N.m.r. assignments, conformation, and absolute configuration of ditryptophenaline and model dioxopiperazines

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Abstract
A detailed 1H n.m.r. analysis was carried out in order to determine the solution conformations of ditryptophenaline (1) and the model dipeptide cyclo-(L-tryptophyl-L-phenylalanyl)(2), as well as two photo-oxidation products (3) and (4). In chloroform solution ditryptophenaline (1) exists in a conformation similar to the solid-state arrangement determined by X-ray crystallography, which involves a positive degree of folding (β) of the 2,5-dioxopiperazine (DOP) ring with the phenyl group in close proximity. The DOP ring of the dipeptide (2) has a negative degree of folding (β) in dimethyl sulphoxide solution with both substituents pseudo-axial. In this conformation the phenyl ring is forced away by the indole group that is situated over the DOP ring. The solution conformations of compounds (3) and (4) in dimethyl sulphoxide are similar to that of compound (1). The assignment of 13C n.m.r. spectra of compounds (1)–(4) is reported as well as the absolute configuration of (1).