1: O2, (EtO)3P, tBuONa / dimethylformamide / 2 h / -40 °C 2: 98.1 percent / DMAP / CH2Cl2 / 4 h / 40 °C 3: aq. K2CO3 / methanol / Ambient temperature 4: 88.9 percent / 80percent aq. CF3COOH / 3 h / Ambient temperature 5: 2.) p-toluenesulfonic acid / 1.) toluene, reflux, 30 min, 2.) toluene, reflux, 3 h View Scheme

1: O2, (EtO)3P, tBuONa / dimethylformamide / 2 h / -40 °C 2: 98.1 percent / DMAP / CH2Cl2 / 4 h / 40 °C 3: aq. K2CO3 / methanol / Ambient temperature 4: 88.9 percent / 80percent aq. CF3COOH / 3 h / Ambient temperature View Scheme

<b>Multi-step reaction with 3 steps<br /> </b>1: 44 percent / papain, L-cysteine hydrochloride, 0.1 M phosphate buffer pH 6.5 / ethyl acetate / 40 h / 40 &deg;C<br />2: 85.9 percent / COCl₂, pyridine / CH₂Cl₂ / 7 h / Heating<br />3: H₂ / 10percent Pd/C / dimethylformamide / 4 h / 760 Torr / Ambient temperature<br /> <br /> &nbsp;