A new catalytic system for the first example of enantioselective Friedel-Crafts-type (FC) addition of indoles to simple enones is described. The use of an equimolar amount of chiral [Al(salen)Cl] and 2,6-lutidine (10 mol %) was found to be effective in promoting the conjugate addition of indoles to (E)-arylcrotyl ketones, furnishing the corresponding beta-indolyl ketones in excellent yield and high enantioselectivity (ee up to 89%). The role of the base was investigated through spectroscopic as well as computational analyses, which suggested that in situ formation of a new chiral (base.[Al(salen)]) complex was operating under our reaction conditions. In particular, a stable cationic [Al(salen)] hexacoordinate trans complex with the additive base and the enone is suggested as being responsible for the stereocontrolled reaction. Finally, detailed monitoring of the reaction course was carried out showing that a conventional FC pathway induced by [Al(salen)Cl] acting as a Lewis acid is operating.

As a part of a program evaluating substituted γ‐lactams as conformationally constrained building blocks of pseudopeptide foldamers, we synthesized the homo‐oligomers of L‐pyroglutamic acid up to the tetramer level by solution methods. The preferred conformation of this pseudopeptide series in structure‐supporting solvents was assessed by FT‐IR absorption, 1H NMR and CD techniques. In addition, the crystal structure of the N α‐protected dimer was established by X‐ray diffraction. A high‐level DFT computational modeling was performed based on the crystallographic parameters. In this analysis, we demonstrated that an αCH⋅⋅⋅OC intramolecular hydrogen bond is responsible for the stabilization of the s‐trans L‐pGlu‐L‐pGlu conformation by 1.4 kcal mol−1. This effect can be easily detected by 1H NMR spectroscopy, owing to the anomalous chemical shifts of the αCH protons present in all of the oligomers. In summary, we have developed a new polyimide‐based, foldameric structure that, if appropriately functionalized, has promise as a rigid scaffold for novel functions and applications.

The liquid-phase synthesis and the conformational analysis of a small library of fully protected tetramers containing L-pyroglutamic acid (L-pGlu), (4S,5R)-4-methyl-5-carboxybenzyloxazolidin-2-one (L-Oxd), or (4R,5S)-4-methyl-5-carboxybenzyloxazolidin-2-one (D-Oxd) as residue i + 1 are reported to test the tendency of these oligomers to assume a -hairpin conformation. The most promising molecule is Boc-L-Val-D-Oxd-Gly-L-Ala-OBn, which assumes a preferential -turn conformation in CDCl3, as shown by IR and 1H NMR analysis. These findings have been confirmed by DFT calculations, which provide an interpretation for the available experimental data and agree with the reported observations.

A 2-oxo-1,3-oxazolidine-4-carboxylic acid was designed as a new, conformationally restricted building block for the construction of pseudopeptide foldamers. IR, 1 H NMR and CD techniques, implemented by detailed DFT computational modeling, were exploited to investigate the preferred threedimensional structure of benzyl (4S,5R)-5-methyl-2-oxo-1,3-oxazolidine-4-carboxylate homo-oligomers synthesized to the pentamer level. The resulting poly(L-Pro) n II like helical

The cover picture shows the side views of the DFT-optimized Boc-(L-Oxd) n -OBn (n ϭ 2Ϫ5) foldameric structures (in the circle) and the front view of Boc-(L-Oxd) 5 -OBn (in the centre). Details are discussed in the article by M. Garavelli, C. Toniolo et al. on p. 259 ff. MICROREVIEW Contents M. Hocek 245Syntheses of Purines Bearing Carbon Substituents in Positions 2, 6 or 8 by Metal-or Organometal-Mediated CϪC Bond-Forming Reactions