Fluorenylmethoxycarbonyl-N-methylamino Acids Synthesized in a Flow Tube-in-Tube Reactor with a Liquid-Liquid Semipermeable Membrane

An Automated Computer-Vision "Bubble-Counting" Technique to Characterise CO2 Dissolution into an Acetonitrile Flow Stream in a Teflon AF-2400 Tube-in-Tube Flow Device

A Teflon AF-2400 based tube-in-tube device was used to generate flow streams of CO₂ in acetonitrile and a computer-vision based 'bubble counting' technique was used to estimate the amount of CO₂ that had passed into solution whilst in the tube-in-tube device by quantifying the amount of CO₂ that left solution (forming separate gas-phase segments) downstream of the back-pressure regulator. For both CO₂ pressures used, there appeared to be a minimum residence time below which no CO₂ was observed to leave solution. This was assumed to be due to residual CO₂ below (or close to) the saturation concentration at atmospheric pressure and, by taking this into account, we were able to fit curves corresponding to simple gradient-driven diffusion and which closely matched previously obtained colorimetric titration data for the same system. The estimated value for the residual concentration of CO₂ (0.37 M) is higher than, but in reasonable general correspondence with, saturation concentrations previously reported for CO₂ in acetonitrile (0.27 M).

Membrane Microreactors for the On-Demand Generation, Separation, and Reaction of Gases

The use of gases as reagents in organic synthesis can be very challenging, particularly at a laboratory scale. This Concept takes into account recent studies to make the case that gases can indeed be efficiently and safely formed from relatively inexpensive commercially available reagents for use in a wide range of organic transformations. In particular, we argue that the exploitation of continuous flow membrane reactors enables the effective separation of the chemistry necessary for gas formation from the chemistry for gas consumption, with these two stages often containing incompatible chemistry. The approach outlined eliminates the need to store and transport excessive amounts of potentially toxic, reactive or explosive gases. The on‐demand generation, separation and reaction of a number of gases, including carbon monoxide, diazomethane, trifluoromethyl diazomethane, hydrogen cyanide, ammonia and formaldehyde, is discussed.

N-methylation in amino acids and peptides: Scope and limitations

Active pharmaceutical ingredients (APIs) can be divided into two types, namely chemical and biological entities. Traditionally, the former has been associated with the so-called small molecules. The revival of peptides in pharmaceutical industry results from their importance in many biological roles. However, low metabolic stability and the lack of oral availability of most peptides is the main drawback for peptide to fulfill that paradigmatic situation. In this regard, efforts are being channeled into addressing this issue by introducing restrictions into the flexible peptide backbone, mainly through N-methyl amino acids (NMAAs) or development of small cyclic peptides. In many cases, both the above restrictions are combined with the aim to enhance oral availability. The synthesis of NMAAs is complex and their introduction into the peptide chain brings additional synthetic challenges and also sometimes leads to side-reactions. Here we discuss the most efficient methods for the synthesis of NMAAs (either in solution or in solid phase) and also their introduction into peptide sequences. Special attention is also given to the detection of side reactions and the most efficient way to prevent them.

Continuous Flow Metathesis for Direct Valorization of Food Waste: An Example of Cocoa Butter Triglyceride

The direct chemical conversion of cocoa butter triglycerides, a material available as a postmanufacture waste stream from the food industry, to 1-decene by way of ethenolysis is reported. The conversion of the raw waste material was made possible by use of 1 mol % of the [RuCl₂(iBu-phoban)₂(3-phenylindenyl)] catalyst. The process has been investigated in both batch and flow conditions, where the latter approach employs a Teflon AF-2400 tube-in-tube gas-liquid membrane contactor to deliver ethylene to the reaction system. These preliminary studies culminate in a continuous processing system, which maintained a constant output over a 150 min period tested.

