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

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.

Mechanistic insights into high mobility group box-1 (HMGb1)-induced Toll-like receptor 4 (TLR4) dimer formation

Supplemental data for this article can be accessed here.High mobility group box-1 (HMGb1), an endogenous danger-associated molecular pattern protein (DAMP) whose extracellular release has been associated with sterile injury and various inflammatory diseases and conditions, has been shown to be a valuable clinical drug target. Elucidation of the specific interactions with the HMGb1 receptor, Toll-like receptor 4 (TLR4) and adaptor protein myeloid differentiation factor-2 (MD-2), will lead to more precisely targeted therapeutics. We sought to examine detailed interactions and dynamics of the HMGb1 A-box and B-box fragments, as well as the intact protein using in silico protein-protein docking (ZDOCK, ZRANK) and molecular dynamics (Schrödinger Desmond, New York, NY). Mutagenesis and SPR-binding studies allowed us to draw further conclusions regarding the details of the HMGb1-TLR4-MD2 interaction and shed light on the reasons for the opposing biological activities of HMGb1 A-box and B-box fragments. From our findings, we hypothesize that disulfide A-box fragment binds as an anchor toward the TLR4-MD-2 but does not facilitate the TLR4 dimer formation, thereby competing with the HMGb1-binding site and preventing HMGb1-induced signaling and downstream inflammation, whereas the pro-inflammatory B-box fragment retains the MD-2 active conformation and binds to both TLR4 proteins in the complex to aid TLR4 dimer formation, which activates the intracellular signaling for downstream inflammatory pathways and cytokine release. Communicated by Ramaswamy H. Sarma.

Synthetic turn mimetics and hairpin nucleators: Quo Vadimus?

Structural mimicry of peptides has witnessed perceptible progress in the last three decades. Reverse turn and β-hairpin units are the smallest secondary structural motifs that are some of the most scrutinized functional cores of peptides and proteins. The practice of mimicking, without altering the function of the bioactive core, ranges from conformational locking of the basic skeleton to total replacement of structural architecture using synthetic analogues. Development of heterogeneous backbones--using unnatural residues in place of natural ones--has broadened further opportunities for efficient structural rigidification. This feature article endeavours to trail the path of progress achieved hitherto and envisage the possibilities that lie ahead in the development of synthetic turn mimetics and hairpin nucleators.

Highly stereoselective one-pot construction of trisubstituted tetrahydro-β-carboline-fused diketopiperazines: a synthetic route towards cialis analogues

A facile and efficient synthetic method for the stereoselective preparation of trisubstituted tetrahydro-β-carboline-fused diketopiperazine derivatives is reported.

Tetrahydro-β-carboline-based spirocyclic lactam as type II' β-turn: application to the synthesis and biological evaluation of somatostatine mimetics

The synthesis of novel spirocyclic lactams, embodying D-tryptophan (Trp) amino acid as the central core and acting as peptidomimetics, is presented. It relies on the strategic combination of Seebach's self-reproduction of chirality chemistry and Pictet-Spengler condensation as key steps. Investigation of the conformational behavior by molecular modeling, X-ray crystallography, and NMR and IR spectroscopies suggests very stable and highly predictable type II' β-turn conformations for all compounds. Relying on this feature, we also pursued their application to two potential mimetics of the hormone somatostatin, a pharmaceutically relevant natural peptide, which contains a Trp-based type II' β-turn pharmacophore.