Synthesis of Protected Derivatives and Short Peptides of AntAib, a Novel Ca-Tetrasubstituted a-Amino Acid of the A5c Type Possessing a Fused Anthracene Fluorophore

The Nα-Boc and Nα-Fmoc protected derivatives of 2-amino-2,3-dihydro-1H-cyclopenta[b]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 Ciα→Ciα cyclized, Cα,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.