Scheme 8.

Synthesis of heterocyclic BA analogues [60]. Reagents and conditions: (i) Jones oxidation, Na₂Cr₂O₇, H₂SO₄, H₂O, acetone, rt, 21 h (44%); (ii) appropriate phenylhydrazine hydrochloride, HOAc, reflux, 3 h, (21–42%); (iii) ethylenediamine, sulfur, morpholine, reflux, 21 h (68%); (iv) NH₂OH/HCl, pyridine, MeOH, reflux, 16 h (84%); (v) propargylamine, Cu(I)Cl, EtOH, reflux, 17 h (11%); (vi) 1. oxalyl chloride, DCM, rt, 3 h; 2. aqueous ammonia, DCM, rt, 1 h, (~100%); (vii) TFAA, DCM, rt, 20 h, (33%); DCM ¼ dichloromethane, TFAA ¼ trifluoroacetic anhydride.