Expanding the Cation Cage: Squalene-Hopene Cyclase-Mediated Enantioselective Semipinacol Rearrangement

Abstract

Squalene-hopene cyclases (SHCs) are the biocatalytic pendant to asymmetric Brønsted-acid catalysis and thus comprise enormous synthetic potential. Nevertheless, their substrate scope is currently limited to terpenes. Herein, we present how to tailor the SHC's cation cage for an enantioselective semipinacol rearrangement of bicyclic allylic alcohols to produce valuable oxa-carbon spirocyclic compounds. Exploiting the subtle divergence of SHC active sites combined with structure-guided semirational engineering, we designed a biocatalyst with a high catalytic performance of ∼4500 TTN and excellent enantioselectivity of 99.5% enantiomeric excess (ee). In silico studies suggest that a broadened active site is pivotal for catalysis. This intricate cationic rearrangement is easily scalable, employing lyophilized cell powder in water. Furthermore, our substrate scope studies demonstrate the acceptance of diverse ring-sized substrates but also reveal that the naturally confined active site limits the function as a general "semipinacolase." This study showcases the ability to harness the SHC's cation cage to tap into the broader field of asymmetric Brønsted-acid catalysis.

Hot off the Press

“…30 Whole-cell, preparativescale hydroxylation of a 3-methylbenzaldehyde intermediate was achieved using an a-ketoglutarate (a-KG)-dependent nonhaem iron (NHI) oxygenase, ClaD, in excellent yield (Scheme 3). An ortho-quinone methide intermediate was generated from this product and captured in a [4 + 2]cycloaddition reaction to generate three rings of (+)-xyloketal B (28).…”

“…Using structure-guided semirational engineering, a squalene-hopene cyclase (SHC) has been designed that can perform enantioselective semipinacol rearrangements of bicyclic allylic alcohols. 28 Expansion of the SHC active site allowed more effective Brønsted acid catalysis and the development of scalable transformations that generate important oxa-carbon spirocyclic compounds with excellent enantioselectivity (Scheme 1). …”

A Career in Catalysis: Bernhard Hauer

“…68 It should be noted that SHCs comprise a broad range in sequence identity but are very similar in their active sites. Taking advantage of this fact, the group of Bernhard improved the activity and selectivity of SHCs in the Friedel− Crafts alkylation 69 and semipinacol rearrangement of allylic alcohol 33 70 (Figure 3B), respectively, by transferring beneficial mutations from one homologue to another, thus allowing more significant jumps in sequence space. In the latter, biocatalytic turnover was improved 174-fold, and docking studies suggest that a broadened active site is key to the selective rearrangement.…”

Harnessing the Structure and Dynamics of the Squalene‐Hopene Cyclase for (−)‐Ambroxide Production**

“…Their unique catalysis is facilitated by a strongly confined cation cage, which nature evolved to perfectly chaperone their substrates and guide the transient cations during cyclization [3] . As a consequence, cyclase enzymologists primarily focused on studying the active site, to shed light on the mechanistic details of a cationic cyclization cascade [4–6] or to expand the substrate and reaction scope, for example, of the squalene‐hopene cyclase (SHC) [7, 8] . Despite substantial progress in this field, a persistent limitation in applying cyclases is their generally low catalytic performance (total turnover numbers, TTNs<10 3 ) [1, 2, 9, 10] as compared to other enzyme families, such as monooxygenases, transaminases or lipases with TTNs in the range of 10 5 –10 7 [11] .…”