Brucine → (R)-1',2',3',4'-tetrahydro-2'-hydroxy-5',8'-dimethoxyspiro[1,3-dithiolane-2,4'-naphthalene]-2'-carboxylic acid

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A suspension of 3.42 g of rac-1',2',3',4'-tetrahydro-2'-hydroxy-5',8'-dimethoxyspiro[1,3-dithiolane-2,4'-naphthalene]-2'-carboxylic acid [prepared as described in Example 4(iii)] and 4.0 g of brucine was heated under reflux until a clear solution was obtained. After seeding, the solution was left to cool slowly to room temperature. The crystalline precipitate (3.6 g) was collected after 2 days. The precipitate was dissolved in 1500 ml of boiling ethyl acetate, the solution was concentrated to 600 ml and left to cool slowly. The crystalline product [2.7 g [α]D20 =-46.6°, c=0.5% in dimethylformamide] was suspended in 150 ml of ethyl acetate and shaken with three 10 ml portions of 5-M hydrochloric acid and with two 100 ml portions of brine. After drying over magnesium sulphate, the solvent was evaporated to give a colourless oil which was crystallised from diethyl ether to yield 1.22 g of (R)-1',2',3',4'-tetrahydro-2'-hydroxy-5',8'-dimethoxyspiro[1,3-dithiolane-2,4'-naphthalene]-2'-carboxylic acid in the form of colourless crystals of melting point 147° -149° C.; [α]D20 =+13.8°, c=0.5% in dioxan.

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