Scheme 4. Synthesis of the EF-building block for E-variations. Conditions: ai27a, SOCl2, 95 °C, then 28a, NEt3, THF, 0 °C → 25 °C, 89% (29a); aii27b, SOCl2, 95 °C, then 28b, NEt3, THF, 0 °C → 25 °C, 11% (29b), 25% (30); b Zn, AcOH/EtOH (1 : 4), 0 °C, 92% (33a); c (COCl)2, THF, cat. DMF, 0 °C → 25 °C, then 28b, NEt3, THF, 0 °C → 25 °C, 89%; d HCl 4 N in dioxane; 0 °C; e TFA/CH2Cl2 (1 : 4), 0 °C; f TBAF, THF, 120 °C, 79% (33b).
