Synthetic Methods for 3,4-Fused Tricyclic Indoles via Indole Ring Formation

3,4-Fused tricyclic indole frameworks are found in various bioactive natural products and pharmaceuticals. The development of an efficient synthetic method for this structural motif has therefore attracted attention in the field of synthetic organic chemistry and medicinal chemistry. Herein, we summarize recent advances in the synthesis of 3,4-fused tricyclic indoles. This class of synthetic methods can be roughly classified into two categories: methods using functionalized indole derivatives as starting materials to construct a fused medium-sized ring (Types A-D: Category I), and methods of constructing a 3,4-fused tricyclic indole skeleton via indole ring formation (Types E-G: Category II). In this focus review, synthetic methods for 3,4-fused tricyclic indoles classified as Category II methods are highlighted, following a brief overview of the Category I methods.