An extension of the 'Bip method':: induced axial chirality in a series of dipeptides based on Bip/β2,2-HBip combined with Ala/β3-HAla

In the search for an extension of the 'Bip, method' for determining the absolute configuration of beta-amino acids and beta-peptides, dipeptides based on beta(2.2)-HBip/L(D)-Ala, Bip/L-beta(3)-HA1a, and p(2,2) -HBip/L-beta(3)-Ha1a were synthesized ill Solution and the induced circular dichroism (ICD) in their biphenyl core evaluated in comparison with the previously investigated Bip/L(D)-A1a series. Weak, poorly informative ICDs were observed in MeOH solution for the linear N-Boc protected dipeptide methyl esters based oil p(2.2)-HBip, as well as for those with A1a/beta(3)-HA1a at the N-terminus of Bip/beta(2,2)-HBip. However, a significant ICD was recorded for Boc-BiP-L-beta(3)-HA1a-OMe. These results were confirmed by low-temperature H-1 NMR spectroscopy Studies of the dipeptides in CDCl3 and CD3OD solutions, showing two diastereoisomeric conformers in significantly different populations for Boc-Bip-L-beta(3)- HA1a-OMe in CD3OD. In general, ICDs were found to be weaker for dipeptides containing beta-amino acids as compared to those of their alpha-amino acid Counterparts. (c) 2006 Elsevier Ltd. All rights reserved.