Roščić, Maja; Horvat, Štefica
Synthesis and peptidyl group migration in isopropylidene acetals derived from the glucose-substituted imidazolidinone related to opioid peptide leucine-enkephalin.
Croatica Chemica Acta, 73
The reaction of the glucose-substituted imidazolidinone 1, structurally related to endogenous opioid pentapeptide leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu), with acetone in the presence of acid catalyst gives 2,3-O-isopropylidene derivative 2 along with the isomeric monoacetal 3. Evidence is presented by using RP HPLC and NMR spectroscopy that, under the conditions of ketal formation, the peptidyl group migrated from the primary (2) to the adjacent secondary hydroxy group of the sugar moiety to give ester 3. The product distribution in the equilibrated reaction mixture (2 :3 approximate to 1.5 : 1) indicated that 5-O-peptidyl derivative 2 is slightly more stable than ester 3. Peptidyl group migration in aqueous solution at pH = 5 shows slow 2 --> 3 rearrangement and markedly faster 3 --> 2 conversion.
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