aReagents: (a) (Bu)2SnO, toluene, 110 °C then BzCl, 5 °C → room temperature; (b) TBDMSCl, imidazole, DMF, room temperature; (c) NaOMe, MeOH, room temperature; (d) Dess-Martin periodinane, CH2Cl2, room temperature; (e) Ph3PCH3Br or Ph3PCH2CH2CH3Br, n-BuLi, THF, −78 °C → room temperature; (f) Pd (10 wt% on carbon) or Pd(OH)2 (20 wt% on carbon), H2 (1 atm), EtOH, room temperature; (g) TFA or 36% aq. HCl in MeOH, room temperature; (h) Formaldehyde, 9-DAA, 1,4-dioxane, H2O, room temperature (for (3R,4S)-69) or 85 °C (for (3R,4S)-70).