Dedicated to Professor Teruaki Mukaiyuma, a dear friend and revered colleague on the occasion of his 70th birthday(1 3. VIII. 96)Structural prerequisites for the stability of the 3, helix of p-peptides can be defined from inspection of models (Figs. 1 and 2 ) : lateral non-Hi-substituents in 2-and 3-position on the 3-amino-acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of hepta- (1) and of the p-heptapeptide H-P-HVal-P-HAla-p-HLeu-(S,S)-~-HAla(a Me)-p-HVal-p-HAla-p-HLeu-OH (22), with a central (2S,3S)-3-amino-2-methylbutanoic-acid residue, confirm the helical structure of such 8-peptides (previously discovered in pyridine solution) (Fig. 3 and Tables 1-5). The CD spectra of helicalp-peptides, the residues of which were prepared by (retentive) Arndt-Eistert homologation of the (S)-or ~-a-amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds 1 S 2 2 and 25 (including partially and fully deprotected forms) (Figs. 4 4 ) . The results fully confirm predicted structural effects: incorporation of a single 'wrong' residue ((R)-P-HAla, p-HAib, (R,S)-B-HAla(aMe), or N-Me-P-HAla) in the central positioh of thep-heptapeptide derivatives A (see 17, 18,20, or 21, resp.) causes the CD minimum to disappear. Also, the P-heptadepsipetide 25 (missing H-bond) and the /I-heptapeptide analogs with a single a-amino-acid moiety in the middle (13 and 14) are not helical, according to this analysis. An interesting case is the heptapeptide 15 with the central achiral, unsubstituted 3-aminopropanoicacid moiety: helical conformation appears to depend upon the presence or absence of terminal protection and upon the solvent (MeOH vs. MeOH/H20).

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of these bonds in the calculated and experimental structures indicate the importance of conjugation in the dilithium compound.The two Li atoms, each solvated by tetramethylethylenediamine (TMEDA), occupy bridging positions on opposite sides of the a,a'-keto dianion system. In addition to the strong coordination to oxygen (d(Li-0)= 1.86 A), each Li atom is in close contact with four C atoms ( Figs. 1 and 2). The LiC distances determined at 117 K by X-ray crystal structure analysis range between 2.42 and 2.73 A.The structure of 1 can be regarded as consisting of two fused enolate units with a common C-0 bond. The lithium enolate units then correspond to the bridged form of acetaldehyde lithium enolate, which is indicated by ab initio calculations to be slightly less stable than the structure with a linear C-0-Li linkage"']. This bridged form has not been observed for monoenolate aggregates, but is clearly favored for "dienolate~" [~].