Hydrochloric Acid → (9S,10S)-2-chloro-9,10-dihydro-9,10-methano-9-anthracenecarboxylic acid

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To a solution of racemic 2-chloro-9,10-dihydro-9,10-methano-9-anthracenecarboxylic acid (described in Example 107) (100 g, 0.37 mol) in ethyl acetate (1.5 L) and methanol (75 mL) was added solid (1S, 2S)-(+)-pseudoephedrine (61.1 g, 0.37 mol). With efficient agitation the mixture was warmed to reflux, held at reflux for 30 min and slowly cooled to 25° C. After a minimum of 2 h the slurry was filtered and washed with ethyl acetate to yield enriched diastereomeric salt (88.6 g, 0.20 mol, 55%; diastereomeric ratio 80:20 as determined by HPLC). The enriched salt was slurried in 3% methanolic ethyl acetate (2.74 L), warmed to reflux, and held at reflux for 30 min. The slurry was cooled to 25° C. slowly, stirred for 2 h, filtered, and washed with ethyl acetate to provide additional enriched salt (70 g, 0.16 mol, 79%, diastereomeric ratio 95:5 as determined by HPLC) Treatment of the enriched salt with 5% methanolic ethyl acetate using the same procedure yielded highly enriched salt (60.0 g, 0.14 mol, 85%, diastereomeric ratio 99:1 as determined by HPLC). This salt (60 g, 0.14 mol) was added to water (1 L) and the resulting suspension acidified to pH 2-3 with concentrated hydrochloric acid (15 mL) and then extracted with diethyl ether (3×500 mL). The combined organic extracts were washed with brine, dried over anhydrous magnesium sulfate and evaporated to an oil. Hexane was added and reduced in vacuo to afford enantiomerically enriched acid (36 g, 0.13 mol, 98% recovery, enantiomeric ratio 99:1 as determined by HPLC) as a white solid. Crystallisation from a mixture of hexane (360 mL) and cyclohexane (720 mL) afforded enantiomerically pure (9S,10S)-2-chloro-9,10-dihydro-9,10-methano-9-anthracenecarboxylic acid as a white solid (30 g, 0.11 mol, 81%) mp 172°-173° C. [α]D =+101° (c=2, CHCl3).

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