Enzyme-catalyzed C-C bond formation using 2-methyltetrahydrofuran (2-MTHF) as (co)solvent: efficient and bio-based alternative to DMSO and MTBE

The enzymatic carboligation of aldehydes (C-C bond formation) catalyzed by benzaldehyde lyase (BAL) affords chiral alpha-hydroxy-ketones under mild reaction conditions in aqueous media. To enhance substrate and product availability under aqueous conditions, processes are often set-up using either DMSO as co-solvent, or MTBE as second organic phase. Although efficient, DMSO leads to difficulties in separation during downstream processing, with wastewater formation. MTBE provides a cleaner and straightforward work-up, but its petrochemical origin, together with its poor degradability, gives rise to environmental concerns. Herein it is reported that 2-methyltetrahydrofuran (2-MTHF) is a promising candidate to substitute DMSO or MTBE in lyase-catalyzed reactions. 2-MTHF can be derived from bio-based resources (e.g. levulinic acid), and it is abiotically degraded by air. When BAL is added to buffer-2-MTHF (5% v/v) mixtures, enzyme remains stable with a half-life of 178 +/- 8 h, with productivities (benzoin synthesis) of 10 g benzoin L(-1)h(-1). Several BAL-catalyzed aldehyde carboligations were assessed under those conditions, leading in all cases to high isolated yield (quantitative in majority), and to high enantioselectivity (up to >99%). Furthermore, preliminary results obtained with two phase systems in the BAL-catalyzed benzoin synthesis afforded 60 g benzoin L-1 in 24 h (ee > 99%). Therefore, 2-MTHF may be a valuable (co)solvent, not only to tackle environmental concerns, but also in terms of practical, efficient biocatalysis.