Scheme 3.

Synthesis of tricyclic 6-Cl-2-indole carboxylic acid derivatives

(a) (1) NaNO₂, HCl, sodium acetate, ethyl 2-oxocyclopentane carboxylate, H₂O, 60% (two steps); (2) EtOH, H₂SO₄; (b) BH₃-THF, THF, 83%; (c) Dt-BuAD, PPh₃, THF, 82%; (d) Cs₂CO₃, DMF, 95%; (e) AIBN, Bu₃SnH, toluene; (f) LiOH (aq.), THF, 50% (two steps); (g) bis(pinacolato)diboron, Pd(dppf)₂Cl₂:CH₂Cl₂, DMF, 35%; (h) H₂O₂, NaOH (aq.), THF, 28%; (i) Cs₂CO₃, DMF, 74%.