Integration of enabling methods for the automated flow preparation of piperazine-2-carboxamide

Summary Here we describe the use of a new open-source software package and a Raspberry Pi® computer for the simultaneous control of multiple flow chemistry devices and its application to a machine-assisted, multi-step flow preparation of pyrazine-2-carboxamide – a component of Rifater®, used in the treatment of tuberculosis – and its reduced derivative piperazine-2-carboxamide.


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H NMR spectra were recorded on a Bruker Avance DPX-400 spectrometer with the residual solvent peak as the internal reference (CDCl 3 = 7.26 ppm, d 6 -DMSO = 2.50 ppm). 1 H resonances are reported to the nearest 0.01 ppm. 13 C NMR spectra were recorded on the same spectrometers with the central resonance of the solvent peak as the internal reference (CDCl 3 = 77.16 ppm, d 6 -DMSO = 39.52 ppm). All 13 C resonances are reported to the nearest 0.1 ppm. DEPT 135, COSY, HMQC, and HMBC experiments were used to aid structural determination and spectral assignment. The multiplicity of 1 H signals are indicated as: s = singlet, d = doublet, dd = doublet of doublet, ddd = doublet of doublet of doublet, t = triplet, q = quadruplet, sext = sextet, m = multiplet, br. = broad, or combinations of thereof. Coupling constants (J) are quoted in Hz and reported to the nearest 0.1 Hz. Where appropriate, averages of the signals from peaks displaying multiplicity were used to calculate the value of the coupling constant. Infrared spectra were recorded neat on a PerkinElmer Spectrum One FT-IR spectrometer using Universal ATR sampling accessories. Unless stated otherwise, reagents were obtained from commercial sources and used without purification. Hydrous zirconia was kindly gifted from MEL Chemicals (cod. XZO 631/01) Using a single stream of a Vapourtec R2/R4+ reactor, material is pumped through a polymer tubing to the glass column reactor. The output of this reactor passes through a second tubing to a 100 psi backpressure regulator (BPR) and then through a third tubing to the switching valve V1, directing it either to waste or to be collected. All tubing is PFA with Ø 1mm.
Flow procedure for the synthesis of (R,S)-piperazine-2-carboxamide. The reagent solution is infused using the Knauer K-120 pump into the H-Cube® via a Ø 0.5 mm PTFE tubing and Ø 0.5 mm stainless steel tubing. The output of the H-Cube® passes through Ø 0.5 mm PTFE tubing through a 100 psi BPR, then the FlowIR™ spectrometer, and then a 75 psi BPR to the collection valve V2 (Valco VICI 10-position switching valve). The second BPR was required to stop the hydrogen from blowing the solution through the FlowIR™ too rapidly, whilst not providing more pressure than the IR head can withstand. used directly without purification in the second step. This could be performed either by matching the flow rates of the two steps, or using a reservoir arrangement as described in the main article. The intermediate solution was delivered to the H-Cube® apparatus (flow rate 0.1 mL min −1 ) using a Knauer K-120 pump. The H-Cube® was loaded with a 10% Pd/C catalyst cartridge, which was heated at 100 °C. After concentration of the reactor output the primary amide 1 was obtained (95% yield).

Collection reservoir
The intermediate solution was directed into a pear shaped flask through a tube (Ø 0.5 mm PTFE, total volume 1 mL) from V1. A bent stainless steel tube allows the solution to be drawn out by the Knauer K-120 pump. An open needle allows the pressure to equalise. A plastic board gives a white background to the image captured by the camera, which is held in position relative to the flask with clamps. Values calculated from NMR, based on relative integration of peaks at 9.24 ppm (starting material), 3.13 ppm (product), 3.79 ppm (side-product 1) and 3.18 ppm (side-product 2).

DoE run results
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Digital Connections
Individual devices were connected as most appropriate to the control computer. The Vapourtec unit was situated a few metres from the control computer and so an Ethernet connection was most convenient. Other devices were closer and were connected by USB or USB/Serial Adapter.
The FlowIR™ has to be controlled by the Mettler-Toledo iC IR software. This is set to perform an autoexport of data to a text file. A small script running on the laptop makes this data accessible to the control computer.
The interface server can be the same machine as the control computer. In this case it was a separate machine outside the lab. The server software can also be run on a virtual machine in the cloud allowing internet access to the experimental data. Importantly the control computer can be behind a firewall and not visible from the internet, increasing the security of the laboratory devices.
This same configuration was used for all of the experiments; a Raspberry Pi® [7] computer was used for experiments not involving a camera.