Cover Picture: Asymmetric Cation‐Olefin Monocyclization by Engineered Squalene–Hopene Cyclases (Angew. Chem. Int. Ed. 50/2021)

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

doi:10.1002/anie.202114610

Abstract

Sperm whale derived ambergris has historically been valued for its distinctive olfactory properties. Today, synthetic routes toward the contained (S)-y-dihydroionone and its optical antipode, emitting a floral orris-type odor, are of interest to the fragrance industry. In their Research Article on page 26080, Rebecca Buller et al. employ engineered squalene–hopene cyclases for the monocyclization of geometric geranylacetone isomers into y-dihydroionone antipodes.

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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Conclusion