Synthesis of protected derivatives and short peptides of antAib, a novel C-tetrasubstituted α-amino acid of the Ac5c type possessing a fused anthracene fluorophore

The N^α-Boc and N^α-Fmoc protected derivatives of 2-amino-2,3-dihydro-1H-cyclopentab]anthracene-2-carboxylic acid (antAib), a novel fluorescent, achiral, α-amino acid, rigid analogue of the known 9-antAla and 2-antAla residues, and belonging to the class of C_i^α→C_i^α cyclized, C^α,α-disubstituted glycines (strong β-turn and helix inducers in peptides), were synthesized in seven steps from 1,2,4-trimethylbenzene. The UV absorption and fluorescence properties of Boc-antAib-OEt and Boc-antAib-OH are also described. Solution syntheses of the short peptides Boc-antAib-l-Ala-OMe, Fmoc-l-Ala-antAib-l-Ala-OMe, as well as Boc-Aib-antAib-l-Ala-OMe and the side product 2,5-dioxopiperazine cyclo-antAib-l-Ala], are presented as examples of the coupling ability at both C- and N-termini of the antAib residue.