Flow chemistry: intelligent processing of gas-liquid transformations using a tube-in-tube reactor

CONSPECTUS: The previous decade has witnessed the expeditious uptake of flow chemistry techniques in modern synthesis laboratories, and flow-based chemistry is poised to significantly impact our approach to chemical preparation. The advantages of moving from classical batch synthesis to flow mode, in order to address the limitations of traditional approaches, particularly within the context of organic synthesis are now well established. Flow chemistry methodology has led to measurable improvements in safety and reduced energy consumption and has enabled the expansion of available reaction conditions. Contributions from our own laboratories have focused on the establishment of flow chemistry methods to address challenges associated with the assembly of complex targets through the development of multistep methods employing supported reagents and in-line monitoring of reaction intermediates to ensure the delivery of high quality target compounds. Recently, flow chemistry approaches have addressed the challenges associated with reactions utilizing reactive gases in classical batch synthesis. The small volumes of microreactors ameliorate the hazards of high-pressure gas reactions and enable improved mixing with the liquid phase. Established strategies for gas-liquid reactions in flow have relied on plug-flow (or segmented flow) regimes in which the gas plugs are introduced to a liquid stream and dissolution of gas relies on interfacial contact of the gas bubble with the liquid phase. This approach confers limited control over gas concentration within the liquid phase and is unsuitable for multistep methods requiring heterogeneous catalysis or solid supported reagents. We have identified the use of a gas-permeable fluoropolymer, Teflon AF-2400, as a simple method of achieving efficient gas-liquid contact to afford homogeneous solutions of reactive gases in flow. The membrane permits the transport of a wide range of gases with significant control of the stoichiometry of reactive gas in a given reaction mixture. We have developed a tube-in-tube reactor device consisting of a pair of concentric capillaries in which pressurized gas permeates through an inner Teflon AF-2400 tube and reacts with dissolved substrate within a liquid phase that flows within a second gas impermeable tube. This Account examines our efforts toward the development of a simple, unified methodology for the processing of gaseous reagents in flow by way of development of a tube-in-tube reactor device and applications to key C-C, C-N, and C-O bond forming and hydrogenation reactions. We further describe the application to multistep reactions using solid-supported reagents and extend the technology to processes utilizing multiple gas reagents. A key feature of our work is the development of computer-aided imaging techniques to allow automated in-line monitoring of gas concentration and stoichiometry in real time. We anticipate that this Account will illustrate the convenience and benefits of membrane tube-in-tube reactor technology to improve and concomitantly broaden the scope of gas/liquid/solid reactions in organic synthesis.

Continuous flow synthesis of α-halo ketones: essential building blocks of antiretroviral agents

The development of a continuous flow process for the multistep synthesis of α-halo ketones starting from N-protected amino acids is described. The obtained α-halo ketones are chiral building blocks for the synthesis of HIV protease inhibitors, such as atazanavir and darunavir. The synthesis starts with the formation of a mixed anhydride in a first tubular reactor. The anhydride is subsequently combined with anhydrous diazomethane in a tube-in-tube reactor. The tube-in-tube reactor consists of an inner tube, made from a gas-permeable, hydrophobic material, enclosed in a thick-walled, impermeable outer tube. Diazomethane is generated in the inner tube in an aqueous medium, and anhydrous diazomethane subsequently diffuses through the permeable membrane into the outer chamber. The α-diazo ketone is produced from the mixed anhydride and diazomethane in the outer chamber, and the resulting diazo ketone is finally converted to the halo ketone with anhydrous ethereal hydrogen halide. This method eliminates the need to store, transport, or handle diazomethane and produces α-halo ketone building blocks in a multistep system without racemization in excellent yields. A fully continuous process allowed the synthesis of 1.84 g of α-chloro ketone from the respective N-protected amino acid within ~4.5 h (87% yield).

Continuous flow generation and reactions of anhydrous diazomethane using a Teflon AF-2400 tube-in-tube reactor

A continuous process for generation, separation, and reactions of anhydrous diazomethane in a tube-in-tube reactor was developed. The inner tube of the reactor is made of hydrophobic, gas-permeable Teflon AF-2400. The diazomethane is formed in the inner tube and then diffuses through the permeable membrane into the outer chamber and subsequently reacts in the solution carried within. This technique allows safe and scalable reactions with dry diazomethane to be performed on a laboratory scale.