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, dinaphthyl and ditolyl) have been synthesised. Studies on the benzylation of the lithium enolates of N-acyl derivatives reveal that the yields obtained were sensitive to the method of quenching the reaction. This was particularly acute for the 5,5-diphenyl system where effective yields (69%) and high diastereoselectivities (dr 98 2) are only observed when the reactions were quenched into aqueous buffer. Methylation studies on the N-acyl derivatives showed that the most advantageous results (58-69%, dr 91 9) were only observed using the sodium enolates. The 5,5-ditolyl-4-isopropyloxazolidin-2-one proved to be more efficacious in terms of efficiency and diastereoselectivity (dr 97 3). Subsequent, simple alkaline hydrolyses of the alkylation products allowed for the high recovery and recyclability of the 5,5-diaryl substituted oxazolidin-2-ones without any deleterious endocyclic cleavage. In addition, the acyl portions were recovered in high yield from the alkaline hydrolyses without any evidence of racemisation