Racemic and (+)-ethyl 2-acetamido-2-carboxy-5-oxohexanoate

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Racemic ethyl 2-acetamido-2-carboxy-5-oxohexanoate has been isolated by single hydrolysis of the corresponding diethyl ester and resolved essentially quantitatively by diastereomeric salt formation with (−)-quinine. The (+)-isomer was retrieved from the less-soluble quininium salt. Racemate crystals, (±)-1, are monoclinic, space group P21/c, a = 7.609(4)Å, b = 16.731(7)Å, c = 10.746(5) Å, β = 99.83(4)°; enantiomeric crystals, (+)-1, are monoclinic, space group P21, a = 5.857(2)Å, b = 15.282(4)Å, c = 7.618(2)Å, β = 95.62(2)°. Molecular packing is similar in the two structures; the enantiomer has a 0.7% higher volume per molecule and the lower fusion temperatures by 3°C. In both structures, carboxylic acids donate hydrogen bonds to the amide carbonyls of adjacent molecules without reciprocation. Amides form three-centered hydrogen bonds to carboxy oxygen of an intramolecular carboxylic acid group and to oxygen of a ketone carbonyl in an adjacent molecule.

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