The epimerization of peptide aldehydes—a systematic study

Synthesis of a BC-Dihydrodipyrrin Building Block of Bacteriochlorophyll a

A strategy for the synthesis of bacteriochlorophyll a relies on joining AD and BC halves that contain the requisite stereochemical configurations of the target macrocycle. The BC half (1) is a dihydrodipyrrin bearing a dimethoxymethyl group at the 1-position, a β-ketoester at the 8-position, and (R)-2-methyl and (R)-3-ethyl substituents in the pyrroline ring. An established route to AD-dihydrodipyrrins (Pd-mediated coupling of a 2-halopyrrole with a chiral 4-pentynoic acid followed by Petasis methenylation, acidic hydrolysis, Paal-Knorr ring closure, and Riley oxidation) proved to be unviable for BC-dihydrodipyrrins given the presence of the β-ketoester unit. A route presented here entails Pd-mediated coupling of a 2-halopyrrole (2) with (3R,4R)-4-ethyl-1,1-dimethoxy-3-methylhex-5-yn-2-one (3), anti-Markovnikov hydration of the alkyne to give the 1,4-diketone, and Paal-Knorr ring closure. Compound 3 was prepared by Schreiber-modified Nicholas reaction beginning with (S)-4-isopropyl-3-propionyloxazolidin-2-one and the hexacarbonyldicobalt complex of (±) 3-methoxy-1-(trimethylsilyl)pentyne followed by transformation of the aldehyde derived therefrom to the 1,1-dimethoxymethylcarbonyl motif. The absolute stereochemical configuration of the Schreiber-Nicholas alkylation product was confirmed by single-crystal X-ray diffraction, whereas the BC half (1) by 1H NMR spectroscopy showed a J value of 2.9 Hz consistent with the trans-configuration. Taken together, the route provides a key chiral building block for the synthesis of photosynthetic tetrapyrroles and analogues.

The role of N-terminal heterocycles in hydrogen bonding to α-chymotrypsin

A series of dipeptide aldehydes containing different N-terminal heterocycles was prepared and assayed in vitro against α-chymotrypsin to ascertain the importance of the heterocycle in maintaining a β-strand geometry while also providing a hydrogen bond donor equivalent to the backbone amide nitrogen of the surrogate amino acid. The dipeptide containing a pyrrole constraint (10) was the most potent inhibitor, with >30-fold improved activity over dipeptides which lacked a nitrogen hydrogen bond donor (namely thiophene 11, furan 12 and pyridine 13). Molecular docking studies of 10 bound to α-chymotrypsin demonstrates a hydrogen bond between the pyrrole nitrogen donor and the backbone carbonyl of Gly₂₁₆ located in the S₃ pocket which is proposed to be critical for overall binding.

Peptides and peptidoaldehydes as substrates for the Pictet-Spengler reaction

The Pictet-Spengler (PS) reaction was performed with various types of substrates: H-Trp-OMe and dipeptides with N-terminal Trp as arylethylamine components and Z-protected amino aldehydes and peptidoaldehydes as carbonyl components. We found that the C-terminal part of Trp derivatives did not have any influence on the stereoselectivity of the reaction and the results are the same for simple esters of Trp and dipeptides. On the contrary, the selectivity of the PS reaction with peptidoaldehydes with L configuration of the C-terminus residue is totally different from that obtained with simple L-amino aldehydes. It allows us to obtain cis stereoisomers, which cannot be isolated from the reaction with amino aldehydes. But the utility of the peptidoaldehydes as substrates for the PS reaction is reduced by the side formation of enamides which decrease the yield of cyclization.

Synthesis of peptide aldehydes

The functionalization of peptides and proteins by aldehyde groups has become the subject of intensive research since the discovery of the inhibition properties of peptide aldehydes towards various enzymes. Furthermore, peptide aldehydes are of great interest for peptide backbone modification or ligation reactions. This review focuses upon their synthesis, which has been developed following two main strategies. The first strategy consists of prior synthesis of the peptide, followed by the introduction of the aldehyde function. The second possible strategy uses alpha-amino aldehydes as starting materials. After protection of the aldehyde, peptide elongation occurs. At the end of the synthesis, the aldehyde function can be unmasked.