Reconstitution of the B_(12)Macrocycle by Radical Ring Closure of a Blue Secocorrin

摘要

The cobyric acid moiety of vitamin B_(12) (1) is fascinating as a highly decorated and stereochemically complex corrin, and its total synthesisis considered by many to be the highlight of organic synthesis. Natural corrins feature a characteristic set of substituents and "strategically" placed methyl groups, seen as a shield against degradation. Indeed, B_(12) cofactors are well equipped for their tasks in the catalysis of "difficult" organometallic enzyme chemistry, and they are strikingly stable and resistant even under the conditions of free radical reactions. However, the enolizable 8-position in ring B of the natural cobalt corrin complexes remains their "Achilles' heel": oxidation of vitamin B_(12) (1) affects the 8-position and efficiently furnishes corrinoid analogues, such as the lactone 2 and the lactam 3 (Scheme 1). Furthermore, a blue corrinoid was formed by a puzzling partial degradation of vitamin B_(12) (1) at ambient temperature in aqueous bicarbonate solution. The blue compound was identified as the secocorrrin 4 (Scheme 2), in which the peripheral C—C bond between the saturated 7- and 8-positions of ring B had been cleaved.