Scheme 1.
Reagents and conditions: a) AgOTf, p-TolSCl, MS 4Å, −78°C, then 11, TTBP, −78°C–0°C; b) (1) Mg(OMe)2, DCM, −20°C–0°C; (2) LevOH, EDC-HCl, DMAP, DCM; (3) DDQ, DCM/H2O; (4) HF/Pyridine; (5) TBDPSCl, imidazole, DCM; c) AgOTf, p-TolSCl, MS 4Å, −78°C, then 13, TTBP, −78°C–0°C; d) AgOTf, p-TolSCl, MS 4Å, −78°C, then 16, TTBP, −78°C–0°C; e) (1) DDQ, DCM/H2O; (2) HF/Pyridine; (3) TBDPSCl, imidazole, DCM.