1999 amino acids, peptides

amino acids, peptidesU 0400

23 - 216Stereocontrol During the Formation of 2-C Mono-Arylated Pseudo-Prolines by Aromatic Stacking Interaction. — Reaction of benzylester-protected dipeptides (I) and (VII) with anisaldehyde dimethylacetal (II)in the presence of pyridinium tosylate proceeds by oxazolidine ring closure tostereoselectively afford the (2R)-diastereomers (III) and (VIII), respectively.This diastereoselectivity is explained by aromatic stacking interaction of benzyland anisyl group in the transition state. Thus, the methyl ester-protectedanalogue (V) lacking aromatic stacking interaction cyclizes to the (2S)-epimer(VI) with high diastereoselectivity. Interestingly, under kinetically controlledconditions the (2S)-epimer (IV) is obtained from benzyl ester peptide (I). Pyri-dinium tosylate, however, catalyzes oxazolidine ring opening and epimerizationin the heat to the thermodynamically stable diastereomer (III). — (KELLER,MICHAEL; LEHMANN, CHRISTIAN; MUTTER, MANFRED; Tetrahedron55 (1999) 2, 413-422; Inst. Chim. Org., Univ. Lausanne, CH-1015 Lausanne,Switz.; EN)

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