Applications of biocatalytic C=C bond reductions in the synthesis of flavours and fragrances

Industrial biotechnology and biocatalysis can provide very effective synthetic tools to increase the sustainability of the production of fine chemicals, especially flavour and fragrance (F&F) ingredients, the market demand of which has been constantly increasing in the last years. One of the most important transformations in F&F chemistry is the reduction of C[dbnd]C bonds, typically carried out with metal-catalysed hydrogenations or hydride-based reagents. Its biocatalytic counterpart is a competitive alternative, showcasing a range of advantages such as excellent chemo-, regio- and stereoselectivity, ease of implementation, mild reaction conditions and modest environmental impact. In the present review, the application of biocatalysed alkene reductions (from microbial fermentations with wild-type strains to engineered isolated ene-reductase enzymes) to synthetic processes useful for the F&F industry will be described, highlighting not only the exquisite stereoselectivity achieved, but also the overall improvement when chirality is not involved. Multi-enzymatic cascades involving C[dbnd]C bioreductions are also examined, which allow much greater chemical complexity to be built in one-pot biocatalytic systems.