A Review of the Synthesis of Oxazoline Derivatives

Oxazolines are important heterocyclic systems due to their biological activities, such as antibacterial, antimalarial, anticancer, antiviral, anti-inflammatory, antifungal, antipyretic, and antileishmanial. They have been widely applied as chiral auxiliaries, polymers, catalysts, protecting groups, building blocks, and ligands in asymmetric synthesis. Due to their importance, many synthetic routes to prepare oxazoline moieties have been investigated and developed by researchers around the world. In this review, we summarized several synthetic methodologies published in the literature. The main substrates are nitriles, carboxylic acids, and acid derivatives, which react with a variety of reactants under conventional heating, microwave irradiation or ultrasound irradiation conditions. Syntheses via intramolecular cyclisation from amides have also been reported. Many publications have highlighted procedures based on solvent-free conditions using eco-friendly, reusable, and easy-availability catalysts.

Asymmetric Henry Reaction Using Cobalt Complexes with Bisoxazoline Ligands Bearing Two Fluorous Tags

The effect of the presence of fluorous tags in bisoxazoline ligands on the stereoselectivity of the cobalt-catalyzed asymmetric Henry reaction was investigated. In contrast to the stereoselectivity obtained with conventional nonfluorous ligands, using bisoxazoline bidentate ligands featuring two fluorous tags in adjacent positions on the aromatic ring yielded a reversed stereoselectivity. The stereoselectivity also reversed when the fluorous tags were replaced with alkyl chains of equivalent length, albeit to a considerably lesser degree, highlighting the effect of the fluorous tags.

Synthesis and Stability Studies of a Simplified, Thiazole-Containing Macrocycle of the Anticancer Agent Salarin C

While salarin C (SalaC) is a potent marine cytotoxin, Kashman demonstrated that congeners which had undergone Wasserman rearrangement exhibit little to no cytotoxicity. Given that thiazoles are known to undergo Wasserman rearrangement at a significantly reduced rate, we hypothesized that a thiazole-containing SalaC would exhibit greater stability without significantly altering the macrocyclic conformation. Herein, we describe the synthesis of a simplified, thiazole-containing macrocycle which demonstrates significantly improved stability under identical aerobic conditions.

Visible-Light 2,4-Dinitrophenol-Mediated Photoannulation of α-Azidochalcones into 2,5-Diaryloxazoles

A novel and efficient visible-light 2,4-dinitrophenol (2,4-DNP) mediated photoannulation of α-azidochalcones into 2,5-diaryloxazoles was developed. The carbon-carbon double bond of α-azidochalcone was cleaved, leading to the formation of new C-O and C-N bonds in the photoannulation. Control experiments were carried out, and a plausible mechanism of the photoannulation was proposed. The scope of the reaction was studied by synthesizing a series of 2,5-diaryloxazoles including two naturally occurring oxazoles (Texamine and Balsoxin) in excellent yields.

Chemo-enzymatic oxidative cleavage of isosafrole for the synthesis of piperonal

Piperonal is a key feedstock for the fine chemical industry. A novel process for its production from isosafrole is described, based on lipase-mediated perhydrolysis in flow, followed by batch alkaline treatment and by MnO2 oxidation in flow.

Novel 2-Aryloxazoline Compounds Exhibit an Inhibitory Effect on Candida spp., Including Antifungal-Resistant Isolates

Because of the increased resistance to currently available antifungals, fungal infections represent a significant challenge to human health. Herein, we report the synthesis of 2-aryloxazoline derivatives from the reaction between l-threonine and derivatives of salicylic or naphthoic acid. In total, 26 compounds were obtained and tested against species of Candida, Cryptococcus, and Aspergillus. We found that all of the compounds inhibited the growth of Candida species at low concentrations (<0.25 μg/mL) and exhibited reduced hemolytic and cytotoxic activities. Additionally, compounds 4i and 9i were especially effective against antifungal-resistant isolates and the emerging fungus Candida auris. However, the compounds were less active on Cryptococcus and Aspergillus. Because of the improved in vitro antifungal efficacy and attenuated cytotoxicity, these two 2-aryloxazolines obtained from salicylic and naphthoic acid derivatives, respectively, may be considered lead molecules for the development of novel antifungal drugs.

Modular continuous flow synthesis of orthogonally protected 6-deoxy glucose glycals

An efficient, modular continuous flow process towards accessing two orthogonally protected glycals is described with the development of reaction conditions for several common protecting group additions in flow, including the addition of benzyl, naphthylmethyl and tert-butyldimethylsilyl ethers. The process affords the desired target compounds in 57-74% overall yield in just 21-37 minutes of flow time. Furthermore, unlike batch conditions, the flow processes avoided the need for active cooling to prevent unwanted exotherms and required shorter reaction times.

Development of a Continuous Flow Photoisomerization Reaction Converting Isoxazoles into Diverse Oxazole Products

A continuous flow process is presented, which directly converts isoxazoles into their oxazole counterparts via a photochemical transposition reaction. This results in the first reported exploitation of this transformation to establish its scope and synthetic utility. A series of various di- and trisubstituted oxazole products bearing different appendages including different heterocyclic moieties were realized through this rapid and mild flow process. Furthermore, the robustness of this approach was demonstrated by generating gram quantities of selected products while also providing insights into likely intermediates.

