Syntheses and Crystal Structures of Three Chiral Oxazolidinones with Different Ring Conformations

The preparation of new, enantiomerically pure, α-amino acids from easily available starting materials is an ongoing challenge in synthetic organic chemistry. Here, we describe the syntheses and crystal structures of three chiral oxazolidinone derivatives prepared from L-alanine and pivalaldehyde to form a Schiff base intermediate and then reaction with the appropriate acid chloride to form the heterocycle. In each compound, the methyl and tert-butyl substituents lie to the same side of the molecule: these homochiral ‘cis’ structures were separated from their trans diastereomers by fractional crystallisation. The five-membered rings in these structures adopt various conformations including envelopes with either a C or O atom as the flap and twisted about a C–O bond. The extended structures of two of these compounds feature C(6) chains of molecules linked by C–H···O hydrogen bonds: one of these has a notably short H···O separation of 2.24 (3) Å.