Investigating the reaction mechanism and organocatalytic synthesis of α,α′-dihydroxy ketones

Abstract

A biomimetic TK one-pot reaction using hydroxypyruvate and aldehydes to generate α,α′-dihydroxy ketones in water has recently been described. To investigate this tertiary-amine mediated reaction mechanism two approaches were used. Firstly, ¹³C labelled lithium hydroxypyruvate was synthesised and used to establish where hydroxypyruvate is incorporated in the product. In separate experiments reaction intermediates were also successfully intercepted and structurally identified using ESI-MS with tandem mass spectrometry ESI-MS/MS. These studies indicated that two mechanisms appear to be operating, one involving the addition of the tertiary amine catalyst to hydroxypyruvate, the other an aldol-based mechanism. Since the first mechanism may enable facial stereodifferentiation in the addition of intermediates to the aldehyde, a preliminary study on the use of chiral catalysts was performed and the first asymmetric organocatalytic synthesis of α,α′-dihydroxy ketones in aqueous media achieved, in up to 50% ee, using a quinine ether catalyst.