Scheme 4.

Reagents and conditions: (i) TrCl, Et₃N, DMAP, CH₂Cl₂, rt, 71%; (ii) (COCl)₂, DMSO, Et₃N, CH₂Cl₂, −78 °C, 92%; (iii) H₂NP(O)Ph₂, p-TolSO₂H, Et₂O, rt, 94%; (iv) MeLi, t-BuLi, Et₂O, THF, −78 °C; 8; (v) Bu₄NHSO₄, 50% aq. NaOH, toluene, rt, 58% (2 steps); (vi) TBAF, THF, rt, 85%; (vii) TPAP, NMO, CH₂Cl₂, benzene, rt to reflux, 67% (6 : 1 dr); (viii) 2,4-dinitrophenylhydrazine, MeOH, rt, 86%.