High-yield synthesis of 2-O-α-D-glucosyl-D-glycerate by a bifunctional glycoside phosphorylase.

Osmolytes are produced by various microorganisms as a defense mechanism to protect cells and macromolecules from damage caused by external stresses in harsh environments. Due to their useful stabilizing properties, these molecules are applied as active ingredients in a wide range of cosmetics and healthcare products. The metabolic pathways and biocatalytic syntheses of glycosidic osmolytes such as 2-O-α-D-glucosyl-D-glycerate often involve the action of a glycoside phosphorylase. Here, we report the discovery of a glucosylglycerate phosphorylase from carbohydrate-active enzyme family GH13 that is also active on sucrose, which contrasts the strict specificity of known glucosylglycerate phosphorylases that can only use α-D-glucose 1-phosphate as glycosyl donor in transglycosylation reactions. The novel enzyme can be distinguished from other phosphorylases from the same family by the presence of an atypical conserved sequence motif at specificity-determining positions in the active site. The promiscuity of the sucrose-active glucosylglycerate phosphorylase can be exploited for the high-yielding and rapid synthesis of 2-O-α-D-glucosyl-D-glycerate from sucrose and D-glycerate. KEY POINTS: • A Xylanimonas protaetiae glycoside phosphorylase can use both d-glycerate and fructose as glucosyl acceptor with high catalytic efficiency • Biocatalytic synthesis of the osmolyte 2-O-α-d-glucosyl-d-glycerate • Positions in the active site of GH13 phosphorylases act as convenient specificity fingerprints.

Engineering Comamonas testosteroni for the production of 2-pyrone-4,6-dicarboxylic acid as a promising building block.

BACKGROUND: Plastics are an indispensable part of our daily life. However, mismanagement at their end-of-life results in severe environmental consequences. The microbial conversion of these polymers into new value-added products offers a promising alternative. In this study, we engineered the soil-bacterium Comamonas testosteroni KF-1, a natural degrader of terephthalic acid, for the conversion of the latter to the high-value product 2-pyrone-4,6-dicarboxylic acid. RESULTS: In order to convert terephthalic acid to 2-pyrone-4,6-dicarboxylic acid, we deleted the native PDC hydrolase and observed only a limited amount of product formation. To test whether this was the result of an inhibition of terephthalic acid uptake by the carbon source for growth (i.e. glycolic acid), the consumption of both carbon sources was monitored in the wild-type strain. Both carbon sources were consumed at the same time, indicating that catabolite repression was not the case. Next, we investigated if the activity of pathway enzymes remained the same in the wild-type and mutant strain. Here again, no statistical differences could be observed. Finally, we hypothesized that the presence of a pmdK variant in the degradation operon could be responsible for the observed phenotype and created a double deletion mutant strain. This newly created strain accumulated PDC to a larger extent and again consumed both carbon sources. The double deletion strain was then used in a bioreactor experiment, leading to the accumulation of 6.5 g/L of product in 24 h with an overall productivity of 0.27 g/L/h. CONCLUSIONS: This study shows the production of the chemical building block 2-pyrone-4,6-dicarboxylic acid from terephthalic acid through an engineered C. testosteroni KF-1 strain. It was observed that both a deletion of the native PDC hydrolase as well as a pmdK variant is needed to achieve high conversion yields. A product titer of 6.5 g/L in 24 h with an overall productivity of 0.27 g/L/h was achieved.

Selective pharmacological inhibitors of HDAC6 reveal biochemical activity but functional tolerance in cancer models.

Our study investigates the biochemical and functional impact of selective histone deacetylase 6 (HDAC6) inhibitors, a promising class of novel therapeutics, in several cancer models. Selective HDAC6 inhibitors (Tubathian A, Tubastatin A, Tubacin and Ricolinostat) and a non-selective HDAC inhibitor (Vorinostat) were evaluated on cancer cell lines derived from multiple tumour types in both an in vitro and in vivo setting as potential cancer therapeutics. Selective HDAC6 inhibitors resulted in α-tubulin acetylation with no impact on histone acetylation but failed to show any anti-cancer properties. Only the use of high concentrations of selective HDAC6 inhibitors resulted in co-inhibition of other HDAC enzymes and consequently in reduced growth, migratory and/or invasive activity of cancer cells in vitro as well as in vivo. The specificity of HDAC6 inhibition was confirmed using a CRISPR/Cas9 knockout cell line. Our results suggest that selective HDAC6 inhibitors may fall short as potential single agent anti-cancer drugs and prove that many previous data regarding this promising class of compounds need to be interpreted with great care due to their use in high concentrations resulting in low selectivity and potential off-target effects.

