On the stereoselective hydrogenation of chiral cyclobutyl dehydro-amino acid derivatives: influence of the catalyst in the π-facial diastereoselection

The Use of Cyclohexyl Isocyanide in the Esterification of N-Benzoyl α-Amino Acids Derivatives

The N-benzoyl of α,β-dehydroamino acids that were obtained by condensation of N-benzoylglycine and an aldehyde, were esterified with an alcohol in high yield using cyclohexyl isocyanide to activate the carboxylic acid under neutral conditions.

1H NMR spectroscopic criteria for the configuration of N-acyl-alpha,beta-dehydro-alpha-amino acid esters

The diagnostic values of the following three spectral criteria for the configuration of N-acyl-alpha,beta-dehydro-alpha-amino acid esters were examined: (i) the proton at the beta-position at the double bond of a Z-isomer is shielded if compared with the respective E-isomer (delta(beta)Z < delta(beta)E); (ii) the proton at the nitrogen atom is shielded in a Z-isomer in comparison with the corresponding E-isomer (delta(NH)Z < delta(NH)E); and (iii) changing of the solvent from CDCl3 to deuterated trifluoroacetic acid (TFA) causes shielding of the H(beta) vinylic proton of an E-isomer or deshielding of the respective proton of the Z-isomer (delta(CDCl3)E > delta(TFA)E or delta(CDCl3)Z < delta(TFA)Z). The investigations were based on a set of 22 (Z)- and (E)-N-acyl-alpha,beta-dehydro-alpha-amino acid esters of diverse structures, with aliphatic, aromatic and heteroaromatic substituents at the vinylic beta-carbon; most of the examined compounds were hitherto unknown. The application of the substituent effect additivity rule given by Pascual et al. for olefinic protons leads to evidently erroneous configuration assignments of N-acyl-alpha,beta-dehydro-alpha-amino acid esters. The considered criteria were fulfilled for all the examined cases with one exception [the second criterion for the alpha-pivaloylamino-beta-(2-furyl)acrylates]. The comparison of changes in the chemical shifts of H(beta) vinylic protons in CDCl3 and deuterated TFA seems to be the most reliable and useful configuration criterion, as it can be used in the case of a single isomer.

Stereoselective Rh-Catalyzed Hydrogenation of Cyclobutyl Chiral Enamides: Double Stereodifferentiation vs Catalyst-Controlled Diastereoselection

The hydrogenation reactions of several cyclobutyl enamides derived from (-)-alpha-pinene or (-)-verbenone have been investigated by using different catalysts. The chiralities of both the substrate and the catalyst as well as the Z/E stereochemistry of the double bond have been considered, and the observed diastereoselectivity has been rationalized. For enamides with the double bond separated from the cyclobutane by a methylene, the Wilkinson catalyst did not induce any diastereoselection, but excellent diastereoselectivity was observed when using Et-DuPHOS-Rh and ChiraPHOS-Rh. The configuration of the new stereogenic center was catalyst-dependent and can be rationalized according to the Halpern mechanism. For (Z)-enamides with the double bond directly linked to the cyclobutane ring, the chirality of the substrate governed the diastereoselection and the Halpern mechanism seemed not to be operative in the hydrogenation with ChiraPHOS, with the configuration of the new stereogenic center being determined by steric effects. On the contrary, the chirality of the catalyst was the factor determining the stereochemistry of the major products with alkyl-DuPHOS-Rh. Z/E stereochemistry influenced the stereodifferentiation, and a different behavior for each Z or E stereoisomer was found. For both (Z)- and (E)-enamides, some instances of match/mismatch between the chirality of the substrate and that of the catalyst were observed. As a result of all of these studies, a series of new cyclobutyl alpha-amino acids has been synthesized. These products are interesting to incorporate into conformationally constrained peptides.

Optically active cyclic hexapeptides with covalently attached pyrene probes: selective alkaline earth metal ion recognition using excimer emission

[structure: see text] The synthesis of two optically active pyrene-modified cyclic hexapetides and their selectivity toward the complexation of alkaline earth metal ions are reported. Complexation was studied by optical and chiroptical methods. The cyclic peptides are forming 2:1 sandwich complexes with the metal ions.