Application of Pictet-Spengler Reaction to Indole-Based Alkaloids Containing Tetrahydro-β-carboline Scaffold in Combinatorial Chemistry

Indole-based alkaloids are well-known in the literature for their diverse biological properties. Polysubstituted optically active tetrahydro-β-carboline derivatives functionalized on C-1 position are the common structural motif in most of the indole-based alkaloids, as well as highly marketed drugs. The stereoselective Pictet-Spengler reaction is one of the currently most important synthetic techniques used for the preparation of these privileged tetrahydro-β-carboline scaffolds. To date, there are numerous research reports that have been published on the synthesis of the tetrahydro-β-carboline scaffold both on solid phase, as well as in solution phase. Moreover rapid growth has been observed for the enantioselective synthesis of tetrahydro-β-carboline scaffold using chiral organocatalysts. In this Review, efforts have been taken to shed light on the latest information available on different strategies to synthesize tetrahydro-β-carboline both on solid phase and in solution phase during the last 20 years. Furthermore, we believe that the present synthetic methodologies covered in this Review will help to improve the status of this privileged tetrahydro-β-carboline scaffold in its use for drug discovery.

Traceless synthesis of diketopiperazine fused tetrahydro-β-carbolines on soluble polymer support

The Pictet-Spengler reaction, using polyethylene glycol immobilized tryptophan ester with a variety of ketones, was achieved by refluxing condition in acidic chloroform. The linear as well as cyclic ketones were employed. All the ketones were reacted within 6-8 h to furnish soluble polymer-supported tetrahydro-β-carboline in good yields. Further expansion at N-terminus of tetrahydro-β-carbolines was achieved through a reaction with chloroacetyl chloride. Finally, the 2,5-diketopiperazine skeleton was constructed over a β-carboline by amination of the resulting N-chloroacetamides and subsequent intramolecular cyclization leading to cleavage of the polymer; constitutes a traceless synthesis of tetracyclic molecular architecture. Significantly, this strategy affords a straightforward and efficient approach for the construction of biological promising molecules with high purity and good yields.