Enantioselective Intermolecular Aldehyde–Ketone Cross‐Coupling through an Enzymatic Carboligation Reaction

Abstract

Thiamine diphosphate (ThDP)-dependent enzymes are wellestablished catalysts in the field of asymmetric synthesis. [1] One of the model reactions catalyzed by these enzymes is the asymmetric CÀC bond-formation reaction between two aldehyde substrates that leads to the formation of 2-hydroxyketones in high enantioselectivities. [2] Exchange of one of the aldehyde substrates in this reaction for a ketone [3] would offer the opportunity for the catalytic asymmetric formation of chiral tertiary alcohols, which are important structural units in natural products and bioactive agents. [4,5] During the last decade, different organocatalysts have been developed for the asymmetric aldehyde-ketone cross-coupling reaction, [6] and intramolecular variants of this reaction have been reported in the literature. [7,8] Most recently, Enders and Henseler described the direct intermolecular cross-coupling between aldehydes and trifluoromethyl ketones using a bicyclic triazolium salt as the catalyst. [9] The asymmetric intermolecular non-enzymatic coupling reaction with ketone acceptors should be more difficult, owing to the lower electrophilicity of the ketone carbonyl group, and the increased steric hindrance compared with aldehyde substrates, and has not yet been reported in the literature. Herein, we present an asymmetric intermolecular aldehyde-ketone carboligation reaction using a ThDP-dependent enzyme as the catalyst (Scheme 1).Branched-chain sugars are important bioactive carbohydrates and are widely represented in nature. Using feeding experiments, it can be shown that the two-carbon branch of several of these sugar derivatives is derived from pyruvate. In 1972, Grisebach and Schmid postulated the participation of ThDP in this carboligation reaction. [10] In the biosynthetic pathway of yersiniose A, a two-carbon branched-chain 3,6di(deoxy)hexose that is found in the O-antigen of Yersinia pseudotuberculosis O:VI, the ThDP-dependent flavoenzyme YerE catalyzes the decarboxylation of pyruvate and the transfer of the activated acetaldehyde onto the carbonyl function of CDP-3,6-di(deoxy)-4-keto-d-glucose (CDP = cytidine-5'-diphosphate). [11,12] The enzymatic activity of YerE was confirmed by incubation of the protein with the enzymatically prepared physiological substrate (starting from CDP-d-glucose). The isolated product CDP-4-aceto-3,6dideoxygalactose was confirmed by NMR spectroscopy.To analyze the substrate range of the enzyme, we amplified the gene yerE from the chromosomal DNA of Y. pseudotuberculosis O:VI using a polymerase chain reaction. The gene was cloned into the pQE-60 expression vector and the recombinant protein was produced in Escherichia coli BL21(DE3) cells. The overexpressed C-terminal His-tagged YerE protein was purified to homogeneity by affinity chromatography using an Ni-NTA purification system (see the Supporting Information).The amino acid sequence of YerE is similar to that of the large subunit of E. coli acetohydroxyacid synthases (EcAHAS) (31 % identity). AHAS successfully catalyzed the ThDP-dependent ...

Catalytic Scope of the Thiamine‐Dependent Multifunctional Enzyme Cyclohexane‐1,2‐dione Hydrolase

“…All 18 PAC products with determined ee values (Table 1) were assigned as R-configured, as judged by the appearance of a negative band centered at 270-290 nm in circular dichroism spectra. [13,14] All 18 products were highly enantioenriched, with ee values in the range of 92-99 %.…”

Structural and Mutagenesis Studies of the Thiamine‐Dependent, Ketone‐Accepting YerE from Pseudomonas protegens

“…YerE from Yersinia pseudotuberculosis (YpYerE) was the firstcharacterized biocatalyst for thiamine diphosphate (ThDP)-dependenta ldehyde-ketonec ross-coupling. [1,2] YpYerE is involved in the biosynthesis of the branched-chain sugar yersiniose A, which is part of the O-antigen of the host and other bacteria. [1,3] The physiologically catalyzed reactioni st he transfer of an activateda cetaldehyde,g enerated by decarboxylation of pyruvate, to a3 ,6-dideoxy-4-keto sugar.…”

“…[1] In addition to pyruvate, YpYerE accepts 2-oxobutyrate and acetaldehyde as donors and ab road range of different acceptors ubstrates with moderate to high enantioselectivity. [2] In addition to YpYerE, other ThDP-dependent enzymes are able to accept ketones.A cetohydroxy acid synthases (AHASs) are al arge group of ThDP-dependent enzymes that are distantly relatedt oY erE. Physiologically,t hey catalyzet he synthesis of acetolactate anda cetohydroxy acids.…”

“…Conversions [%] weredeterminedafter 24 hbyG C-MSa nalysis; ee values [%] (in brackets)w ere determined by chiral-phase HPLCa nalysis (n.d.:n ot determined).R esults for YpYerE were published by Lehwaldetal. and added for comparison [2].…”

The Enzymatic Asymmetric Conjugate Umpolung Reaction

“…This is in contrast to YerE, another ThDP-dependent enzyme isolated from Yersinia pseudotuberculosis. [11] Although both YerE and PigD catalyze 1,2-additions using 2-oxoacids as donor substrates and aldehydes as acceptors, in the case of ketones as acceptors YerE selectively catalyzes the 1,2-addition, whereas PigD catalyzes only the 1,4-addition. Interestingly, not only aliphatic, but also aromatic and some heterocyclic a,bunsaturated ketones selectively reacted with pyruvate in the presence of PigD to give 1,4-adducts (Scheme 1, Table 1).…”