Recent Advances and Strategies towards Synthesis of Indolyl and Tryptophan-C-Glycoside Scaffolds

Recently, C-glycosidic bonds have emerged as pivotal elementary units present in many naturally occurring alkaloids as well as in pharmaceutically active molecules. Because of the stability and great utility of C-glycosides, synthetic approaches en route to the C-C bond formation have gradually become a center of attraction in carbohydrate chemistry. Towards these researches, C-glycosylation with heteroaryl moieties like indole, and tryptophan can remarkably expand the functional and structural evolution, which has great value for medicinal chemists and chemical biologists. The indolyl and tryptophan-C-glycosides have unique structures and biological activities that have attracted a number of synthetic studies. Therefore, these motifs have possessed an extensive interest in the direction of efficient synthesis. Herein, this review systematically summarizes the classical approaches to C-heteroaryl glycosides mainly focusing on indole and tryptophan synthesis. Furthermore, highlighted the recently developed metal-catalyzed C-H functionalization and photoinduced cross-coupling strategies that aim to control the regioselectivity and diastereoselectivity of the reaction in carbohydrate chemistry. In view of reaction type, mechanism, and status, this review will cover the synthesis of indole and tryptophan C-heteroaryl glycosides with diverse regio- and diastereoselectivity from four perspectives which include a) conventional approach, b) heterocyclization and C-glycosylation sequence, c) metal-catalyzed C-H functionalization, and d) photoinduced cross-coupling. 1. Introduction 2. Strategies for the synthesis of indolyl and tryptophan-C-glycosides 2.1. Conventional approach 2.2. Heterocyclization and C-glycosylation sequence 2.3. Metal-catalyzed C-H functionalization 2.4. Photoinduced cross-coupling 3. Summary and Outlook