Scheme 4.

Synthesis of conformationally restricted analogs^a

^aReagents and Conditions: (a) PhB(OH)₂, Cu(OAc)₂, pyridine, 3Å MS, O₂, DCE; (b) Na₂S, DMF, 140 °C; (c) ethyl-2-bromo-2-phenylacetate, K₂CO₃, DMF; (d) Aq. LiOH, EtOH, THF; (e) (COCl)₂, cat. DMF, DCM; (f) 3-bromo-3-phenylpropanoic acid, NaHCO₃, DMF, 50 °C; (g) methanesulfonic anhydride, DCE, 85 °C; (h) NaBH₄, EtOH; (i) chlorodiphenyl silane, InCl₃, DCM; (j) CS₂, NaH, MeI, THF, 0 °C; (k) tributyltin hydride, AIBN, toluene, reflux.