Facile synthesis of orthogonally protected amino acid building blocks for combinatorial N-backbone cyclic peptide chemistry

Protected N^α-(aminoallyloxycarbonyl) and N^α-(carboxyallyl) derivatives of all natural amino acids (except proline), and their chiral inverters, were synthesized using facile and efficient methods and were then used in the synthesis of N^α-backbone cyclic peptides. Synthetic pathways for the preparation of the amino acid building units included alkylation, reductive amination and Michael addition using alkylhalides, aldehydes and α,β-unsaturated carbonyl compounds, and the corresponding amino acids. The resulting amino acid prounits were then subjected to Fmoc protection affording optically pure amino acid building units. The appropriate synthetic pathway for each amino acid was chosen according to the nature of the side-chain, resulting in fully orthogonal trifunctional building units for the solid-phase peptide synthesis of small cyclic analogs of peptide loops (SCAPLs™). N^α-amino groups of building units were protected by Fmoc, functional side-chains were protected by t-Bu/Boc/Trt and N-alkylamino or N-alkylcarboxyl were protected by Alloc or Allyl, respectively. This facile method allows easy production of a large variety of amino acid building units in a short time, and is successfully employed in combinatorial chemistry as well as in large-scale solid-phase peptide synthesis. These building units have significant advantage in the synthesis of peptido-related drugs.