Scheme 3.
Reagents and conditions: (i) HBTU, i-Pr2NEt, DMF, THPONH2, 0 °C – 23 °C; (ii) TFA/IPA (1:2), 0 °C – 23 °C, 29% (over two steps); (iii) TFA/CH2Cl2 (1:1), 23 °C, 74%.
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