Microwave-assisted synthesis of azetidines in aqueous media

Green Synthesis of Morpholines via Selective Monoalkylation of Amines

Morpholines are common heterocycles in pharmaceutical and agricultural products, yet methods to synthesize them from 1,2-amino alcohols are inefficient. We report the simple, high yielding, one or two-step, redox neutral protocol using inexpensive reagents (ethylene sulfate and tBuOK) for the conversion of 1,2-amino alcohols to morpholines. Key to this methodology is the identification of general conditions that allow for the clean isolation of monoalkylation products derived from a simple SN2 reaction between an amine and ethylene sulfate. Experiments suggest that the degree of selectivity is dependent upon the structure of reacting 1,2-amino alcohol as well as the unique properties of ethylene sulfate. This method can be used for the synthesis of a variety of morpholines containing substituents at various positions, including 28 examples derived from primary amines and multiple examples contained in known active pharmaceutical ingredients. We have conducted multiple examples on >50 g scale. We have also demonstrated the formal synthesis of a morpholine from a simple primary amine using ethylene sulfate. Overall, while this new methodology has many environmental and safety benefits relative to the traditional methods used to prepare morpholines from 1,2-amino alcohols, the most striking feature is the facile selective monoalkylation of a variety of primary amines. We have also explored various reactions beyond those related to the synthesis of morpholines, including obtaining proof-of-principle that ethylene sulfate can be used for the synthesis of piperazines and as a 2-carbon electrophile for fragment couplings.

Construction of Azacycles by Intramolecular Amination of Organoboronates and Organobis(boronates)

Intramolecular amination of organoboronates occurs with a 1,2-metalate shift of an aminoboron "ate" complex to form azetidines, pyrrolidines, and piperidines. Bis(boronates) undergo site-selective amination to form boronate-containing azacycles. Enantiomerically enriched azacycles are formed with high stereospecificity.

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.

Facile and Green Synthesis of Saturated Cyclic Amines

Single-nitrogen containing saturated cyclic amines are an important part of both natural and synthetic bioactive compounds. A number of methodologies have been developed for the synthesis of aziridines, azetidines, pyrrolidines, piperidines, azepanes and azocanes. This review highlights some facile and green synthetic routes for the synthesis of unsubstituted, multisubstituted and highly functionalized saturated cyclic amines including one-pot, microwave assisted, metal-free, solvent-free and in aqueous media.