Oligomers of a 5-Carboxy-methanopyrrolidine β-Amino Acid. A Search for Order

CD spectra for homooligomers (n = 4, 6, 8) of (1S,4R,5R)-5-syn-carboxy-2-azabicyclo[2.1.1]hexane (MPCA), a methano-bridged pyrrolidine β-carboxylic acid, suggest an ordered secondary structure. Even in the absence of internal hydrogen bonding, solution NMR, X-ray, and in silico analyses of the tetramer are indicative of conformations with trans-amides and C5-amide-carbonyls oriented toward the C4 bridgehead. This highly constrained β-amino acid could prove useful in the ongoing development of well-defined foldamers.