Compound 1b

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Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

The peptide NH-Pro-Gly-Leu-Gly-Phe-CO₂H (24.5 mg, 0.05 mmol, 1 eq) was dissolved in a mixture of 1:1 dichloroethane:acetonitrile (1 mL). Propionaldehyde (5.4 μL, 0.075 mmol, 1.5 eq) and (N-isocyanimino)triphenylphosphorane (15.1 mg, 0.05 mmol, 1 eq) were added to the reaction mixture. After cyclization and reverse phase purification the pure fractions were pooled and lyophilized to afford 14mg of the title compound major diastereomer 1a (50% yield) and 5 mg of the minor diastereomer 1b (18 % yield) for a total 68% overall yield. Minor (R,S)- 1b: HR-MS (ESI) calculated for C₂₈H₄₀N₇O₅[M+H]⁺: 554.3085found: 554.3083. HPLC method used was the same as applied in the purification of 1a, retention time 8.15 min.