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

Cyclic Dipeptides: The Biological and Structural Landscape with Special Focus on the Anti-Cancer Proline-Based Scaffold

Cyclic dipeptides, also know as diketopiperazines (DKP), the simplest cyclic forms of peptides widespread in nature, are unsurpassed in their structural and bio-functional diversity. DKPs, especially those containing proline, due to their unique features such as, inter alia, extra-rigid conformation, high resistance to enzyme degradation, increased cell permeability, and expandable ability to bind a diverse of targets with better affinity, have emerged in the last years as biologically pre-validated platforms for the drug discovery. Recent advances have revealed their enormous potential in the development of next-generation theranostics, smart delivery systems, and biomaterials. Here, we present an updated review on the biological and structural profile of these appealing biomolecules, with a particular emphasis on those with anticancer properties, since cancers are the main cause of death all over the world. Additionally, we provide a consideration on supramolecular structuring and synthons, based on the proline-based DKP privileged scaffold, for inspiration in the design of compound libraries in search of ideal ligands, innovative self-assembled nanomaterials, and bio-functional architectures.

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

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.

The diketopiperazine-fused tetrahydro-β-carboline scaffold as a model peptidomimetic with an unusual α-turn secondary structure

Aiming at restricting the conformational freedom of tryptophan-containing peptide ligands, we designed a THBC (tetrahydro-β-carboline)-DKP (diketopiperazine)-based peptidomimetic scaffold capable of arranging in an unusual α-turn conformation. The synthesis is based on a diastereoselective Pictet–Spengler condensation to give the THBC core, followed by an intramolecular lactamization to complete the tetracyclic THBC-DKP fused ring system. The presence of conformers bearing the intramolecular thirteen-membered hydrogen bond that characterizes the α-turn structure is confirmed by ¹H NMR conformational studies. To the best of our knowledge, this scaffold represents one of the rare examples of a designed constrained α-turn mimic.