Method:
3-Azido-but-2-enoic acid benzyl ester (33): A solution of tetramethylguanidinium azide (1.1g, 6.9 mmol, 2.0 equiv) in CHCl3 (10 mL) was added dropwise to a stirring solution. Of 1,4-But-2-ynoic acid benzyl ester 32 (0.6 g, 2.9 mmol, 1.0 equiv) in CHCl3 (20 mL) at -10°C and then diluted with water (50 mL). The product was then extracted with dichloromethane (3 x 100 mL). The combined organic layers were dried (sodium sulfate), filtered and concentrated. The clear light yellow oil was purified by flash column chromatography (7 % EtOAc in Hex) to provide E-33 (0.421 g, 56 %) as a clear light yellow liquid and Z-33 (0.210 g, 28%) as a white solid.

3-Azio-but-2-enoic benzyl ester (S6): 3-Azido-but-2-enoic acid benzyl ester (S6): A solution of azide E-33 (0.421 g, 1.9 mmol, 1 equiv) in dichloromethane (10 mL) was added dropwise to a solution of triphenylphosphine (0.50 g, 1.9 mmol, 1.0 equiv) in dichloromethane (10 mL) over 15 minutes. The resulting bright yellow solution was stirred for 4 h at 23 °C. The mixture was then concentrated and recrystallized (CH2CL2/Hex) to afford iminophosphorane Z-S6 (0.656 g , 75%) as a light brown solid. The same procedure was used to prepare iminophosphorane Z-S6 in 58% yield.

S6 to 34 3-(4-Methoxy-benzyliminomethyleneamino)-but-2-enoic acid benzyl ester (34). 4-Methoxybenzylisocyanate (0.150 mL, 1.05 mmol, 1.1 equiv) was added to a solution of iminophosphorane S6 (0.434 g, 0.96 mmol, 1.0 equiv) in toluene (4mL). The reaction was stirred at 80 °C for 4h. The resulting mixture was cooled to ambient temperature and concentrated in vacuo. The residue was purified by silica gel flash chromatography (88 % Hex in EtOAc) to provide carbodiimide 34 (0.236g, 73%) as a clear oil that turns light yellow upon standing.