the combined extracts are washed with saturated sodium bicarbonate dried over anhydrous sodium sulfate and evaporated to give crude 12 sublimation on a steam bath gives pure 12 a solution of exo 11 1 05 mmol in 5 ml of acetone is stirred in an erlenmeyer flask at 25 u00b0 c to this solution is added dropwise 8 n chromic acid until the orange color persisted the temperature being kept at 25 u00b0 c the orange solution is then stirred at 25 u00b0 c for about additional 3 h most of the acetone is removed and 5 ml of water is added to the residue the aqueous mixture is extracted twice with ether and the combined extracts are washed with saturated sodium bicarbonate dried over anhydrous sodium sulfate and evaporated to give crude 12 sublimation on a steam bath gives pure 12 a solution of the carboxylic acid 9 4 59 mmol in 15 ml of dry thf is stirred under dry argon and cooled to 0 u00b0 c a solution of 1 5 g 13 76 mmol of lithium diisopropylamide in 25 ml of dry thf under argon is added through a syringe to the solution of 9 at such a rate that the temperature does not rise above 10 u00b0 c the resulting solution of the dianion of 9 is stirred at 0 u00b0 c for about 3 h it is then cooled.