The cyclization reaction of aldehydes with Z-and Boc-protectedB -hydroxyamino-acid esters (Tables 1 and 2), or of dipeptides containing serine or threonine at the N-terminus (Table 3), to give oxazolidine derivatives, occurs in the presence of isopropyl orthoformate and catalytic amounts of [Rh(MeCN)3(triphos)](CF3S03)3. The reaction may be carried out under kinetic or under thermodynamic control, so that the ratio of the two possible epimeric products can be changed. The protecting group can be removed from the 3-position of the oxazolidine ring, and the resulting NH group can be coupled with another amino acid. Thus, a new method for the preparation of peptides containing B -hydroxy-amino acid-derived oxazolidines ('pseudo-prolines') is available. N-Neopentyl-substituted tripeptides are also described.Serine-Derived Oxazolidines and Transition-Metal Acetalization Catalysts. -Oxazolidines, prepared from the j3 -hydroxyamino acids serine and threonine, have found multiple uses in stereoselective syntheses [l-lo]. They have been employed by one of our groups to synthesize a-branched analogs of serine and threonine, as well as other synthetically interesting building blocks [ 1 1-14], applying the principle of self-regeneration of stereogenic centers [15-171. Not only have these amino acids shown themselves as useful building blocks in diastereoselective syntheses, but the concomitant mechanistic and structural investigations have led to better understanding of the role amide groups play in modern organic synthesis, as discussed in a recently published review article [18]. Following a totally different line of work, Mutter et al. showed that the solubility problems often encountered both in solid and solution-phase peptide synthesis can be overcome by incorporating serines, as oxazolidine derivatives ('pseudo-prolines'), thus preventing the formation of poorly solublej3 -strands [ 191. For this purpose, the pivalaldehyde acetal of serine methyl ester was used as a building block in an otherwise conventional peptide synthesis.Recent work on transition-metal-catalyzed acetalization indicated that it might be possible to obtain oxazolidines directly from a given peptide. For over a decade, a ') Part of the projected Ph. D. theses of T. S. and of J. Q., ETH-Zurich. The authors gratefully acknowledge a stipend given to T. S. by the Verband der Chemisehen Industrrr (D-Frankfurt).