To a solution of thioanilide 7 (2 mmol) dissolved
by gentle warming at 40 °C and then cooled to 0 °C in glacial
acetic acid (diluted with 5% water, 15 mL) was added NaNO2
(0.21 g, 3 mmol) in portions over 5 min with stirring. After 30
min, ice water (～100 mL) was added, and the precipitated
product was filtered and washed with water. The orange solid
was dried in vacuo at rt overnight and then at 50 °C for 4 h to
afford benzotriazoles 1-(N-BOC-L-thionophenylalaninyl)-6-nitrobenzotriazole (9b): yield 81%. as amorphous solids of
sufficient purity for use in the next step.
