Highly effective and recyclable chiral auxiliaries: a study of the synthesis and use of three 4-isopropyl-5,5-diaryloxazolidin-2-ones

A series of three 5,5-diaryl substituted oxazolidin-2-ones (diphenyl, dinap hthyl and ditolyl) have been synthesised. Studies on the benzylation of the lithium enolates of N-acyl derivatives reveal that the yields obtained wer e sensitive to the method of quenching the reaction. This was particularly acute for the 5,5-diphenyl system where effective yields (69%) and high dia stereoselectivities (dr 98:2) are only observed when the reactions were que nched into aqueous buffer. Methylation studies on the N-acyl derivatives sh owed that the most advantageous results (58-69%, dr greater than or equal t o 91:9) were only observed using the sodium enolates. The 5,5-ditolyl-4-iso propyloxazolidin-2-one proved to be more efficacious in terms of efficiency and diastereoselectivity (dr greater than or equal to 97:3). Subsequent, s imple alkaline hydrolyses of the alkylation products allowed for the high r ecovery and recyclability of the 5,5-diaryl substituted oxazolidin-2-ones w ithout any deleterious endocyclic cleavage. In addition, the acyl portions were recovered in high yield from the alkaline hydrolyses without any evide nce of racemisation.