Synthesis of 1,4-Thiazepane-Based Curcuminoids with Promising Anticancer Activity.

Curcumin, the main component of turmeric (Curcuma longa) is known to display an interesting bioactivity profile, including pronounced anticancer properties. However, its low bioavailability, metabolic instability and nonspecific activity are concerns that have to be addressed before curcuminoids can be considered for therapeutic applications. Within that framework, intensive research has been carried out in the last decades to develop new curcumin derivatives, generally centered on standard modifications of the sp2 curcumin framework, with the aim to augment its bioavailability while maintaining or improving its anticancer properties. To find potential hit molecules by moving away from the classical flat curcumin framework, we investigated an unexplored modification to produce novel, out-of-plane 1,4-thiazepane-based curcuminoids and assessed the impact of this modification on the biological activity. In this way, 21 new, structurally diverse thiazepane scaffolds (4-aryl-1-(7-aryl-1,4-thiazepan-5-ylidene)but-3-en-2-ones) were synthesized, as well as some biologically interesting unexpected reaction products (such as 5-aryl-6-arylmethylene-3-ethoxycyclohex-2-en-1-ones and 4-acetyl-5-aryl-2-(3-arylacryloyl)-3-methylcyclohex-2-en-1-ones). All these analogues were subsequently tested on their antioxidant capacity, their cytotoxicity properties and their ROS (reactive oxygen species) production. Many compounds demonstrated interesting activities, with ten curcuminoids, whereof eight 1,4-thiazepane-based, showing better antiproliferative properties compared to their mother compounds, as well as an increased ROS production. This unprecedented 3D curcumin modification has thus delivered promising new hit compounds with good activity profiles eligible for further exploration.

α-Unsaturated 3-Amino-1-carboxymethyl-ß-lactams as Bacterial PBP Inhibitors: Synthesis and Biochemical Assessment.

Innovative monocyclic ß-lactam entities create opportunities in the battle against resistant bacteria because of their PBP acylation potential, intrinsically high ß-lactamase stability and compact scaffold. α-Benzylidene-substituted 3-amino-1-carboxymethyl-ß-lactams were recently shown to be potent PBP inhibitors and constitute eligible anchor points for synthetic elaboration of the chemical space around the central ß-lactam ring. The present study discloses a 12-step synthesis of ten α-arylmethylidenecarboxylates using a microwave-assisted Wittig olefination as the crucial reaction step. The library was designed aiming at enhanced ß-lactam electrophilicity and extended electron flow after enzymatic attack. Additionally, increased ß-lactamase stability and intermolecular target interaction were envisioned by tackling both the substitution pattern of the aromatic ring and the ß-lactam C4-position. The significance of α-unsaturation was validated and the R39/PBP3 inhibitory potency shown to be augmented the most through decoration of the aromatic ring with electron-withdrawing groups. Furthermore, ring cleavage by representative ß-lactamases was ruled out, providing new insights in the SAR landscape of monocyclic ß-lactams as eligible PBP or ß-lactamase inhibitors.

Electrophilic Bromination in Flow: A Safe and Sustainable Alternative to the Use of Molecular Bromine in Batch.

Bromination reactions are crucial in today's chemical industry since the versatility of the formed organobromides makes them suitable building blocks for numerous syntheses. However, the use of the toxic and highly reactive molecular bromine (Br2) makes these brominations very challenging and hazardous. We describe here a safe and straightforward protocol for bromination in continuous flow. The hazardous Br2 or KOBr is generated in situ by reacting an oxidant (NaOCl) with HBr or KBr, respectively, which is directly coupled to the bromination reaction and a quench of residual bromine. This protocol was demonstrated by polybrominating both alkenes and aromatic substrates in a wide variety of solvents, with yields ranging from 78% to 99%. The protocol can easily be adapted for the bromination of other substrates in an academic and industrial environment.

Antibacterial and ß-Lactamase Inhibitory Activity of Monocyclic ß-Lactams.

