Regioselective synthesis of 1,7-dioxaspiro[4.4]nonanes from a trimethylenemethane dianion synthon

Reagents and conditions: (i) Li, C10H8 (Cat.), R1R2CO, THF, −78 °C; (ii) R3R4C(O)CHR5, 0 °C; (iii) H2O; (iv) I2, Ag2O, THF or dioxane-H2O, rt; (RuO2) (cat.), NaIO4, CCl4-H2O, rt.