Process development for enzyme catalysed asymmetric C–C-bond formation

Benzaldehyde lyase (BAL, EC: 4.1.2.38) is a highly enantioselective enzyme, which catalyses the formation and cleavage of (R)-benzoin derivatives. The asymmetric synthesis of (R  )-3,3′3,3′-dimethoxybenzoin (2a) and (R  )-3-methoxy-2′2′-chlorobenzoin (2b) was investigated by means of reaction engineering and a reactor concept for the preparative synthesis was developed. For the synthesis of (2b), the optimal ratio of 3-methoxybenzaldehyde (1a) and 2-chlorobenzaldehyde (1b) was found to be equimolar.An aqueous–organic two-phase system was found to be the best reaction system. The aqueous phase consists of phosphate buffer; the organic phase is formed by the substrate. As the catalyst is hydrophilic, the reaction takes place in the aqueous phase. As a result of their low solubility in the aqueous media the formed benzoins precipitate. Centrifugation and filtration were used to separate the product from the reaction media. In order to eliminate the by-products (2b) was recrystallised. The synthesis of (2a) was carried out with a high volumetric productivity of 2304gL-1d-1. In a further experiment the enzyme consumption could be minimised (ttn=250ttn=250,100). Twelve gram of (2a) (yield 82%, purity 97%, ee 99%) and 19.5 g of (2b) (yield 86%, purity 97%, ee 99%) were isolated.