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Stereoselectivity in Phthalimide Initiated Photocyclization of Peptides

Phthalimide in the triplet excited state is a good oxidizing agent. Therefore, upon photoexcitation of the phthalimide to the triplet state, alkyl-carboxylates are easily oxidized by inter- or intramolecular single electron transfer (SET) from the carboxylate to the phthalimide [1]. The SET promotes irreversible decarboxylation leading to radical intermediates, which was implemented in different synthetic applications. For example, such a decarboxylation has been used in the photocyclization of crown ethers or peptides [1, 2]. We have become interested in photoinduced decarboxylation reactions of different phthalimide derivatives of adamantane aminoacids [3, 4]. Within the scope of that research, we have found an example of highly enantioselective dipeptide cyclization with a memory of chirality [5]. In the continuation of the research we investigated photocyclization of tetra- and pentapeptides 1-6 (Scheme 1). The photocyclizations of 1-6 to peptides 7-12 proceed in moderate yields, but with high diastereoselectivity. Interestingly, preliminary results indicate different stereochemistry in the cyclic products in tetra- and pentapeptides and suggest that the chiral information in the peptide chains leads to a complete inversion or retention of configuration.

peptides ; phthalimide ; photochemistry ; stereoselectivity

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