A solution of TBS-enol ether 8 (6.35g, 19.7mmol) and CH2Cl2 (57mL) was added dropwise to a solution of trifluoroacetic acid (TFA) (1.70 mL, 22.1 mmol, 1.1 eqiv) and CH2Cl2(13mL) at ambient temperature. The deep-red mixture that resulted was stirred at ambient temperature for 2 hours before it was diluted with 400mL of diethyl ether and then washed with NaHCO3 solution (3 x 30 mL saturated aqueous). The organic layer was dried over anhydrous MgSO4, then filtered and concentrated under reduced pressure. The product was purified using column chromatography (EtOAc/hexane, 20:80) resulted in the (3aSR,7aRS)-7a-acetyl-7,7- dimethylhexahydro-1H-inden-5(6H)-one (3.65 g, 89 %), a white amorphous solid .

A solution of diketone 4 (3.10 mL, 14.9 mmol) and 2-methyl-2-ethyl- 1,3-dioxolane (MED) (38.0 mL, 304 mmol, 20 equiv) was added to ethylene glycol (0.1 mL, 1.79 mmol, 12 mol %) and p-toluenesulfonic acid monohydrate (0.708 g, 3.72 mmol, 25 mol %) at ambient temperature. The mixture was then stirred for 45 minutes at ambient temperature before it was added into a NaHCO3 solution (50 mL, saturated aqueous). The aqueous layer was extracted using diethyl ether (3 x 150 mL). The organic layers were combined and washed with NaCl solution (1 x 80 mL, saturated aqueous). The washed organic layer was then dried over MgSO4, filtered and concentrated under reduced pressure. The resulting product was purified using column chromatography (EtOAc/hexane, 20:80) and resulted in 1-((3a’SR,7a’RS)-7’,7’-dimethyloctahydrospiro-[[1,3]dioxolane-2,5’-indene]-7a’-yl)- ethanone 9 (3.55 g, 95 %) , a clear oil.