Oxalic Acid → Oxalic acid--tert-butyl (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylate (1/1)

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Reaction Conditions

Reaction Steps

97.9 g (463.3 mmol) of (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid t-butyl ester from the two preparations according to Step 1 were dissolved in 750 mL t-butyl acetate and charged to a 3 L four-neck flask equipped with overhead mechanical stirring, a thermometer, addition funnel, and reflux condenser. With stirring at ambient temperature (−21° C.), a solution of 44.0 g (488.6 mmol) of oxalic acid in 750 mL 2-propanol was added dropwise over 37 minutes, increasing the mixture's temperature to 31° C. Solids began to precipitate after addition of ˜50 mL of the oxalic acid solution, and resulted in a thick suspension after the addition of 450 mL. After addition of 500 mL of the oxalate solution, the precipitated solids redissolved to provide a dark yellow solution. Solids precipitated again rapidly after the addition of 600 mL of the oxalic solution and persisted through the end of the oxalic acid addition. This suspension was then heated 78° C. to provide a thin suspension which was allowed to cool passively with stirring to ambient temperature (˜21° C.). After 16 hours since the cooling began, the precipitated solids were collected by filtration and washed successively with isopropanol (450 mL), isopropyl acetate (450 mL), and methyl t-butyl methyl ether (450 mL). The solids were dried in a vacuum oven (30° C., 25″ vacuum, N2 stream) to provide 118.1 g (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid t-butyl ester oxalic acid 1:1 salt (64% yield from (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid hydrochloride) as a dense, tan free flowing powder (99.7% purity by GC analysis), which exhibited the expected 1H-NMR spectrum for (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid t-butyl ester oxalic acid (1:1) salt.

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