Resolution of 5-hydroxymethyl-2-oxazolidinone by preferential crystallization[ 1 ] and investigations on the nature of the racemates of some 2-oxazolidinone derivatives

After ascertaining its conglomerate nature by DSC and solid-state IR analyses, 5-hydroxymethyl-2-oxazolidinone 1, whose enantiomers are very important synthons, was efficiently resolved without chiral auxiliaries by preferential crystallization from a supersaturated isopropanolic solution of the racemate, slightly enriched in one enantiomer (3.7% ee). Favourable conditions to the entrainment were defined utilizing the previously constructed ternary phase diagram {( R)-1, ( S)-1, 2-propanol}. Furthermore, the investigations were extended to other chiral 2-oxazolidinones with a functionalized methyl at the 5- or 4-position finding that 5-tosyloxymethyl-2-oxazolidinone is a racemic compound, whereas just the corresponding mesylate is a conglomerate as the parent alcohol 1. Interestingly, 4-hydroxymethyl-2-oxazolidinone 4 proved to be a racemic compound in contrast with its positional isomer 1 demonstrating how a relatively fine variation in the molecular structure can unpredictably influence the crystalline nature of the racemate. The X-ray structure determination carried out on ( S)-(+)-1, (±)-4 and ( R)-(+)-4 enlightened the importance of the hydrogen bond in determining different supramolecular assembling in the two homochiral compounds with respect to the racemic one and allowed a correlation with the stability of the crystal to be made.