Isotopically Labeled and Unlabeled β-Peptides with Geminal Dimethyl Substitution in 2-Position of Each Residue: Synthesis and NMR Investigation in Solution and in the Solid State.

The preparation of (S)-beta(2.2.3) -amino acids with two Me groups in the a-position and the side chains of Ala, Val, and Len in the P-position (double methylation of Boc-beta-HAla-OMe, Boc-beta-Val-OMe, and Boc-beta-LeuOMe, Scheme 2) is described. These beta-amino acids and unlabelled as well as specifically C-13- and (15)labelled 2,2-dimethyl-3-amino acid (beta(2.2)-HAib) derivatives have been coupled in solution (Schemes 1, 3 and 4) to give protected (N-Boc, C-OMe), partially protected (N-Boc/C-OH, N-H/C-OMe), and unprotected beta(2.2) - and beta(2.2.3)- hexapeptides, and beta(2.2) and beta(2.2.3)-heptapeptides 1-7. NMR Analyses in solution (Tables 1 and 2, and Figs. 2-4) and in the solid state (2D-MAS NMR measurements of the fully labelled BOC-(beta(2.2)-HAib)(6)-OMe ([C-13(30), N-15(6)]-1e; Fig. 5), and TEDOR/REDOR NMR investigations of mixtures (Fig. 6) of the unlabelled AC-([beta(2.2)-HAib)(7)- OMe (4) and of a labelled derivative ([C-13(4),N-15(2)]-5; Figs. 7- 11, and 19), a molecular-modeling study (Figs. 13 15), and a search in the Cambridge Crystallographic Data Base (Fig. 16) allow the following conclusions: i) there is no evidence for folding (helix or turn) or for aggregation to sheets of the geminally dimethyl substituted peptide chains in solution; ii) there are distinct conformational preferences of the individual beta(2.2) and beta(2.2.3)-amino acid residues: close to eclipsing around the C(O) - C(Me-2(CHR)) bond (tau(1.2)), almost perfect staggering around the C(2)-C(3) ethane bond (tau(2,3)), and antiperiplanar arrangement of H(C3) and H(N) (TIN; Fig. 12) in the solid state; iii) the beta(2,2)-peptides may be part of a turn structure with a ten-membered H-bonded ring; iv) the main structure present in the solid state of F3CCO(beta(2,2) -HAib)(7)-OMe is a nonfolded chain (>30 Angstrom between the termini and >20 Angstrom between the N-terminus and the CH2 group of residue 5) with all C = O bonds in a parallel alignment (+/-10degrees). With these structural parameters, a simple modelling was performed producing three (maybe four) possible chain geometries: one fully extended, two with parallel peptide planes (with zick-zack and crankshaft-type arrangement of the peptide bonds). and (possibly) a fourth with meander-like winding (D-G in Figs. 17 and 18).