Highly efficient transformation of lignin major segments into quinolines

The development of a sustainable synthesis route for functional quinolines from lignin beta-O-4 segments containing gamma-OH group is of high importance to meet biorefinery demands, but remains challenge due to the low cleavage efficiency of this specific motif and the incompatible catalytic systems. In this work a highly efficient transformation of beta-O-4 lignin major segments into functional quinolines catalyzed by vanadium-based complex and copper has been developed, obtaining a wide range of substituted quinolines with good yields, which is a two-fold increase of quinolines yields when compared with previous sustainable routes. The pathway of this one-pot cascade reaction is investigated and a potential mechanism is proposed, involving the selective aryl ether cleavage, dehydrogenation, aza-Michael addition, dehydration and oxidation process. This work features a highly efficient transformation of lignin beta-O-4 segments into N-heterocyclics without additional an oxidant or reductant agent, providing a concise and sustainable route for functional quinolines synthesis.