Synthesis of Conformationally Constrained <span style="font-variant: small-caps">d</span>-Glu-<i>meso</i>-DAP Analogs as Innate Immune Agonists

The dipeptide <span style="font-variant: small-caps;">d</span>-Glu-<i>mes</i>o-DAP (iE-DAP) is the minimal structural fragment capable of activating the innate immune receptor nucleotide-binding oligomerization domain protein (NOD1). The <i>meso</i>-diaminopimelic acid (<i>meso</i>-DAP) moiety is known to be very stringent in terms of the allowed structural modifications which still retain the NOD1 activity. The aim of our study was to further explore the chemical space around the <i>meso</i>-DAP portion and provide a deeper understanding of the structural features required for NOD1 agonism. In order to achieve the rigidization of the terminal amine functionality of <i>meso</i>-DAP, isoxazoline and pyridine heterocycles were introduced into its side-chain. Further, we incorporated the obtained <i>meso</i>-DAP mimetics into the structure of iE-DAP. Collectively, nine innovative iE-DAP derivatives additionally equipped with lauroyl or didodecyl moieties at the α-amino group of <span style="font-variant: small-caps;">d</span>-Glu have been prepared and examined for their NOD1 activating capacity. Overall, the results obtained indicate that constraining the terminal amino group of <i>meso</i>-DAP abrogates the compounds’ ability to activate NOD1, since only compound <b>6b</b> retained noteworthy NOD1 agonistic activity, and underpin the stringent nature of this amino acid with regard to the allowed structural modifications.