Novel chemistry of β-carbolines. Expedient synthesis of polycyclic scaffolds

Functionalization of β-carbolines is a challenge as numerous natural alkaloids with different biological activities present this heterocycle. The RCM is used herein with allyl-, vinyl-, ethynyl-, and propargyl-β-carbolines to generate additionally fused hetero- and carbocycles, and it is combined with other cyclization processes to achieve great molecular complexity in one synthetic step. Thus, an RCM–Diels–Alder sequence gives pentacyclic compounds related with certain alkaloids. On the other hand, vinylpyrrolo[2,1-a]-β-carbolines and vinyl-β-carbolines give different products upon reaction with activated dienophiles. Thus, a novel domino processes affords complex polycycles like 35–38. Other alkynes like 3-butyn-2-one give a Stevens rearrangement.