Synthesis and conformational analysis of efrapeptins.

The efrapeptin family of peptide antibiotics produced by the fungus Tolypocladium niveum, and the neo-efrapeptins from the fungus Geotrichum candidumare inhibitors of F(1)-ATPase with promising antitumor, antimalaria, and insecticidal activity. They are rich in C(α)-dialkyl amino acids (Aib, Iva, Acc) and contain one ß-alanine and several pipecolic acid residues. The C-terminus bears an unusual heterocyclic cationic cap. The efrapeptins C-G and three analogues of efrapeptin C were synthesized using α-azido carboxylic acids as masked amino acid derivatives. All compounds display inhibitory activity toward F(1)-ATPase. The conformation in solution of the peptides was investigated with electronic CD spectroscopy, FT-IR spectroscopy, and VCD spectroscopy. All efrapeptins and most efrapeptin analogues were shown to adopt helical conformations in solution. In the case of efrapeptin C, VCD spectra proved that a 3(10)-helix prevails. In addition, efrapeptin C was conformationally studied in detail with NMR and molecular modeling. Besides NOE distance restraints, residual dipolar couplings (RDC) observed upon partial alignment with stretched PDMS gels were used for the conformational analysis and confirmed the 3(10)-helical conformation.

Chiral, fully extended helical peptides.

The synthesis of the N-protected (blocked) homo-peptide esters from the chiral C(α)-ethyl, C(α)-n-pentylglycine was performed in solution to the hexapeptide level. The conformational propensity exhibited by these oligomers in chloroform solution and in the crystal state was assessed by use of FTIR absorption, NMR, and X-ray diffraction. The results indicated that fully extended helical structures (2.0(5)-helices) are overwhelmingly adopted irrespective of the peptide main-chain length. This oligomeric series is of great interest as it is characterized by the longest C ( i )(α) ,…, C ( i+1 )(α) (per residue) separation achievable in the class of chiral, rigid, helical peptide spacers based on α-amino acids.

Enantioselective RH-catalyzed hydrogenation of enol acetates and enol carbamates with monodentate phosphoramidites.

[reaction: see text] Monodentate phosphoramidites, in particular PipPhos and its octahydro analogue, are excellent ligands for the rhodium-catalyzed asymmetric hydrogenation of aromatic enol acetates, enol carbamates, and 2-dienol carbamates up to 98% ee. These latter substrates were hydrogenated selectively to the carbamates of the allyl alcohol.

Merging homogeneous catalysis with biocatalysis; papain as hydrogenation catalyst.

Papain, modified at Cys-25 with a monodentate phosphite ligand and complexed with Rh(COD)2BF4, is an active catalyst in the hydrogenation of methyl 2-acetamidoacrylate.

Enantioselective Rh-catalyzed hydrogenation of N-formyl dehydroamino esters with monodentate phosphoramidite ligands.

Enantioselectivities up to >99% ee were achieved in the rhodium-catalyzed asymmetric hydrogenation of N-formyl dehydroamino esters using monodentate phosphoramidites as chiral ligands. The substrates were synthesized by condensation of methyl isocyanoacetate with a range of aldehydes and with cyclohexanone. A highly convenient multigram scale one step synthesis of methyl 2-(formamido)acrylate was developed. This compound was used in the synthesis of methyl 2-(formamido)cinnamate via a solvent free Heck reaction. Moreover, full conversion and >99% ee were obtained in 1 h in the hydrogenation of methyl 2-(formamido)acrylate with 0.2 mol % catalyst and 2 bar hydrogen pressure. The versatility of the formyl protection was established by its removal under mild conditions.