Synthesis of racemic orobanchols via acid-mediated cascade cyclization: Insight into the process of BC-ring formation in strigolactone biosynthesis

Abstract Strigolactones, a class of apocarotenoids, are known to function as rhizosphere semiochemicals and plant hormones. Canonical strigolactones consist of a tricyclic lactone (ABC-rings) and a butenolide moiety (D-ring), and their biosynthesis has been extensively studied. However, the process of BC-ring formation is yet to be clarified. The conversion of an 18-oxocarlactonoate derivative into racemic orobanchols was performed in the same way as in our previous study, which demonstrated that the BC-ring formation is possible “in flask” via acid-mediated cascade cyclization. The results strongly suggested that the mechanism of acid-mediated BC-ring formation may be based on the conrotatory electrocyclic reaction, and may provide significant insights into the process of BC-ring formation in strigolactone biosynthesis.