Scheme 5.
Reagents and conditions: (i) EtOH, reflux, 18 h (47: 70%; 48: 44%); (ii) TFA, DCM dry, 2 h, quantitative; (iii) DIPEA, DMF dry, (13m: 40%; 13o: 41%); and (iv) N-hydroxysuccinimide, EDC, DCM dry, 18 h, 71%.
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