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Conditions: (i) 1.7 equiv 1–4Rp/Sp, 2.7 equiv DBU (equiv/5′-OH group), Py/MeCN (2:3, v/v), for 15–30 min at r.t., followed by addition of acetic acid (3 equiv); (ii) product mixture precipitated in 2-propanol; (iii) 5% DCA in MeOH/DCM, for 3–6 min at r.t., followed by addition of pyridine (2 equiv in comparison to DCA); (iv) 25% aqueous ammonia, for 5 h at 55 °C.
