Asymmetric Cation-Olefin Monocyclization by Engineered Squalene-Hopene Cyclases

Squalene-hopene cyclases (SHCs) have great potential for the industrial synthesis of enantiopure cyclic terpenoids. A limitation of SHC catalysis so far, however, has been the enzymes' strict ( S )-enantioselectivity at the stereocenter formed after the first cyclization step. To gain enantio-complementary access to valuable monocyclic terpenoids such as ( R )- and ( S )-γ-dihydroionone, an SHC wild-type library including 18 novel homologs was set up. A previously not described SHC from Acidothermus cellulolyticus ( Aci SHC) was found to synthesize small amounts of the desired monocyclic ( R )-γ-dihydroionone from ( E/Z )-geranylacetone. Using enzyme and process optimization, the conversion to the desired product was increased by two orders of magnitude to 79%. Notably, analyzed Aci SHC variants could finely differentiate between the geometric geranylacetone isomers: While the Z -isomer yielded the desired monocyclic ( R )-γ-dihydroionone (>99% ee ), the E -isomer was converted to the ( S , S )-bicyclic ether (>95% ee ). Applying the knowledge gained from the observed stereodivergent and enantioselective transformations to an additional SHC-substrate pair, access to the complementary ( S )-γ-dihydroionone (>99.9% ee ) could be obtained through substrate engineering.