Photooxygenolytic Degradation of the Vitamin‐B 12 Derivative Heptamethyl Co α, Co β‐Dicyanocobyrinate. Efficient Preparation of Bicyclic Fragments

The methylene‐blue sensitized photooxygenation of heptamethyl Co α, Co β‐dicyanocobyrinate ( 1 , cobester) at ca. −45° and in (D 3 )acetonitrile solution proceeds readily to the stage of selective double cleavage of the corrin macrocycle. It furnishes the bisected heptamethyl Co α, Co β‐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 Co α, Co β‐dicyano‐5,6‐dioxo‐5,6‐secocobyrinate ( 2a ) and of its isomer heptamethyl Co α, Co β‐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 Co α, Co β‐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 (H 2 , 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.