Designing for sustainability with biocatalytic and chemoenzymatic cascade processes

Cascade reactions have been widely recognized to cut costs, decrease solvent usage, and reduce cycle times in chemical processes. Recently, biocatalytic cascades have altered how we design synthetic routes to complex molecules to achieve sustainable commercial processes for pharmaceutical, agricultural, and fine chemical industries. With advancements in protein engineering and an increase in the number of enzyme classes available to chemists, industrial and academic groups alike have endeavored to expand the scope of biocatalysis from single reactions to multi-enzyme cascades to rapidly build complex molecular structures. Recent reports have drawn inspiration from biosynthetic pathways and have applied engineered enzymes to in vitro enzymatic cascades. Furthermore, combining transition-metal catalysis and enzymes in one-pot chemoenzymatic cascades likewise serves to broaden the scope of biocatalysis, enabling traditional chemical reactions to be performed under mild aqueous conditions. In this article, we review recent biocatalytic and chemoenzymatic cascades from 2019 to 2021.     

Optically active dioxatetraazamacrocycles: chemoenzymatic syntheses and applications in chiral anion recognition

Two new C2 and D2 symmetrical dioxatetraaza 18-membered macrocycles [(R,R)-1 and (S,S,S,S)-2] are efficiently synthesized in enantiomerically pure forms by a chemoenzymatic method starting from (+/-)-trans-cyclohexane-1,2-diamine. The protonation constants and the binding constants with different chiral dicarboxylates are determined in aqueous solution by means of pH-metric titrations. The triprotonated form of (S,S,S,S)-2 shows moderate enantioselectivity with malate and tartrate anions (deltadeltaG=0.62 and 0.66 kcal mol(-1), respectively), being the strongest binding observed in both cases with the L enantiomer. Good enantiomeric discrimination is obtained with tetraprotonated (R,R)-1 and N-acetyl aspartate, the complex with the D-enantiomer being 0.92 kcalmol(-1) more stable than its diastereomeric counterpart. Despite the lack of enantioselectivity of tri- and tetraprotonated (R,R)-1 for the tartrate anion, a very good diastereopreference for meso-tartrate is found. All these experimental results allow us to propose a model for the host-guest structure based on coulombic interactions and hydrogen bonds.     

Diastereoselective syntheses of new analogues of the farnesyltransferase inhibitor RPR 130401

The access to several benzo[f]perhydroisoindolic analogues of farnesyltransferase inhibitors from a single dienic precursor is reported. An initial [4 + 2] cycloaddition between diphenylisobenzofuran6 and pyrrolines 11, 14, and 15 led to either the syn or the anti isomers, depending on the mode of activation of the cycloaddition. The syn diastereomers were isolated in 90% de under 12 kbar at room temperature, while their anti counterparts were obtained with the same selectivity by warming the reaction mixture to 110 degrees C in toluene at atmospheric pressure. Both syn and anti adducts were separately N-deprotected, and the resulting amines reacted with an activated ester derived from the acid (20) to afford the final targets (5). Two new analogues (5a and 5b) of the FT inhibitor RPR 130401 were thus synthesized in a mere three-step synthetic scheme with overall yields from 30 to 60%.     

Efficient syntheses of rugulosin analogues

Oxidation of 3b or 14c,d with Pb(OAc)4 in AcOH for 20 min at 25 degrees C and 1 h at 75 degrees C gave flavoskyrin-type dimers 6b and 15c,d in 53-86% yield. Heating a solution of 6b or 15c,d in pyridine under air for 1 h at 75-80 degrees C and then for 1-2 h at 110 degrees C afforded rugulosin-type dimers 10b and 17c,d in 61-88% yield. This two-step sequence provides a practical route to this unusual natural product skeleton.     