Due to the widespread emergence of resistant bacterial strains, an urgent need for the development of new antibacterial agents with novel modes of action has emerged. The discovery of naturally occurring monocyclic ß-lactams in the late 1970s, mainly active against aerobic Gram-negative bacteria, has introduced a new approach in the design and development of novel antibacterial ß-lactam agents. The main goal was the derivatization of the azetidin-2-one core in order to improve their antibacterial potency, broaden their spectrum of activity, and enhance their ß-lactamase stability. In that respect, our review covers the updates in the field of monocyclic ß-lactam antibiotics during the last three decades, taking into account an extensive collection of references. An overview of the relationships between the structural features of these monocyclic ß-lactams, classified according to their N-substituent, and the associated antibacterial or ß-lactamase inhibitory activities is provided. The different paragraphs disclose a number of well-established classes of compounds, such as monobactams, monosulfactams, monocarbams, monophosphams, nocardicins, as well as other known representative classes. Moreover, this review draws attention to some less common but, nevertheless, possibly important types of monocyclic ß-lactams and concludes by highlighting the recent developments on siderophore-conjugated classes of monocyclic ß-lactams.

In Silico Design and Enantioselective Synthesis of Functionalized Monocyclic 3-Amino-1-carboxymethyl-ß-lactams as Inhibitors of Penicillin-Binding Proteins of Resistant Bacteria.

As a complement to the renowned bicyclic ß-lactam antibiotics, monocyclic analogues provide a breath of fresh air in the battle against resistant bacteria. In that framework, the present study discloses the in silico design and unprecedented ten-step synthesis of eleven nocardicin-like enantiomerically pure 2-{3-[2-(2-aminothiazol-4-yl)-2-(methoxyimino)acetamido]-2-oxoazetidin-1-yl}acetic acids starting from serine as a readily accessible precursor. The capability of this novel class of monocyclic 3-amino-ß-lactams to inhibit penicillin-binding proteins (PBPs) of various (resistant) bacteria was assessed, revealing the potential of α-benzylidenecarboxylates as interesting leads in the pursuit of novel PBP inhibitors. No deactivation by representative enzymes belonging to the four ß-lactamase classes was observed, while weak inhibition of class C ß-lactamase P99 was demonstrated.

Theoretical insight into the regioselective ring-expansions of bicyclic aziridinium ions.

Transient bicyclic aziridinium ions are known to undergo ring-expansion reactions, paving the way to functionalized nitrogen-containing heterocycles. In this study, the regioselectivity observed in the ring-expansion reactions of 1-azoniabicyclo[n.1.0]alkanes was investigated from a computational viewpoint to study the ring-expansion pathways of two bicyclic systems with different ring sizes. Moreover, several nucleophiles leading to different experimental results were investigated. The effect of solvation was taken into account using both explicit and implicit solvent models. This theoretical rationalization provides valuable insight into the observed regioselectivity and may be used as a predictive tool in future studies.

Exploring the sequence diversity in glycoside hydrolase family 13_18 reveals a novel glucosylglycerol phosphorylase.

In the carbohydrate-active enzyme database, GH13_18 is a family of retaining glycoside phosphorylases that act on α-glucosides. In this work, we explored the functional diversity of this family by comparing distinctive sequence motifs in different branches of its phylogenetic tree. A glycoside phosphorylase from Marinobacter adhaerens HP15 that was predicted to have a novel function was expressed and characterised. The enzyme was found to catalyse the reversible phosphorolysis of 2-O-α-D-glucosylglycerol with retention of the anomeric configuration, a specificity that has never been described before. Homology modelling, docking and mutagenesis were performed to pinpoint particular acceptor site residues (Tyr194, Ala333, Gln336) involved in the binding of glycerol. The new enzyme specificity provides additional insights into bacterial metabolic routes, being the first report of a phosphorolytic route for glucosylglycerol in a glucosylglycerol-producing organism. Furthermore, glucosylglycerol phosphorylase might be an attractive biocatalyst for the production of the osmolyte glucosylglycerol, which is currently produced on industrial scale by exploiting a side activity of the closely related sucrose phosphorylase. Family GH13_18 has clearly proven to be more diverse than was initially assumed, and the analysis of specificity-determining sequence motifs has shown to be a straightforward and fruitful tool for enzyme discovery.

Synthesis of Potent and Selective HDAC6 Inhibitors Bearing a Cyclohexane- or Cycloheptane-Annulated 1,5-Benzothiazepine Scaffold.

