Squaramide-functionalised amino acids and their application in solid-phase peptide synthesis

Squaramides are well-known charge-neutral bioisosteres of many nitrogenous functional groups, including those found in the side chains of amino acids (e.g., Arg, Asn, Glu, and Lys). However, there are very few reports of squaramide-functionalisation of these side chains. Herein, we report a convenient three-step synthesis for Boc, Alloc, or Cbz protected squaramide-functionalised Fmoc amino acids (SqAAs) derived from Dap, Dab, Orn and Lys in good yields and high enantiopurity (>95% ee), in milligram to multigram scale (38-93% over three steps for Boc-protected amino acids). These amino acids were found to be compatible with a variety of commonly used amide coupling reagents and green solvents, as well as being extremely resistant to epimerisation in solution-phase coupling. Orn and Dab derivatives (7 and 8) were able to be incorporated into solid-phase Fmoc SPPS workflows in the construction of four tetrapeptides, Arg/Asp-SqAA-Gly-Leu, in good yield (23-57%) and with no significant byproduct formation. We propose that these squaramide-functionalised amino acids could serve as convenient building blocks for constructing bioisosteres of bioactive peptides.