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

Abstract

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.