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Preparation, isolation, and characterization of Nalpha-Fmoc-peptide isocyanates: solution synthesis of oligo-alpha-peptidyl ureas.

The N(alpha)-Fmoc-peptide isocyanates 3a-q, 4a-c, and 5a-c were prepared by the Curtius rearrangement of N(alpha)-Fmoc-peptide acid azides in toluene under thermal, microwave, and ultrasonic conditions. All the N(alpha)-Fmoc-oligo-peptide isocyanates made were isolated as stable crystalline solids with 71 to 94% yield and were fully characterized by 1H NMR, 13C NMR, and mass spectroscopy. Their utility for the synthesis of oligo-alpha-peptidyl ureas 7a-f and 8a-c by the divergent coupling approach was demonstrated. The coupling of N(alpha)-Fmoc-dipeptide isocyanates with amino acid ester or with N,O-bis(trimethylsilyl)amino acids resulted in N(alpha)-Fmoc-tripeptidyl urea ester and acids containing one each of peptide bond and urea bond. The divergent approach is extended to the synthesis of tetrapeptidyl ureas by the 2 + 2 strategy using bis-TMS-peptide acid as an amino component. To incorporate urea bonds in adjacent positions, N(alpha)-Fmoc-peptidyl urea isocyanates 9a-d were prepared and employed in the synthesis of three tetrapeptidyl ureas 10a-b and 11 containing one peptide bond and two urea bonds in series from the N-terminal end. The protocol was then employed for the synthesis of five urea analogues 13-15, 18, and 21 of [Leu5]enkephalin containing urea bonds at the 2, 3, 4 positions as well as at the 2, 4 and 2, 3, 4 positions. The analogue 2l was made by the convergent synthesis by the N --> C terminal chain extension. Finally, two urea analogues 22 and 23 of repeat units of bioelasto polymers, namely Val-Pro-Gly-Val-Gly-OH and Pro-Gly-Val-Gly-Val-OH, were synthesized incorporating the urea bond by the concomitant isocyanate generation and urea bond formation under thermal conditions.