Comparative Study of the Synthesis and Structural and Physicochemical Properties of Diketopiperazines vs Aza-diketopiperazines

Three cheers for nitrogen: aza-DKPs, the aza analogues of 2,5-diketopiperazines

Nitrogen-containing heterocycles represent a major source of pharmacological probes and drug candidates. To extend their molecular diversity and their potential biological activities, it is of importance to design and synthesize new N-heterocyclic scaffolds. Therefore, aza-diketopiperazines (aza-DKPs), the aza analogues of well-known 2,5-diketopiperazines (DKPs), emerged as a promising new scaffold. Although the first synthesis of an aza-DKP dates from 1951, significant developments have been made during the last decade. This feature article summarizes the different synthetic strategies to access and functionalise aza-DKPs. Their biological properties and potential applications in medicinal chemistry and drug discovery are discussed as well.

Solid-Phase Synthesis of Hybrid 2,5-Diketopiperazines Using Acylhydrazide, Carbazate, Semicarbazide, Amino Acid, and Primary Amine Submonomers

We report the solid-phase synthesis of N,N'-di(acylamino)-2,5-diketopiperazine, an acylhydrazide-based conformationally rigid 2,5-DKP scaffold having exocyclic N-N bonds. We also show that different combinations of acylhydrazides, carbazates, semicarbazides, amino acids, and primary amines can be used to synthesize a highly diverse collection of hybrid DKP molecules via the solid-phase submonomer synthesis route. Finally, we show incorporation of a methyl substituent in one of the carbon atoms of the DKP ring to generate chiral daa- and hybrid-DKPs without compromising the synthetic efficiency.

Isoform-selective HDAC1/6/8 inhibitors with an imidazo-ketopiperazine cap containing stereochemical diversity

A series of hydroxamic acids linked by different lengths to a chiral imidazo-ketopiperazine scaffold were synthesized. The compounds with linker lengths of 6 and 7 carbon atoms were the most potent in histone deacetylase (HDAC) inhibition, and were specific submicromolar inhibitors of the HDAC1, HDAC6 and HDAC8 isoforms. A docking model for the binding mode predicts binding of the hydroxamic acid to the active site zinc cation and additional interactions between the imidazo-ketopiperazine and the enzyme rim. The compounds were micromolar inhibitors of the MV4-11, THP-1 and U937 cancer cell lines. Increased levels of histone H3 and tubulin acetylation support a cellular mechanism of action through HDAC inhibition.This article is part of a discussion meeting issue 'Frontiers in epigenetic chemical biology'.

Versatile Synthetic Approach for Selective Diversification of Bicyclic Aza-Diketopiperazines

Herein, we report a convenient synthesis of unprecedented aza-diketopiperazines (aza-DKPs). The strategy is based on selective diversification of bicyclic aza-DKP scaffolds by click reaction, N-acylation, and/or N-alkylation. These scaffolds containing either azido or amino groups were obtained by a key Rh(I)-catalyzed hydroformylative cyclohydrocarbonylation reaction of allyl-substituted aza-DKP. The methodology is readily amenable to the parallel synthesis of original aza-DKPs to enlarge the chemical diversity of aza-heterocycles.

N, N'-Di(acylamino)-2,5-diketopiperazines: Strategic Incorporation of Reciprocal n → π* Interactions in a Druglike Scaffold

The incorporation of the recently discovered reciprocal n → π* interactions in 2,5-diketopiperazines (DKPs) is reported to design a novel N, N'-di(acylamino)-2,5-diketopiperazine (daa-DKP) scaffold. The design, synthesis, and structural features of daa-DKPs and the effect of reciprocal n → π* interactions in their structural rigidity is discussed.