Stereocontrolled syntheses of alpha-C-mannosyltryptophan and its analogues

The total synthesis of alpha-C-mannosyltryptophan (C-Man-Trp), a naturally occurring C-glycosylamino acid, was achieved from a commercially available alpha-methyl-D-mannoside in 10 steps including the following key steps: the C-glycosidation of a mannose derivative with a stannylacetylene, Castro indole synthesis, and Sc(ClO4)3-promoted coupling with L-serine-derived aziridine carboxylate. The glucose- and galactose-analogues of C-Man-Trp were also synthesized in a similar manner. Conformational analyses of the synthesized C-glycosyltryptophan and its synthetic intermediate are briefly discussed.     

A new chemoenzymatic synthesis of d-cloprostenol

A new chemoenzymatic synthesis of d-cloprostenol based on the biocatalytical resolution of anti-2-oxotricyclo[2.2.1.0]heptan-7-carboxylic acid 1 has been developed. The resolution was attempted by different approaches: esterification or reduction of the acid and hydrolysis or reduction of the corresponding esters. The most efficient method proved to be the reduction of the propyl esters of 1 catalysed by the yeast Kluyveromyces marxianus, which allowed for the recovery of the enantiomerically pure ester of anti-2-oxotricyclo[2.2.1.0]heptan-(R)-7-carboxylic acid (R)-3 at 60% molar conversion of 3.0 g/l of racemic substrate acid under optimised conditions. anti-2-Oxotricyclo[2.2.1.0]heptan-(R)-7-carboxylic acid was obtained by alkaline hydrolysis and employed for the synthesis of d-cloprostenol.     

In vitro and in vivo metabolisms of K-48

Metabolic pathways of the oxime K-48 have been elucidated by means of in vitro and in vivo experiments. K-48 exposure to rat liver microsomal fraction resulted in the formation of a hydroxylated derivative, in addition to a small molecular fragment. The in vivo metabolism in rats was investigated after intramuscular administration of 50 μmol oxime. K-48 was present in the rat serum in unchanged form. Similarly, the analysis of rat cerebrospinal fluid indicated the sole occurrence of unchanged K-48. In contrast, unchanged K-48 was not found in the rat urine, where only the metabolite generated by epoxidation on the alkyl chain connecting the two pyridinium rings was present. The presence of unchanged K-48 in the serum and cerebrospinal fluid facilitates quantitative determination using HPLC separation and ultraviolet absorbance detection. Figure Suggested metabolic pathways of K-48     

In vitro and in vivo fluorescence monitoring of photosensitizers

The use of steady state fluorescence spectroscopy in the detection and monitoring of potential photochemotherapeutic agents is examined. Problems associated with both in vitro and in vivo fluorescence measurements are investigated, and typical data are presented. Recent results on the use of fluorescence in pharmacokinetic studies are discussed, and the relative merits of in vitro vs. in vivo methods are outlined.     

In vitro and in vivo metabolism studies of dimethazine

The use of anabolic steroids is prohibited in sports. Effective control is done by monitoring their metabolites in urine samples collected from athletes. Ethical objections however restrict the use of designer steroids in human administration studies. To overcome these problems alternative in vitro and in vivo models were developed to identify metabolites and to assure a fast response by anti‐doping laboratories to evolutions on the steroid market. In this study human liver microsomes and an uPA^+/+‐SCID chimeric mouse model were used to elucidate the metabolism of a steroid product called ‘Xtreme DMZ’. This product contains the designer steroid dimethazine (DMZ), which consists of two methasterone molecules linked by an azine group. In the performed stability study, degradation from dimethazine to methasterone was observed. By a combination of LC‐High Resolution Mass Spectrometry (HRMS) and GC‐MS(/MS) analysis methasterone and six other dimethazine metabolites (M1–M6), which are all methasterone metabolites, could be detected besides the parent compound in both models. The phase II metabolism of dimethazine was also investigated in the mouse urine samples. Only metabolites M1 and M2 were exclusively detected in the glucuro‐conjugated fraction; all other compounds were also found in the free fraction. For effective control of DMZ misuse in doping control samples, screening for methasterone and methasterone metabolites should be sufficient. Copyright © 2016 John Wiley & Sons, Ltd.     

