Synthesis of enantiomerically pure cis- and trans-4-amino-l-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acid: A spin-labelled, cyclic, chiral beta-amino acid, and 3D-Structural analysis of a doubly spin-labelled beta-hexapeptide

Amination of 3-carboxymethyl-1-oxyl-2,2,6,6-tetramethyl-4-piperidone (1) with either (R)- or (S)-α-methylbenzylamine gave corresponding enamines 2. Whereas the reduction with NaBH3CN/CH3COOH afforded predominantly a mixture of two possible cis diastereomers of 3, (1′R,3S,4S)/(1′R,3R,4R) or (1′S,3R,4R)/(1′S,3S,4S), which could be separated by crystallisation of their HCl salts, the use of NaBH4/(CH3)2CHCOOH as the reducing agent resulted in a mixture of one trans- and one cis diastereomer of 3 (1′R,3S,4R)/(1′R,3R,4R) or (1′S,3R,4S)/(1′S,3S,4S) in varying proportions depending upon the conditions used. The stereochemistry of the four diastereomers of 3 was clearly established by X-ray diffraction analysis of one of them, combined with 1H NMR spectroscopic studies after nitroxide reduction. Removal of the chiral auxiliary from the separated diastereomers of 3 by hydrogenation and regeneration of the nitroxide radical gave expected amino esters 4. A model β-hexapeptide containing (3R,4S)-β-TOAC combined with (1S,2S)-2-aminocyclohexane carboxylic acid was synthesised by solution methods and its preferred conformation (314-helix) was assessed by FTIR absorption, CD, and EPR spectroscopy