Scheme 3.

Synthetic route to 11–14^a

^a Reagents and conditions: (a) 50% aq. H₃PO₂, conc. H₂SO₄, Cu₂O, 1M aq. NaNO₂, MeOH, 0 °C to rt, 2 h 30 min, 22%; (b) triphosgene, THF, rt, 1 h, 98%; (c) chloromethylsulfonyl chloride, Et₃N, DCM, 0 °C to rt, 12 h then 2N NaOH, rt, 2 h 30 min, 60%-72%; (d) 25, NaOEt, DMF, 130 °C, 1 h, 33%-72%.