A Dieckmann Cyclization Route to Piperazine-2,5-diones

Synthesis of Monodehydro-Diketopiperazines Enabled by Cp*Rh(III)-Catalyzed Amine-Directed N-H Functionalization

A two-step, diversity-building sequence to prepare monodehydro-diketopiperazines from readily accessible materials is reported. Rh(III)-catalyzed, amine-directed N-H functionalization of a variety of α-amino amides with a diazophosphonate ester and subsequent cyclization gives phosphonate-substituted diketopiperazines. A Horner-Wadsworth-Emmons reaction then provides monodehydro-diketopiperazines with high E-alkene selectivity. This transformation was used to incorporate a variety of groups originating from diverse aldehydes and ketones with different steric and electronic properties. Face-selective hydrogenation to diketopiperazines is also disclosed.

Backbone-Enabled and Ester Groups Switched δ-C(sp2)-H Amination/Fluorination: Cyclic Dipeptides Synthesis

An efficient and concise strategy for the synthesis of cyclic dipeptides via Pd-catalyzed site-selective δ-C(sp2)-H amination/fluorination and N-to-C cyclization is disclosed. The backbone amides within the dipeptides serves as endogenous directing groups, while the desired products were switched by the C-terminal ester group. This chemistry presents a novel and robust alternative to construct cyclodipeptide fragments.

Concise Synthesis of 2,5-Diketopiperazines via Catalytic Hydroxy-Directed Peptide Bond Formations

2,5-Diketopiperazines (DKPs) with hydroxymethyl functional groups are essential structures found in many bioactive molecules and functional materials. We have established a simple protocol for the concise synthesis of this type of DKPs through diboronic acid anhydride-catalyzed hydroxy-directed peptide bond formations. The sequential reactions in this report, which consist of three steps, an intermolecular catalytic condensation reaction in which water is the only byproduct, a simple deprotection of the nitrogen-protecting group, and an intramolecular cyclization, enabled the synthesis of functionalized DKPs in high to excellent yields without any intermediate purification. The utility of this protocol has been demonstrated by synthesizing natural products, phomamide and Cyclo(Deala-l-Leu).

Structural and Thermodynamics Studies on Polyaminophosphonate Ligands for Uranyl Decorporation

The development of actinide decorporation agents with high complexation affinity, high tissue specificity, and low biological toxicity is of vital importance for the sustained and healthy development of nuclear energy. After accidental actinide intake, sequestration by chelation therapy to reduce acute damage is considered as the most effective method. In this work, a series of bis- and tetra-phosphonated pyridine ligands have been designed, synthesized, and characterized for uranyl (UO₂²⁺) decorporation. Owing to the absorption of the ligand and the luminescence of the uranyl ion, UV-vis spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were used to probe in situ complexation and structure variation of the complexes formed by the ligands with uranyl. Density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) spectroscopy on uranyl-ligand complexes revealed the coordination geometry around the uranyl center at pH 3 and 7.4. High affinity constants (log K ∼17) toward the uranyl ion were determined by displacement titration. A preliminary in vitro chelation study proves that bis-phosphonated pyridine ligands can remove uranium from calmodulin (CaM) at a low dose and in the short term, which supports further uranyl decorporation applications of these ligands.

Construction of Spirooxindole Skeleton Through Intramolecular Dieckmann Cyclization

A highly efficient and direct approach was developed to construct the structurally diverse spirooxindole skeleton, which is an important basic motif in natural products. Both the 3,3'-pyrrolidonyl spirooxindoles and spiroindolin-2-one δ-lactones were smoothly obtained by the intramolecular Dieckmann cyclization of oxindoles in excellent yield under mild conditions.

Unified Approach to Prenylated Indole Alkaloids: Total Syntheses of (-)-17-Hydroxy-Citrinalin B, (+)-Stephacidin A, and (+)-Notoamide I

A unified strategy for the synthesis of congeners of the prenylated indole alkaloids is presented. This strategy has yielded the first synthesis of the natural product (-)-17-hydroxy-citrinalin B as well as syntheses of (+)-stephacidin A and (+)-notoamide I. An enolate addition to an in situ generated isocyanate was utilized in forging a key bicyclo[2.2.2]diazaoctane moiety, and in this way connected the two structural classes of the prenylated indole alkaloids through synthesis.

A benzotriazole-mediated route to protected marine-derived hetero-2,5-diketopiperazines containing proline

A procedure for the cyclization of dipeptidoyl benzotriazolides containing proline derivatives promoted by triethylamine under MW activation is introduced. The reaction is general for a variety of dipeptidoyl benzotriazolides and represents a very practical and convenient method for the preparation of Pro- or Hyp-derived 2,5-diketopiperazines (2,5-DKPs) and bis-DKPs with a disulfide linker. This method can be used for the construction of 2,5-DKP compound libraries and for the synthesis of natural products with diketopiperazine cores.