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A new route to completely protected alpha-methylated alpha-amino acids starting from alanine is described (see Scheme). These derivatives, which are obtained via base-catalyzed opening of the oxazolidinones (2S,4R)- and (2R,4S)-2, can be directly employed in peptide synthesis. The synthesis of both enantiomers of Z-protected alpha-methylaspartic acid beta-(tert-butyl)ester (O4-(tert-butyl) hydrogen 2-methylaspartates (R) or (S)4a, alpha-methylglutamic acid gamma-(tert-butyl) ester (O5-(tert-butyl) hydrogen 2-methylglutamate (R)- or (S)-4b), and of N-epsilon-bis-Boc-protected alpha-methyllysine (N6 ,N6-bis[(tert-butyloxy)carbonyl]-2-methyllysine (R)-or (S)-4c) is described in full detail.

The molecular structures of four protected isovaline- (Iva-)containing peptides to the pentamer level have been determined by x-ray diffraction. The peptides are t-Boc-Ala(S)-Iva-Ala-OMe (t-Boc : tert-butyloxycarbonyl; OMe : methoxy) and its (R)-Iva diastereomer, and t-Boc-[Ala-(R)-Iva]2-Ala-OH and its (S)-Iva diastereomeric methyl ester analogue. The two tripeptides are folded in an open type II beta-bend conformation. The fully developed right-handed 3(10)-helix formed by the (R) -Iva pentapeptide, which includes an unusual intramolecular (acid) O-H ... O = C (peptide) H bond, is partially unfolded (near the C-terminus) in the (S)-Iva pentapeptide. H-1-nmr and Fourier transform ir absorption studies suggest that in CDCl3 solution (a) the two tripeptides maintain a type II beta-bend conformation of comparable stability and (b) both diastereomeric pentapeptide sequences adopt a fully developed 3(10)-helix. A comparison with the preferred conformation of other extensively investigated C-alpha,alpha-disubstituted glycines is made and the implications for the use of the Iva residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.