Continuous Flow Processes as an Enabling Tool for the Synthesis of Constrained Pseudopeptidic Macrocycles

Continuous Synthesis of a Macrocyclic Sulfite of Polyethylene Glycol by Cascaded Continuous Stirred Tank Reactors (CSTRs)

Many macrocyclic compounds are attractive drug-like molecules or intermediates due to their special properties. However, the bulk synthesis of such compounds are hindered by the necessity of using diluted solutions, in order to prevent intermolecular reactions that yields oligomer impurities, thereby resulting in a low production efficiency. Such challenge can be adequately addressed by using continuous reactors, allowing improved efficiency with smaller space footprints. In this work, we proposed a novel continuous process for the synthesis of a macrocyclic sulfite of tetraethylene glycol (PEG4-MCSi), which is a precursor to a very useful building block, PEG4-macrocyclic sulfate (PEG4-MCS). The basic reaction parameters, including stoichiometry and temperature, were first confirmed with small batch reactions, and the effectiveness of coiled reactors and continuous stirred tank reactors (CSTRs) were compared. Cascaded CSTRs were proven to be suitable, and the reaction parameters were subject to further optimization to give a robust continuous process. The process was then tested with 4 parallel runs for up to 64 h. Finally, the merits and demerits of batch and continuous reactions were also compared, demonstrating the suitability of latter in the bulk production of macrocyclic PEG-MCSi compounds.

Basically, nucleophilicity matters little: towards unravelling the supramolecular driving forces in enzyme-like CO2 conversion

The reaction mechanism for the cycloaddition of CO₂ to styrene oxide in the presence of macrocyclic pseudopeptides has been studied using DFT methods. Computational calculations indicate that the unprecedented catalytic behaviour previously observed experimentally, in which the most reactive species was not the most nucleophilic but the most basic one, can be associated to the tight cooperativity between several supramolecular interactions promoted by simple peptidomimetics able to display a synzymatic behaviour. This bizarre catalytic performance afforded remarkable conversions of a sluggish substrate like styrene oxide into the desired cyclic carbonate, even under relatively mild reaction conditions, opening the way for the practical use of CO₂ as a raw material in the preparation of valuable chemicals. Furthermore, the remote modification of essential structural features of the macrocycle (synzyme engineering) permitted the driving forces of the synzymatic system to be analyzed, stressing the crucial synergic effect between an elegantly preorganized oxyanion hole and additional aromatic interactions.