Cyclization under mild conditions of salicyloyl‐dipeptides

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Abstract
Carboxy‐activated linear peptides 6(a‐c) of general formula Sal‐Xaa‐Pro‐ONp (Xaa = Phe; Gly; Aib) were synthesized and treated at room temperature with 1,8‐diazabicyclo [5,4,0] undec‐7‐ene (DBU) in benzene solution. The tetrahedral adducts (oxa‐cyclols) 7, 11 and 12, tautomeric forms of the corresponding 10‐membered cyclodepsitripeptides, have been isolated in each of the three models examined. These adducts, which contain the hydroxylated carbon atom located at the junction between two 6‐membered rings, do not show a tendency to isomcrize into the corresponding macrocyclic lactones, regardless of the nature of the substituents on the Cα carbon atom of the central residue. Partially cyclized dimeric products 8 and 13, identified as N‐(Sal‐Xaa‐Pra)‐dioxopiperazines (Xaa = Phe: Aib), have been also isolated from the cyclization reactions.

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