To investigate diazepinone analogues as γ-turn mimics, seven 1,4-benzodiazepin-2-ones 6 and fourteen pyrrolo[1,2-d][1,4]benzodiazepin-6-ones 4 and 5 were synthesized from 1-(2-aminophenyl)pent-4-en-1-one (7). Acylation of aniline 7 with N-Boc-amino acids, olefin oxidation, Boc removal, and intramolecular Paal-Knorr condensation gave 4 and 5. Alternatively, Boc removal prior to oxidation gave benzodiazepinones 6, which were converted to 4 by ozonolysis and cyclization. Comparison of dihedral angle values for the amino acid component from X-ray analyses of 4g, 5f, and 6f and related diazepinones has catalogued the manner by which ring substituents affect the component's ability to mimic the central residues of γ-turns.
Enantiopure (2S)-N-(Boc)-3-(6-methylpyridazinyl)alanine (14) has been synthesized to serve as a phenylalanine analog lacking significant pi-donor capability. Two approaches were developed to furnish the target compound from L-aspartic acid as chiral educt in respectively six and nine steps and 13% and 12% yields. In both routes, a key homoallylic ketone intermediate was synthesized by a copper-catalyzed cascade addition of vinylmagnesium bromide to a carboxylic ester. Dipeptide models Ac-Xaa-Pro-NHMe (21a-c) were prepared and the relative populations of prolyl cis- and trans-amide isomers were measured in chloroform, dimethylsulfoxide, and water by proton NMR spectroscopy in order to assess the significance of the electron density of the neighboring aromatic residue on the prolyl amide geometry.
Exploring the scope of the copper-catalyzed cascade addition of vinyl Grignard reagents to carboxylic esters, a set of substituted homoallylic ketones (γ,δ-unsaturated ketones) have been synthesized in 11%–94% yields from treatment of methyl 4-methoxybenzoate and methyl N-Boc-β-alaninate with different methyl-, dimethyl-, and phenyl-substituted vinyl Grignard reagents in the presence of catalytic amounts of CuCN in THF. The respective 2,4-di-, 2,3,5-tri-, and 2,3,4,5-tetrasubstituted pyrroles were obtained in 47%–93% yields from the homoallylic ketones by a sequence featuring ozonolysis followed by Paal–Knorr condensation with ammonium formate.Key words: copper-catalyzed cascade addition, homoallylic ketone, alkyl-substituted vinyl Grignard reagents, ozonolysis, pyrrole, γ,δ-unsaturated ketone.
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