Efficient synthesis of heterocyclic compounds derived from 2,6-dioxopiperazine derivatives and their evaluation for anti-inflammatory and anticancer activities

Recent advances in the synthesis of piperazine based ligands and metal complexes and their applications

The piperazine scaffold is a privileged structure frequently found in biologically active compounds. Piperazine nucleus is found in many marketed drugs in the realm of antidepressants (amoxapine), antipsychotics (bifeprunox), antihistamines (cyclizine and oxatomide), antifungals (itraconazole), antibiotics (ciprofloxacin), etc. This is one of the reasons why piperazine based compounds are gaining prominence in today's research. In addition to the ring carbons, substitution in the nitrogen atom of piperazine not only creates potential drug molecules but also makes it unique with versatile binding possibilities with metal ions. Piperazine ring-based compounds find their application in biological systems with antihistamine, anticancer, antimicrobial and antioxidant properties. They have also been successfully used in the field of catalysis and metal organic frameworks (MOFs). The present review focuses on the synthesis and application of different piperazine derivatives and their metal complexes having diverse applications.

Rigid Scaffolds: Synthesis of 2,6-Bridged Piperazines with Functional Groups in all three Bridges

The activity of pharmacologically active compounds can be increased by presenting a drug in a defined conformation, which fits exactly into the binding pocket of its target. Herein, the piperazine scaffold was conformationally restricted by substituted C2- or C3-bridges across the 2- and 6-position. At first, a three-step, one-pot procedure was developed to obtain reproducibly piperazine-2,6-diones with various substituents at the N-atoms in high yields. Three strategies for bridging of piperazine-2,6-diones were pursued: 1. The bicyclic mixed ketals 8-benzyl-6-ethoxy-3-(4-methoxybenzyl)-6-(trimethylsilyloxy)-3,8-diazabicyclo[3.2.1]octane-2,4-diones were prepared by Dieckmann analogous cyclization of 2-(3,5-dioxopiperazin-2-yl)acetates. 2. Stepwise allylation, hydroboration and oxidation of piperazine-2,6-diones led to 3-(3,5-dioxopiperazin-2-yl)propionaldehydes. Whereas reaction of such an aldehyde with base provided the bicyclic alcohol 9-benzyl-6-hydroxy-3-(4-methoxybenzyl)-3,9-diazabicyclo[3.3.1]nonane-2,4-dione in only 10 % yield, the corresponding sulfinylimines reacted with base to give N-(2,4-dioxo-3,9-diazabicyclo[3.3.1]nonan-6-yl)-2-methylpropane-2-sulfinamides in >66 % yield. 3. Transformation of a piperazine-2,6-dione with 1,4-dibromobut-2-ene and 3-halo-2-halomethylprop-1-enes provided 3,8-diazabicyclo[3.2.1]octane-2,4-dione and 3,9-diazabicyclo[3.3.1]nonane-2,4-dione with a vinyl group at the C2- or a methylene group at the C3-bridge, respectively. Since bridging via sulfinylimines and the one-pot bridging with 3-bromo-2-bromomethylprop-1-ene gave promising yields, these strategies will be exploited for the synthesis of novel receptor ligands bearing various substituents in a defined orientation at the carbon bridge.