Photooxygenolytic Degradation of the Vitamin-B12 Derivative Heptamethyl Coα,Coβ-Dicyanocobyrinate. Efficient Preparation of Bicyclic Fragments

The methylene-blue sensitized photooxygenation of heptamethyl Coα,Coβ-dicyanocobyrinate ( 1 , cobester) at ca. ?45° and in (D₃)acetonitrile solution proceeds readily to the stage of selective double cleavage of the corrin macrocycle. It furnishes the bisected heptamethyl Coα,Coβ-dicyano-5,6:14,15-tetraoxo-5,6:14,15-disecocobyrinate ( 3 ) in 91% yield after warming the photooxygenation mixture to room temperature. Complex 3 is also obtained by photooxygenation of the secocorrinoid oxygenation products of 1 , namely of heptamethyl Coα,Coβ-dicyano-5,6-dioxo-5,6-secocobyrinate ( 2a ) and of its isomer heptamethyl Coα,Coβ-dicyano-14,15-dioxo-14,15-secocobyrinate ( 2b ). When the raw photooxygenation product of 1 is kept at low temperature, 3 is not formed in a significant amount; spectral analysis reveals 4 as intermediate that is transformed into 3 quantitatively upon warm-up and storage at r.t. Compound 4 is assigned the structure of heptamethyl Coα,Coβ-dicyano-5,6-epidioxy-5,6-dihydro-14,15-dioxo-14,15-secocobyrinate, based on NMR-spectral data and since 4 is also formed cleanly in the corresponding low-temperature photooxygenation of 2b . Catalytic reduction of the Co(III) complex 3 (H₂, Pt/C) in the presence of EDTA produces a colourless oil, from which the bicyclic fragments 5 (corresponding to rings A and D of 1 ) and 6 (corresponding to rings B and C of 1 ) are obtained in 99 and 91% yield, respectively, after chromatographic separation.