Selective inhibitors of histone deacetylase 6 (HDAC6) are an emerging class of pharmaceuticals due to the involvement of HDAC6 in different pathways related to neurodegenerative diseases, cancer, and immunology. Herein, the synthesis of ten new benzohydroxamic acids, constructed by employing the tetrahydrobenzothiazepine core as a privileged pharmacophoric unit, is described. This is the first report on the synthesis and isolation of octahydrodibenzothiazepines and octahydro-6H-benzocycloheptathiazepines, which were then used to develop a new class of HDAC6 inhibitors. Evaluations of their HDAC-inhibiting activity resulted in the identification of cis-N-(4-hydroxycarbamoylbenzyl)-1,2,3,4,4a,5,11,11a-octahydrodibenzo[b,e][1,4]thiazepine-10,10-dioxide and cis-N-(4-hydroxycarbamoylbenzyl)-7-trifluoromethyl-1,2,3,4,4a,5,11,11a-octahydrodibenzo[b,e][1,4]thiazepine-10,10-dioxide as highly potent and selective HDAC6 inhibitors with activity in the low nanomolar range, which also show excellent selectivity on the enzymatic and cellular levels. Furthermore, four promising inhibitors were subjected to an Ames fluctuation assay, which revealed no mutagenic effects associated with these structures.

Reactivity of 3-Oxo-ß-lactams with Respect to Primary Amines-An Experimental and Computational Approach.

The reactivity of 3-oxo-ß-lactams with respect to primary amines was investigated in depth. Depending on the specific azetidin-2-one C4 substituent, this reaction was shown to selectively produce 3-imino-ß-lactams (through dehydration), α-aminoamides (through CO elimination), or ethanediamides (through an unprecedented C3-C4 ring opening). In addition to the experimental results, the mechanisms and factors governing these peculiar transformations were also examined and elucidated by means of DFT calculations.

Asymmetric Synthesis of 3,4-Disubstituted 2-(Trifluoromethyl)pyrrolidines through Rearrangement of Chiral 2-(2,2,2-Trifluoro-1-hydroxyethyl)azetidines.

Enantiopure 4-formyl-ß-lactams were deployed as synthons for the diastereoselective formation of chiral 2-(2,2,2-trifluoro-1-hydroxyethyl)azetidines via trifluoromethylation through aldehyde modification followed by reductive removal of the ß-lactam carbonyl moiety. Subsequent treatment of the (in situ) activated 2-trifluoroethylated azetidines with a variety of nitrogen, oxygen, sulfur, and fluorine nucleophiles afforded chiral 3,4-disubstituted 2-(trifluoromethyl)pyrrolidines in good to excellent yields (45-99%) and high diastereoselectivities (dr >99/1, 1H NMR) via interception of bicyclic aziridinium intermediates. Furthermore, representative pyrrolidines were N,O-debenzylated in a selective way and used for further synthetic elaboration to produce, for example, a CF3-substituted 2-oxa-4,7-diazabicyclo[3.3.0]octan-3-one system.

Cobalt carbonyl-catalyzed carbonylation of functionalized aziridines to versatile ß-lactam building blocks.

The Co2(CO)8-catalyzed carbonylation of different classes of non-activated aziridines with diverse substitution patterns was investigated. Special attention was devoted to selectivity issues and reaction optimization. This study resulted in the regio- and stereospecific synthesis of 24 novel ß-lactam target structures in high yields on a multigram scale. The synthetic potential of the newly obtained azetidin-2-ones was illustrated via ring-expansion, ring-closure, and/or side chain-functionalization protocols to provide a straightforward entry to novel pyrrolidines, C-fused bi- and tricyclic ß-lactams and monocyclic carbapenem analogs.

LiAlH4 -Induced Selective Ring Rearrangement of 2-(2-Cyanoethyl)aziridines toward 2-(Aminomethyl)pyrrolidines and 3-Aminopiperidines as Eligible Heterocyclic Building Blocks.

2-(2-Cyanoethyl)aziridines and 2-aryl-3-(2-cyanoethyl)aziridines were deployed as substrates for an In(OTf)3 -mediated regio- and stereoselective ring rearrangement upon treatment with LiAlH4, affording a variety of novel 2-(aminomethyl)pyrrolidines and 3-aminopiperidines, respectively. Further synthetic elaboration of the obtained 3-aminopiperidines resulted in the formation of a peculiar and unexplored conformationally constrained imidazolidinone and diketopiperazine scaffold.

Synthesis and biological assessment of novel N-(hydroxy/methoxy)alkyl ß-enaminone curcuminoids.

