A New Racemic Conglomerate and Its Characterization by the Melting Point Diagram – Formation of the Metastable Racemic Compound
The highly branched ester rac-1 has been prepared. It exists in the form of a conglomerate (m.p. 90–92 °C) in which the two enantiomers are present in separate crystal lattices, as indicated by the melting point diagram. Both enantiomers of 1 have been isolated by crystallization [m.p. 112–114 °C, [α]D22 = +24.4 ± 0.7 (c = 0.45, Et2O), –25.4 ± 0.8 (c = 0.44, Et2O)]; mechanical manipulation of the conglomerate provided samples enriched in one or the other enantiomer and subsequent recrystallization of these partially resolved samples furnished the isolated enantiomers. The high value of the enthalpy of fusion of enantiomers 1 (ΔHfenant = 8.8 ± 1.4 kcal mol–1) and the moderate value of the corresponding entropy (ΔSfenant = 23 ± 4 cal K–1 mol–1), both derived from the melting point diagram, are ascribed to the relative compactness of the highly branched molecule and to its reduced conformational flexibility, respectively. Ester rac-1 has been serendipitously obtained in the form of a racemic compound (m.p. 82–84 °C) by crystallization of the racemate. The racemic compound transforms into the conglomerate at 70 °C. This transformation may be regarded as a spontaneous optical resolution occurring in the solid crystalline state.