Gaschromatographische Untersuchung der Racemisierung bei Peptidsynthesen

Structural biochemistry. 17. Synthesis of tryptophan dipeptides

Boc-Trp-Leu hydrazide has been found to significantly inhibit growth of the L-1210 and P388 murine lymphocytic leukemias. A series of related tryptophan dipeptides were prepared and found to be devoid of such activity. The tryptophan dipeptides were most conveniently obtained employing the DCCI-NHS peptide bond-forming procedure.

Highly Stereoselective Peptide Modifications through Pd-Catalyzed Allylic Alkylations of Chelated Peptide Enolates

Deprotonation of peptides in the presence of zinc chloride gives rise to highly reactive nucleophiles that can be subjected to palladium-catalyzed allylic alkylation reactions. Excellent diastereoselectivities are obtained that are nearly independent of the allylic substrate used. By using this protocol, highly functionalized side chains can also be incorporated in excellent yields and selectivities. The stereochemical outcome of the reaction is exclusively controlled by the peptide chain as long as terminal pi-allyl-palladium complexes are involved. Probably, there is a threefold coordination, at least, of the deprotonated peptide chain to the chelating zinc ion. In such metal peptide complexes, one face of the generated enolate is shielded by the side chain of the adjacent amino acid, thus directing the electrophilic attack onto the opposite face. This behavior explains why an S amino acid always generates an R amino acid (and the other way round).

Chlorofluoroacetic Acid as a Highly Versatile Derivatizing Agent: Assignment of Stereochemistry to Esters of Chiral Alcohols

Stereochemistry of a series of diastereomeric esters obtained from chiral alcohols 1a-21a by derivatization with (S)- or (R)-chlorofluoroacetic acid was correlated with their LC and GC separation (∆tr) and NMR resolution (∆δ). Both the chromatographic and NMR spectral behavior of respective diastereomers was found to follow systematic rules reflecting their steric arrangement. Moreover, identical conformations of the esters seem to be preferred in solution as well as in the chromatographic processes. Reasons underlying this behavior are discussed.