The Curtius rearrangement of Fmoc-amino acid azides 1 was carried out in toluene by refluxing the solution for 30 min. The resulting isocyanates 2 have been isolated as crystalline solids and are fully characterized by IR, (1)H NMR, (13)C NMR, and mass spectra. They are found to be stable for several months when stored at 4 degrees C. The acyl azides of Asp, Glu, Ser, Tyr, and Lys with side-chain protection having tert-butyl, benzyl, and Boc groups were also converted to the corresponding isocyanates 2h-m. The rearrangement of Fmoc-amino acid azides in toluene to isocyanates 2 under microwave irradiation was also accomplished. The direct exposure of solid azides to microwaves for 60 s led to the completion of the rearrangement. The resulting isocyanates, after recrystallization, were found to be analytically pure. The scale-up of the rearrangement, under microwave irradiation as tested up to 0.75 mol, posed no problems and led to the isolation of the isocyanates in 91-96% yield. The utility of isocyanates as building blocks in the synthesis of urea peptides 4 is demonstrated. Further, the coupling of isocyantes 2 directly with N,O-bis(trimethylsilyl) derivatives of amino acids 6 resulted in urea peptide acids 7 with good yield in high purity. Thus, the synthesis of urea peptide acids 7d-g containing Asp, Glu, Ser, and Tyr with a free side-chain functional group have been carried out.

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.

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The use of Boc 2 O as a coupling agent in the homologation of N-urethane protected-α-amino acid to its β-homomers by the Arndt-Eistert method is described. The homologation gives good yields without racemization. The use of Boc 2 O as a coupling agent not only allows the easy scale up of the process but also it is cost effective. CO urethane); 1 H NMR (δ, CDCl 3 ): 0.9 (3H, J = 7.5, t), 0.93 (3H, J = 7.1, d), 1.32 (11H, m), 1.5 (1H, m), 4.24 (1H, m), 5.3 (1H, s) and 5.42 (1H, J = 8.3, d).Boc-D-isoleucyldiazomethane (II g). 1.55 g (5 mmol) of I g, after the reaction gave 1.18 g (92.5%) of II g; mp 85-86 ЊC; Anal. Calc. for C 12 H 21 N 3 O 3 (255.28): C, 56.45; H, 8.28; N, 16.45; Found C, 56.32; H, 8.30; N,16.50%; IR ν max. cm Ϫ1 2107 (CHN 2 ), 1700 (CO urethane); 1 H NMR (δ, CDCl 3 ): 0.9 (3H, J = 7.5, t), 0.93 (3H, J = 7.1, d), 1.3 (11H, m), 1.55 (1H, m), 4.24 (1H, m), 5.35 (1H, s) and 5.45 (1H, J = 8.3, d).