Multigram amounts of suitably protected b2 -amino acids with 17 of the 20 proteinogenic side chains are prepared by diastereoselective reactions of Li, B, or Ti enolates of the corresponding 3-acyl-4-isopropyl-5,5-diphenyloxazolidin-2-ones (acyl-DIOZ; 1) with appropriate electrophiles (amidomethylation, hydroxyalkylation, (benzyloxycarbonyl)methylation) in yields of 55 ± 90% and with diastereoselectivities of 80 to > 97% (Scheme). The primary products 2 ± 8 thus obtained are converted to protected b 2 -amino acids by standard procedures ( Table 1). Many of the DIOZ derivatives are highly crystalline compounds (31 X-ray crystal structures in Table 2). The chiral auxiliary DIOZ, readily prepared in either enantiomeric form, is recovered with high yield.b-Peptides, which consist of homologs of a-amino acids, have turned out to present a new world of peptide chemistry. While the functional groups in the chain (the amide bonds) and in the side chains (of Arg, Asp, Glu, His, Lys, Met, Ser, Thr, Trp, and Tyr) are identical, there is just an additional backbone CH 2 group in each amino acid moiety. Even this causes dramatic effects: secondary structures differ fundamentally (in number, shape, size, and polarity) and are more stable and predictable at short chain length [1] [2]; there is total enzymatic stability (against peptidases and proteases) [3], metabolic stability in rats has been demonstrated [4], even microbial degradation is slow [5]. On the other hand, the activities of an a-peptidic hormone and of an apeptidic amphipathic helix can be mimicked by b-peptides [6], and strong binding of certain b-peptides to DNA of mammalian cells has been detected [7].There are two ways of inserting CH 2 groups into the backbone of peptides: between the CO group and the a-C-atom ( 3 b 3 -amino acid) or between the a-C-atom and the N-atom ( 3 b 2 -amino acid), and it turns out that the latter change has the more intriguing effects, leading to turn structures [2] or to a novel type of helix, which consists of alternating ten-and twelve-membered H-bonded rings [1] [8].