A tutored discourse on microcontrollers, single board computers and their applications to monitor and control chemical reactions

This Tutored Discourse constitutes a preliminary exposure on how synthesis chemists can engage positively with inexpensive, low-power microcontrollers to aid control, monitoring and optimisation of chemical reactions.

Mechanistic insight into the oxazoline decomposition of DFC-M, a synthetic intermediate of florfenicol

2-(dichloromethyl)-5[4-(methylsulfonyl)-phenyl]-4-(fluoromethyl)-oxazoline (DFC-M, 1) is a key oxazoline-containing intermediate in commercial process for the synthesis of Florfenicol (3), a marketed broad spectrum veterinary antibiotic. DFC-M was not stable in solution due to the presence of oxazoline moiety, which provided further hindrance for analytical sample preparation and HPLC analysis. Hence, the mechanistic study on the in-solution degradation of DFC-M was carried out via online and offline UPLC-HR-ESI-MS as well as in-situ NMR, and the degradation pathways were proposed. This mechanistic information, together with the follow-up solution stability study, provided crucial information regarding the solution handling and mobile phase selection for DFC-M analysis during commercial processing.

Synthesis of protected glucose derivatives from levoglucosan by development of common carbohydrate protecting group reactions under continuous flow conditions

Common carbohydrate protecting group reactions under continuous flow processes are reported in the context of producing partially-protected glucose building blocks from levoglucosan. Benzyl ether protection was demonstrated without the use of NaH using barium oxide, which, however, pointed to the need for forms of this catalyst not as susceptible to close packing under flow. Acylation conditions were developed under continuous flow in acetonitrile and avoiding pyridine. Ring-opening the derivatized levoglucosan with propanethiol was also demonstrated producing S-alkyl 2,4-di-O-benzyl-glucopyranoside building block in 2 rather than 12 steps in increased overall yield.

Metal-free iodine(iii)-promoted synthesis of 2,5-diaryloxazoles

A nonmetal-catalyzed oxidative cyclization to achieve 2,5-disubstituted oxazoles from inexpensive and readily available substituted chalcone, (diacetoxyiodo)benzene (PIDA) and ammonium acetate (NH4OAc) at room temperature is described. The reaction forms a variety of 2,5-diaryloxazoles in good to excellent yields with broad substrate scope under mild conditions without the requirement of ligands and additional bases.

Multi-step continuous-flow synthesis

Organic chemistry is continually evolving to improve the syntheses of value added and bioactive compounds. Through this progression, a concomitant advancement in laboratory technology has occurred. Many researchers now choose to mediate transformations in continuous-flow systems given the many benefits over round bottom flasks. Furthermore, reaction scale up is often less problematic as this is addressed at the inception of the science. Although single-step transformations in continuous-flow systems are common, multi-step transformations are more valuable. In these systems, molecular complexity is accrued through sequential transformations to a mobile scaffold, much like an in vitro version of Nature's polyketide synthases. Utilizing this methodology, multi-step continuous-flow systems have improved the syntheses of active pharmaceutical ingredients (APIs), natural products, and commodity chemicals. This Review details these advancements while highlighting the rapid progress, benefits, and diversification of this expanding field.

In-line separation of multicomponent reaction mixtures using a new semi-continuous supercritical fluid chromatography system

A new bespoke semi-continuous parallel column supercritical fluid chromatography unit has been developed that solves the problem of effective separation of continuous, multicomponent reaction mixtures.

Metal-free annulation of β-acylamino ketones: facile access to spirooxazolines and oxazolines via oxidative C–O bond formation

A metal-free annulation reaction of β-acylamino ketone derivatives has been reported for the synthesis of a group of functionalized spirooxazolines and oxazolines in good to excellent yields.

Telescoped continuous flow generation of a library of highly substituted 3-thio-1,2,4-triazoles

An integrated continuous flow process for the synthesis of 3-thio-1,2,4-triazoles is reported. A small library of 18 compounds was prepared in just 48 minutes of residence time in moderate to excellent yields.

Continuous flow of nitroso Diels-Alder reaction

Our flow reaction systems have provided quantitative yields of nitroso Diels-Alder products with no byproducts in cases of cyclic dienes without temperature and pressure controls. Additionally, the reaction times were significantly shortened by using homogeneous catalyst (CuCl) or heterogeneous reagent (MnO2) in comparison with batch reaction.

A Versatile Room-Temperature Route to Di- and Trisubstituted Allenes Using Flow-Generated Diazo Compounds

A copper-catalyzed coupling reaction between flow-generated unstabilized diazo compounds and terminal alkynes provides di- and trisubstituted allenes. This extremely mild and rapid transformation is highly tolerant of several functional groups.

Cyclopropanation using flow-generated diazo compounds

A practical and mild protocol for the cyclopropanation of unstabilised diazo compounds is reported.

Flow chemistry as a discovery tool to access sp2-sp3 cross-coupling reactions via diazo compounds

The room temperature sp²–sp³ cross-coupling of flow-generated diazo compounds with boronic acids is reported.

The work takes advantage of an important feature of flow chemistry, whereby the generation of a transient species (or reactive intermediate) can be followed by a transfer step into another chemical environment, before the intermediate is reacted with a coupling partner. This concept is successfully applied to achieve a room temperature sp²–sp³ cross coupling of boronic acids with diazo compounds, these latter species being generated from hydrazones under flow conditions using MnO₂ as the oxidant.