Journal of Chemical Crystallography, Vol. 30, No. 9, 2000 Racemic and (+)-ethyl 2-acetamido-2-carboxy-5-oxohexanoate Daniel A. Osborne, (1) W. Reid Dreher, Jr., (1) H. Ray Gore, Jr., (1) Andrew Stoddard, (1) Edward J. Valente (1) * Received September 5, 2000 Racemic ethyl 2-acetamido-2-carboxy-5-oxohexanoate has been isolated by single hydrol- ysis of the corresponding diethyl ester and resolved essentially quantitatively by diastere- omeric salt formation with (−)-quinine. The (+)-isomer was retrieved from the less-soluble quininium salt. Racemate crystals, (±)-1, are monoclinic, space group P2 1 /c, a = 7.609(4) ˚ A, b = 16.731(7) ˚ A, c = 10.746(5) ˚ A, β = 99.83(4) ◦ ; enantiomeric crystals, (+)-1, are monoclinic, space group P2 1 , a = 5.857(2) ˚ A, b = 15.282(4) ˚ A, c = 7.618(2) ˚ A, 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 hydro- gen 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. KEY WORDS: Racemate; enantiomer; hydrogen-bonding; oxocarboxylic acid. Introduction Certain 4- and 5-oxocarboxylic acids can form cyclic pseudoacids. 1 Structures intervening between the two functional groups may improve the like- lihood of cyclization, such as ortho aryl and cis alkene disubstitution. 2 Among susceptible aliphatic oxocarboxylic acids, substitution along the aliphatic chain produces a relatively small influence on the open-cyclic equilibrium. 3,4 Cyclic structures can be stabilized in aliphatic oxocarboxylic acids by fa- voring cyclic conformations that diminish steric ef- fects of adjacent substituents. 1 As part of a study of 2-amino-5-oxocarboxylic acids, the structure of ethyl 2-acetamido-2-carboxy-5-oxohexanoate was ex- amined (1). This compound has four different func- tional groups (carboxylic acid, carboxylate ester, sec- ondary amide and ketone functions) substituted on a (1) Department of Chemistry, Mississippi College, Clinton, Missis- sippi 39058. * To whom correspondence should be addressed at e-mail valente@mc.edu single chiral carbon and its enantiomers may be useful chiral resolving agents. The resolution of 1, and struc- tures for both the racemate [(±)-1] and the resolved enantiomer [(+)-1] are reported. Experimental Solvents and reagents used were obtained from Aldrich Chemical Co., and were of the highest avail- able purity. Melting points (reported as endotherm peaks) were determined by differential scanning calorimetry on a Shimadzu DSC-50, standardized by 99.999% indium and tin; infrared spectra were recorded on a Perkin-Elmer 1600 FT-IR spectrom- eter on 2% solid solutions in dry KBr. Diethyl 2-acetamido-2-(3 ′ -oxobutyl)malonate (2) To a stirred solution of 10.0 g (46 mmol) diethyl 2-acetamidomalonate in absolute ethanol containing a small amount (<50 mg) of sodium ethoxide was added 3.23 g (46.0 mmol) methyl vinyl ketone under 583 1074-1542/00/0900-0583$18.00/0 C  2000 Plenum Publishing Corporation