Asymmetric Cation‐Olefin Monocyclization by Engineered Squalene–Hopene Cyclases

Michael Eichenberger,Raphael Berweger,Sandro Dossenbach,Rebecca Buller,Sean Hüppi,Uwe T Bornscheuer,Lucas Hortencio,Francis Voirol,Felix Flachsmann,Natalie Aeberli,Sabine Vollenweider,Eric Eichhorn,David Patsch

doi:10.1002/ange.202108037

Michael Eichenberger, Raphael Berweger + Show 11 more

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https://doi.org/10.1002/ange.202108037

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Abstract

AbstractSqualene–hopene cyclases (SHCs) have great potential for the industrial synthesis of enantiopure cyclic terpenoids. A limitation of SHC catalysis 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, an SHC‐wild‐type library including 18 novel homologs was set up. A previously not described SHC (AciSHC) was found to synthesize small amounts of monocyclic (R)‐γ‐dihydroionone from (E/Z)‐geranylacetone. Using enzyme and process optimization, the conversion to the desired product was increased to 79 %. Notably, analyzedAciSHC 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.

Highlights

Ionones are significant contributors to the appealing scents of many flowers and fruits, including violets, roses, or raspberries.[1]
In our quest to create an efficient biocatalyst for the enantioselective production ofionones, we aimed to identify an SHC enzyme with the capability to generate monocyclic products from either (E/Z)-geranylacetone (2) or (E/Z)-pseudoionone (1)
The screening library consisted of 13 previously characterized class II terpene cyclases from the SHC-family and 18 novel SHC homologs, which were identified through the presence of two defining PFAM domains for type II triterpene cyclases (PF13249, PF13243) and the SHC-family specific DXDD active site motif (Table S2)

Summary

Introduction

Ionones are significant contributors to the appealing scents of many flowers and fruits, including violets, roses, or raspberries.[1] They belong to a family of natural products known as apocarotenoids, which are derived from carotenoids by oxidative cleavage catalyzed by carotenoid oxygenases.[2] An efficient synthetic access to racemic ionones by cationolefin cyclization of pseudoionone (1) was discovered already in the late 19th century by Tiemann and Krüger (Scheme 1).[3] ionones were among the first commercially. Cation-olefin cyclizations of pseudoionone (1) and geranylacetone (2) to racemic ionones (3) and bicyclic enolether (4), respectively. The natural (S)-(+)-g-dihydroionone ((S)-5) occurs for example in Ambergris and is of interest as an intermediate for the synthesis of (À)-aambrinol (6), which exhibits a highly appreciated animalic scent typical for aged Ambergris tincture (Scheme 2)

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