Scheme 1.
Reagents and conditions: (i) PPh3, LiN3, CBr4, DMF, r.t., 3 days; (ii) conc. NH4OH, MeOH, r.t., 6 days; (iii) PPh3, DIAD, DPPA, THF, r.t., 68 h; (iv) TFA/H2O (9:1), 0 °C–r.t., 25 min.
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