Enantioselective total syntheses of ropivacaine and its analogues

An alternative asymmetric synthesis of ropivacaine and analogues employing the ‘cation pool’ strategy and host/guest supramolecular co-catalysis approach is presented. In this study, chiral auxiliaries, several soft nucleophiles as well as one-pot conditions for anodic oxidation, followed by nucleophilic addition, have been applied.     

In vitro and in vivo imaging with quantum dots

Quantum dots (QDs), also named semiconductor nanocrystals, have initiated a new realm of bioscience by combining nanomaterials with biology, which will profoundly influence future biological and biomedical research. In this review, we describe the extraordinary optical properties of QDs and developments in methods for their synthesis. We focus on fluorescent imaging with QDs both in vitro and in vivo, and the cytotoxicity of QDs and potential barriers to their use in practical biomedical applications. Finally, we provide insights into improvements aimed at decreasing the cytotoxicity of QDs and the future outlook of QD applications in biomedical fields.

Total syntheses of ellipticine alkaloids and their amino analogues

Staudinger reaction of 9 with triphenylphosphine gave 2,4-dihydropyrrolo[3,4-b]indole 10. Treatment of 10 with di-tert-butyl dicarbonate and 4-dimethylaminopyridine gave 11. Diels-Alder reaction of 11 with 3,4-pyridyne gave cycloadducts 12 and 13, which were converted into ellipticine 1 and isoellipticine 2. New amino analogues of ellipticine 27 and 28 were also synthesized by this new route.     

New syntheses of unsymmetrical thiepins and their selenium analogues

New routes toward fused thiepins and their selenium analogues are described, wherein initial cleavage of suitable diaryl disulfides or selenides bearing masked aldehyde functionalities with an N-protected and metalated indole-3-carbaldehyde acetal or a lithiated 2-bromobenzaldehyde acetal derivative gave a set of unsymmetrical diacetals. Subsequent deacetalization, followed by McMurry coupling, afforded the target unsymmetrical thiepins and selenepins.     

Practical enantiospecific syntheses of lysobisphosphatidic acid and its analogues

We describe a versatile, efficient, and practical method for the preparation of enantiomerically pure lysobisphosphatidic acid (LBPA), bisether analogues, and phosphorothioate analogues of LBPA from solketal. Phosphorylation of a protected sn-2-O-oleoyl glycerol with 2-cyanoethyl bis(N,N-diisopropylamino)phosphite, followed by oxidation and deprotection, generated the enantiomers of 2,2'-LBPA. The corresponding phosphorothioate analogues were obtained by oxidation with sulfur. The (R,R) and (S,S) enantiomers of both LBPA and phosphorothioate LBPA were synthesized from (S)- and (R)-solketal, respectively. The ether analogue of (S,S)-lysobisphosphatidic acid (LBPA) and its enantiomer were synthesized from the same enantiomer (S)-solketal by simply changing the sequence of deprotection steps.     

In vivo and in vitro production of alkaloids by Haplophyllum patavinum

A protocol for shoot induction from callus of Haplophyllum patavinum was established. Two known furoquinoline (skimmianine and haplopine), and three quinolone (edulinine, ribalinine and isoplatydesmine) alkaloids were isolated for the first time from plant material, callus and shoot cultures of this species. The structures of these compounds have been characterised on the basis of spectroscopic evidence.     

Concise syntheses of trifluoromethylated cyclic and acyclic analogues of cADPR

A novel trifluoromethylated analogue of cADPR, 8-CF3-cIDPDE (5) was designed and synthesized via construction of N1,N9-disubstituted hypoxanthine, trifluoromethylation and intramolecular condensation. A series of acyclic analogues of cADPR were also designed and synthesized. These compounds could be useful molecules for studying the structure-activity relationship of cADPR analogues and exploring the cADPR/RyR Ca2+ signalling system.