Scheme 3. Structural Variation of the Trigger at the 3 Position.

Reagents and conditions: (i) ethyl chloroformate, Et₃N, THF, 0 °C, 30 min, then NaBH₄, THF, H₂O, rt, 2 h, 72%; (ii) HCl, Et₂O, MeOH, 0 °C to rt, 3 h; (iii) diazo transfer reagent, CuSO₄·5H₂O, K₂CO₃, MeOH, rt, 14 h; (iv) TsCl, Et₃N, DMAP, CH₂Cl₂, rt, 24 h, 53% (3 steps); (v) 4, Cs₂CO₃, DMF, 40 °C, 20 h, 58%; (vi) SnCl₂, PhSH, Et₃N, MeCN, rt, 2.5 h, 33%; (vii) NH₃, MeOH, then allyl pinacol boronate; (viii) diazo transfer reagent, CuSO₄·5H₂O, K₂CO₃, MeOH, rt, 78% (2 steps); (ix) TsCl, Et₃N, DMAP, CH₂Cl₂, rt, quantitative; (x) 4, Cs₂CO₃, DMF, 45 °C, 33%; (xi) SnCl₂, PhSH, Et₃N, MeCN, rt, 74%.