subsequent NaH-promoted 5-exo-dig cyclization gave imi- dazolin-2-one 9 in 78% yield with 10% racemization (see SI). Olefin migration occurred upon hydrazone removal, using hydroxylamine hydrochloride in pyridine,9 to afford N-amino-imidazolone hydrochloride which, with- out further purification, was treated with 4-methoxybenzoyl chloride to provide N-acyl dipeptide amide 10 in 56% overall yield. Crystals were grown by slow diffusion of hexanes into an ethyl acetate/chloroform solution of 10. X-ray diffraction revealed two turn conformations in the solid state (Figure 2): 10a exhibiting a type II0 β-turn with an intra- molecular ten-membered hydrogen bond between residues i and i þ 3, and 10b showing a seven-membered hydrogen bonded conformer in an inverse γ turn. The X-ray struc- tures for 10 deviate primarily by rotation of the ψiþ2 dihedral angle, as shown by comparison of their j and ψ dihedral angles with an ideal turn geometry and crystal structures of azapeptide and R-amino-γ-lactams, which adopted a turn geometry (Table 1).21 23 In contrast to amino lactams, the planar geometry of the N-amino- imidazalone causes the ψiþ1 dihedral angle to deviate by 33 46 from that of an ideal type II0 β-turn, the geometry of which is contingent on the stereochemistry of the C-terminal residue (i.e., Phe). Scheme 2. Synthesis of N-(p-Methoxybenzamido)imidazolin-2- one Isopropyl Amide (10) Measurement of the amide chemical shift values of 10 as a function of DMSO-d6 % (1 to 100%) in CDCl3 indicated relatively little variation (0.45 ppm) for the isopropylamide NH signal compared to the benzamide chemical shift (1.21 ppm; see SI), consistent with solvent-shielded (21) (a) Andre, F.; Boussard, G.; Bayeul, D.; Didierjean, C.; Aubry, A.; Marraud, M. J. Pept. Res. 1997, 49, 556–562. (b) Andre, F.; Vicherat, A.; Boussard, G.; Aubry, A.; Marraud, M. J. Pept. Res. 1997, 50, 372– 381. (c) Lecoq, A.; Boussard, G.; Marraud, M.; Aubry, A. Biopolymers 1993, 33, 1051–1059. (d) Marraud, M.; Aubry, A. Pept. Sci 1996, 40, 45– 83. (e) Benatalah, Z.; Aubry, A.; Boussard, G.; Marraud, M. Int. J. Pept. Protein Res. 1991, 38, 603–605. (22) Baures, P. W.; Ojala, W. H.; Gleason, W. B.; Mishra, R. K.; Johnson, R. L. J. Med. Chem. 1994, 37, 3677–3683. (23) St-Cyr, D. J.; Maris, T.; Lubell, W. D. Heterocycles 2010, 82, 729–737. 4554 Figure 2. X-ray structures of N-(amido)imidazolin-2-one amide 10. Broken lines represent inferred hydrogen bonds. Table 1. Structures 10 13 and Their φ and ψ Dihedral Angles (in degrees) from Crystal Analyses Compared with Ideal Turns type of turn β-II0 inverse γ 10a 10b β-II 11 12 13 jiþ1 60 58.9 62.1 60 55.4 42 40 ψiþ1 120 153.3 166.1 120 120.9 133 116 jiþ2 80 70 69.1 71.7 80 89.3 89 96 ψiþ2 0 60 4.6 65.7 0 17.8 6.9 97 (hydrogen-bonded) and exposed hydrogens,24 as found in the X-ray structure. To access constrained Phe, Trp, and His mimics, Sono- gashira couplings were performed on dipeptide 1, using various aryl iodides, Pd(PPh3)2Cl2, and CuI in a 1:1 DMF/Et2NH mixture (Scheme 3, Table 2). Electron-rich and -poor aryl iodides as well as N-protected indole and imidazole iodides all reacted in the coupling reaction to furnish aza-arylpropargylglycines 14 in 50 93% yields. Exposure of 14 to the NaH-promoted 5-exo-dig cycliza- tion produced mixtures possessing endo- and exocyclic double bonds. For example, imidazolin-2-ones 15a and 16a were isolated as isomeric mixtures in 69% yield. Although either Z or E geometry were possible for 15, a (24) For a selected example, see: Lee, H.-J.; Ahn, I.-A.; Ro, S.; Choi, K.-H.; Choi, Y.-S.; Lee, K.-B. J. Pept. Res. 2000, 56, 35–46. Org. Lett., Vol. 14, No. 17, 2012