins

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Publication details

The article was received on 29 May 2013, accepted on 09 Sep 2013 and first published on 23 Sep 2013

Article type: Paper
DOI: 10.1039/C3NJ00574G
Citation: New J. Chem., 2013,37, 3964-3975
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    Serendipitous synthetic entrée to tetradehydro analogues of cobalamins

    R. M. Deans, O. Mass, J. R. Diers, D. F. Bocian and J. S. Lindsey, New J. Chem., 2013, 37, 3964
    DOI: 10.1039/C3NJ00574G

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