Synthesis, resolution and assignment of absolute configuration of trans 3-amino-1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (POAC), a cyclic, spin-labelled beta-amino acid

Racemic trans 3-(9-fluorenylmethyloxycarbonylamino)-1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (Fmoc-POAC-OH), prepared by conventional methods, was resolved upon esterification with (aR)-2,2'-dihydroxy-1,1'-binaphthyl. Separation of the obtained diastereomeric monoesters Fmoc-(+/-)-trans-POAC-O-(aR)-binaphthol by crystallization/chromatography, and removal of the chiral auxiliary by saponification of the aryl ester function furnished both enantiomers (+)-(3R,4R)-Fmoc-POAC-OH and (-)-(3S,4S)-Fmoc-POAC-OH. The absolute configuration of the asymmetric C(3), C(4) carbons of POAC were assigned from the induced circular dichroism of a flexible biphenyl probe present in the terminally protected dipeptide derivatives Boc-Bip-(+)-POAC-OMe and Boc-Bip-(-)-POAC-OMe (Bip, 2',1':1,2;1 '',2 '':3,4-dibenzcyclohepta-1,3-diene-6-amino-6-carboxylic acid). This assignment was confirmed by X-ray diffraction analysis of the diastereomeric monoester Fmoc-(+)-trans-POAC-O-(aR)-binaphthol, shown to be (aR,3R,4R). Solution synthesis of peptides to the hexamer level, based on the (3RAR)-POAC enantiomer combined with (1S,2S)-2-aminocyclopentane-l-carboxylic acid, was carried out to examine coupling conditions at both C- and N-termini of the POAC residue, in view of further syntheses and 3D-structural investigations.