Curcumin, a natural compound extracted from the rhizomes of Curcuma Longa, is known to display pronounced anticancer activity but lacks good pharmacokinetic properties. In that respect, augmenting the water solubility by structural modification of the curcumin scaffold may result in improved bioavailability and pharmacokinetics. A possible scaffold modification, especially important for this study, concerns the imination of the labile ß-diketone moiety in curcumin. Previous work revealed that novel N-alkyl ß-enaminones showed a similar water solubility as compared to curcumin, accompanied by a stronger anti-proliferative activity. To extend this ß-enaminone compound library, new analogues were prepared in this work using more polar amines (hydroxyalkylamines and methoxyalkylamines instead of alkylamines) with the main purpose to improve the water solubility without compromising the biological activity of the resulting curcuminoids. Compared to their respective parent compounds, i.e. curcumin and bisdemethoxycurcumin, the bisdemethoxycurcumin N-(hydroxy/methoxy)alkyl enaminone analogues showed better water solubility, antioxidant and anti-proliferative activities. In addition, the curcumin enaminones displayed activities comparable to or better than curcumin, and the water solubility was improved significantly. The constructed new analogues may thus be of interest for further exploration concerning their impact on oxidative stress related diseases such as cancer.

Diastereoselective synthesis of 3-acetoxy-4-(3-aryloxiran-2-yl)azetidin-2-ones and their transformation into 3,4-oxolane-fused bicyclic ß-lactams.

cis-3-Acetoxy-4-(3-aryloxiran-2-yl)azetidin-2-ones were prepared through a Staudinger [2+2]-cyclocondensation between acetoxyketene and the appropriate epoxyimines in a highly diastereoselective way. Subsequent potassium carbonate-mediated acetate hydrolysis, followed by intramolecular ring closure through epoxide ring opening, afforded stereodefined 3-aryl-4-hydroxy-2-oxa-6-azabicyclo[3.2.0]heptan-7-ones as a novel class of C-fused bicyclic ß-lactams. Selective benzylic oxidation of bicyclic N-(4-methoxybenzyl)-ß-lactams with potassium persulfate and potassium dihydrogen phosphate provided the corresponding N-aroyl derivatives as interesting leads for further ß-lactamase inhibitor development.

Synthesis and SAR assessment of novel Tubathian analogs in the pursuit of potent and selective HDAC6 inhibitors.

The synthesis of novel isoform-selective HDAC inhibitors is considered to be an important, emerging field in medicinal chemistry. In this paper, the preparation and assessment of thirteen selective HDAC6 inhibitors is disclosed, elaborating on a previously developed thiaheterocyclic Tubathian series. All compounds were evaluated in vitro for their ability to inhibit HDAC6, and a selection of five potent compounds was further screened toward all HDAC isoforms (HDAC1-11). The capability of these Tubathian analogs to inhibit α-tubulin deacetylation was assessed as well, and ADME/Tox data were collected. This thorough SAR evaluation revealed that the oxidized, para-substituted hydroxamic acids can be recognized as valuable lead structures in the pursuit of novel potent and selective HDAC6 inhibitors.

Synthesis and anticancer properties of new (dihydro)pyranonaphthoquinones and their epoxy analogs.

1,4-Dihydroxy-2-naphthoic acid was used as a substrate for a straightforward five-step synthesis of 3-substituted 1H-benzo[g]isochromene-5,10-diones, with a Michael addition of N-acylmethylpyridinium ylides across 2-hydroxymethyl-1,4-naphthoquinone and a subsequent acid-mediated dehydratation of intermediate hemiacetals as the key steps. The obtained benzo[g]isochromene-5,10-diones were subsequently deployed for further synthetic elaboration to produce new 3,4-dihydrobenzo[g]isochromene-5,10-diones and (3,4-dihydro-)4a,10a-epoxybenzo[g]isochromene-5,10-diones. All compounds were screened for their cytotoxic and antimicrobial effects, revealing an interesting cytotoxic activity of 1H-benzo[g]isochromene-5,10-diones against different cancer cell lines.

Synthesis of 2-aryl-3-(2-cyanoethyl)aziridines and their chemical and enzymatic hydrolysis towards γ-lactams and γ-lactones.

Trans- and cis-2-aryl-3-(2-cyanoethyl)aziridines, prepared via alkylation of the corresponding 2-aryl-3-(tosyloxymethyl)aziridines with the sodium salt of trimethylsilylacetonitrile, were transformed into variable mixtures of 4-[aryl(alkylamino)methyl]butyrolactones and 5-[aryl(hydroxy)methyl]pyrrolidin-2-ones via KOH-mediated hydrolysis of the cyano group, followed by ring expansion. In addition, next to this chemical approach, enzymatic hydrolysis of the former aziridinyl nitriles by means of a nitrilase was performed as well, interestingly providing a selective route towards the above-mentioned functionalized γ-lactams.