Inhibiting the protein-protein interaction (PPI) between the transcription factor Nrf2 and its repressor protein Keap1 has emerged as a promising strategy to target oxidative stress in diseases, including CNS disorders. Numerous non-covalent small-molecule Keap1-Nrf2 PPI inhibitors have been reported to date, but many feature suboptimal physicochemical properties for permeating the blood-brain barrier, while others contain problematic structural moieties. Here, we present the first side-by-side assessment of all reported Keap1-Nrf2 PPI inhibitor classes using fluorescence polarization (FP), thermal shift assay (TSA), and surface plasmon resonance (SPR)-and further evaluate the compounds in an NQO1 induction cell assay and in counter tests for non-boronate ester building block 41 as the crucial carbon skeleton-building step. Synthesis of the enol triflate using the one-step NaHMDS-mediated enolization/PhNTf2-induced trapping procedure reported in the patent application 39 was not efficient in our hands, giving low yield (43% vs. 88% reported in literature 39) and significant byproduct formation. We found that using a freshly-made LiHMDS as an alternative base gave a cleaner reaction and excellent yield (quantitative). The converging SM reaction step between 38 and 41 gave several wellknown by-products, including the boronic acid, aryl boronate homo-coupling product and protodeboronation product, but could still afford the desired cross-coupling product 42 in good yield (69% vs. 33% reported in literature 39). Catalytic hydrogenation to deprotect the carboxylic acid and reduce the alkene double bond diastereoselectively furnished only the cis-cyclohexane 43 in accordance with literature; 39 this was revealed by nuclear Overhauser effect (NOE) NMR (Supporting Information Figure S1). This facial selectivity can be explained by a steric directing effect of the carboxybenzyl group. The cyclohexane carboxylic acid of 43 was finally coupled with 2-butylpyrrolidine and the pyrazole carboxylic acid deprotected to give 10 as a mixture of four stereoisomers. Attempted separation of the two diastereoisomers by preparative HPLC was unsuccessful. In the patent application, purification by HPLC is reported to give two different fractions, each containing all four stereoisomers in slightly different proportions, of which one was directly tested as a mixture. 39 Having this literature result as a reference point, we did not proceed with further purification of 10.

The growing impetus to develop greener and more cost-efficient synthetic methods has prompted chemists to look for new ways to activate small organic molecules. In this review, we cover the most recent industrial developments in electrosynthesis.

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