reductase P2-G03, giving rise to g-lactone 7a in enantiopure form. The ADH evo030 distinguished itself with a great performance and the P2-G03/evo030/iPrOH assembly delivered the butenolide in 90% yield. Further refinement of the system by modulation of the redox mediator revealed the actual impact of isopropanol (Scheme 3c). In absence of any alcohol, the P2-G03/evo030 couple conducted the redox isomerization without significant ring contraction and high selectivity for the dlactone (operating through an oxidation-induced pathway, see Scheme 2b). However, already at 0.2% isopropanol, the desired g-lactone was produced as major entity, and at 2% of the redox mediator, an excellent selectivity of 95% 7a at 99% ee was observed. Further increase of the mediator led to overall inhibition of the process and somewhat surprisingly to a loss in selectivity.