Microwave-Assisted Ring-Opening of Activated Aziridines with Resin-Bound Amines

The microwave-assisted syntheses and applications of non-fused single-nitrogen-containing heterocycles

Microwave technology has emerged as a great tool for the efficient synthesis of organic compounds and it provides opportunities for chemists to achieve chemical transformations that tend to be challenging using classical approaches. Additionally, N-heterocycles are well-known for their medicinal/biological significance, along with their applications as excellent building blocks in chemical synthesis. The dominance of N-heterocycles in drug molecules and other pharmacological agents makes them attractive scaffolds, which encourages chemists to develop a wide range of strategies towards the greener synthesis and functionalization of these heterocycles. In this regard, we have collated and discussed literature relating to the microwave-assisted synthesis and the modification of non-(benzo)fused single-nitrogen-containing N-heterocycles from the past decade. The role of the microwave technique and its benefits over the conventional approach have also been emphasized in terms of overall reaction efficiency, reaction time, yield, reduced side-product generation, neat and clean reactions, chemo-/regio-/enantio-selectivity, and the use of mild reagents/reaction conditions to achieve the objectives of green and sustainable chemistry.

Aziridine based electrophilic handle for aspartic acid ligation

A one-pot ligation strategy at aspartic acid junction has been described by incorporating aziridin-2,3-dicarboxylate to the N-side of a peptide fragment that ligates with a variety of small peptide thio acids to afford native peptides in good yields.

Regioselective ring-opening of amino acid-derived chiral aziridines: an easy access to cis-2,5-disubstituted chiral piperazines

An efficient four-step synthetic strategy for cis-2,5-disubstituted chiral piperazines derived from amino-acid-based aziridines is described. The key steps in this strategy are the highly regioselective boron trifluoride diethyl etherate (BF(3)·OEt(2))-mediated ring-opening of less-reactive N-Ts chiral aziridines by α-amino acid methyl ester hydrochloride followed by Mitsunobu cyclization. This protocol has been used in an attempt to construct the piperazine core framework of natural product (+)-piperazinomycin.

Selectively N-protected enantiopure 2,5-disubstituted piperazines: avoiding the pitfalls in solid-phase Fukuyama-Mitsunobu cyclizations

An efficient solid-phase route to ring-substituted piperazines from O-linked resin-bound (S)-aziridine-2-methanol is described. Regioselective microwave-assisted aminolysis followed by intramolecular Fukuyama-Mitsunobu cyclization constitute the key features of the protocol. Simple piperazines and diazepanes were readily obtained without preceding N-protection of the acyclic intermediate, whereas attempts to extend this protocol to chiral 2,5-disubstituted piperazines failed. Modifications encompassing N-carbamoylation prior to ring-closure were therefore investigated. However, standard carbamoylating agents, for example, Fmoc-Cl and Alloc-Cl tended to give bis-protected by-products. Thus, novel microwave-assisted solid-phase N-protection procedures were developed for efficient introduction of Fmoc, Boc and Alloc groups. The subsequent cyclization proceeded in moderate to excellent yields depending on the bulk of the side chain and type of N-protecting group. This protocol readily provided novel cis- and trans-2,5-disubstituted piperazines displaying a variety of N-protecting group patterns after further on-resin manipulations. Also, unexpected by-products obtained during these optimization studies were identified and characterized. This includes nosylated ureas arising from an alternative cyclization pathway. Finally, post-cleavage oxidation gave access to the Fmoc/Boc-protected alpha-amino acid as well as the corresponding aldehyde. The chiral piperazines described in this work will enable construction of combinatorial libraries with a higher chemical diversity compared to those containing simple N,N'-difunctionalized piperazines, often present in drug-like compounds.

Unprecedented carbon-carbon bond cleavage in nucleophilic aziridine ring opening reaction, efficient ring transformation of aziridines to imidazolidin-4-ones

N-Styryl-3-aryl-1-methylaziridine-2-carboxamides, which were readily obtained from the cross coupling reaction between 3-aryl-1-methylaziridine-2-carboxamides and 1-aryl-2-bromoethenes catalyzed by CuI/N,N-dimethylglycine in the presence of Cs(2)CO(3), underwent a base-mediated intramolecular nucleophilic aziridine ring opening reaction effectively via the carbon-carbon bond cleavage of aziridine to afford the ring expanded imidazolidin-4-one products in good yields.