Unraveling the Mechanism and Regioselectivity of the B12‐Dependent Reductive Dehalogenase PceA

PceA is a cobalamin‐dependent reductive dehalogenase that catalyzes the dechlorination of perchloroethylene to trichloroethylene and then to cis‐dichloroethylene as the sole final product. The reaction mechanism and the regioselectivity of this enzyme are investigated by using density functional calculations. Four different substrates, namely, perchloroethylene, trichloroethylene, cis‐dichloroethylene, and chlorotheylene, have been considered and were found to follow the same reaction mechanism pattern. The reaction starts with the reduction of Co^II to Co^I through a proton‐coupled electron transfer process, with the proton delivered to a Tyr246 anion. This is followed by concerted C?Cl bond heterolytic cleavage and proton transfer from Tyr246 to the substrate carbon atom, generating a Co^III?Cl intermediate. Subsequently, a one‐electron transfer leads to the formation of the Co^II?Cl product, from which the chloride and the dehalogenated product can be released from the active site. The substrate reactivity follows the trend perchloroethylene>trichloroethylene?cis‐dichloroethylene?chlorotheylene. The barriers for the latter two substrates are significantly higher compared with those for perchloroethylene and trichloroethylene, implying that PceA does not catalyze their degradation. In addition, the formation of cis‐dichloroethylene has a lower barrier by 3.8 kcal mol^?1 than the formation of trans‐dichloroethylene and 1,1‐dichloroethylene, reproducing the regioselectivity. These results agree quite well with the experimental findings, which show cis‐dichloroethylene as the sole product in the PceA‐catalyzed dechlorination of perchloethylene and trichloroethylene.     

Non‐natural Cofactor and Formate‐Driven Reductive Carboxylation of Pyruvate

A non‐natural cofactor and formate driven system for reductive carboxylation of pyruvate is presented. A formate dehydrogenase (FDH) mutant, FDH*, that favors a non‐natural redox cofactor, nicotinamide cytosine dinucleotide (NCD), for generation of a dedicated reducing equivalent at the expense of formate were acquired. By coupling FDH* and NCD‐dependent malic enzyme (ME*), the successful utilization of formate is demonstrated as both CO₂ source and electron donor for reductive carboxylation of pyruvate with a perfect stoichiometry between formate and malate. When ¹³C‐isotope‐labeled formate was used in in vitro trials, up to 53 % of malate had labeled carbon atom. Upon expression of FDH* and ME* in the model host E. coli, the engineered strain produced more malate in the presence of formate and NCD. This work provides an alternative and atom‐economic strategy for CO₂ fixation where formate is used in lieu of CO₂ and offers dedicated reducing power.     

Conversion of a Dehalogenase into a Nitroreductase by Swapping its Flavin Cofactor with a 5‐Deazaflavin Analogue

Natural and engineered nitroreductases have rarely supported full reduction of nitroaromatics to their amine products, and more typically, transformations are limited to formation of the hydroxylamine intermediates. Efficient use of these enzymes also requires a regenerating system for NAD(P)H to avoid the costs associated with this natural reductant. Iodotyrosine deiodinase is a member of the same structural superfamily as many nitroreductases but does not directly consume reducing equivalents from NAD(P)H, nor demonstrate nitroreductase activity. However, exchange of its flavin cofactor with a 5‐deazaflavin analogue dramatically suppresses its native deiodinase activity and leads to significant nitroreductase activity that supports full reduction to an amine product in the presence of the convenient and inexpensive NaBH₄.     

Catalytic Reductive Pinacol‐Type Rearrangement of Unactivated 1,2‐Diols through a Concerted,Stereoinvertive Mechanism

A catalytic pinacol‐type reductive rearrangement reaction of internal 1,2‐diols is reported herein. Several scaffolds not usually amenable to pinacol‐type reactions, such as aliphatic secondary–secondary diols, undergo the transformation well without the need for prefunctionalization. The reaction uses a simple boron catalyst and two silanes and proceeds through a concerted, stereoinvertive mechanism that enables the preparation of highly enantiomerically enriched products. Computational studies have been used to rationalize the preference for migration over direct deoxygenation.     

Catalytic Reductive Pinacol‐Type Rearrangement of Unactivated 1,2‐Diols through a Concerted,Stereoinvertive Mechanism

A catalytic pinacol‐type reductive rearrangement reaction of internal 1,2‐diols is reported herein. Several scaffolds not usually amenable to pinacol‐type reactions, such as aliphatic secondary–secondary diols, undergo the transformation well without the need for prefunctionalization. The reaction uses a simple boron catalyst and two silanes and proceeds through a concerted, stereoinvertive mechanism that enables the preparation of highly enantiomerically enriched products. Computational studies have been used to rationalize the preference for migration over direct deoxygenation.     

Combretins A and B,New Cycloartane‐Type Triterpenes from Combretum fragrans

The phytochemical studies on the leaves of the traditionally used medicinal plant Combretum fragrans F. Hoffm (Combretaceae) from Cameroon have led to the isolation of combretins A and B ( 1 and 2 , resp.), two new cycloartane‐type triterpenes from the AcOEt‐soluble subfraction along with β‐sitosterol ( 3 ), oleanolic acid ( 4 ), ursolic acid ( 5 ), and pratensein ( 6 ). The compounds 4  –  6 are reported for the first time from this species. The structures of the new triterpenes were elucidated by spectroscopic techniques including ¹H‐ and ¹³C‐NMR (DEPT), and 2D‐NMR experiments.     

nido‐Undecaboranes and ‐borates of the Type B11H13L and B11H12L‐

The Lewis base SMe₂ in 7‐B₁₁H₁₃(SMe₂) ( 1a ) can be replaced by the amines L = NH₂(CH₂tBu), NH₂Cy, NH₂Ph, NH₂(4‐C₆H₄Me), py, chinoline or the phosphanes L = PPh₃, PMePh₂, yielding 7‐B₁₁H₁₃L ( 1b ‐ i ). The borane 1a can be deprotonated by certain amines, alkanides, or hydrides to give the anion 7‐B₁₁H₁₂(SMe₂)^‐ ( 2a ). Replacing the base SMe₂ in the anion 2a by weak bases gives B₁₁H₁₂L^‐ (L = PPh₃, MeCN; 2h , j ). Upon reaction of 1a with the amine NH₂(CH₂tBu) in the ratio 1:2, a deprotonation and the substitution of SMe₂ by the amine are observed, 7‐B₁₁H₁₂[NH₂(CH₂tBu)]^‐ ( 2b ) being formed. At 170 °C, the 7‐isomers 1b , f are isomerized into a mixture of the corresponding 1‐ and 2‐isomers ( 1b′ , f′ and 1b″ , f″ , respectively).     

Metal‐Catalyzed Reductive Coupling Reactions of Organic Halides with Carbonyl‐Type Compounds

Metal‐catalyzed reductive coupling reactions of aryl halides and (pseudo)halides with carbonyl‐type compounds have undergone an impressive development within the last years. These methodologies have shown to be a powerful alternate strategy, practicality aside, to the use of stoichiometric, well‐defined, and, in some cases, air‐sensitive organometallic species. In this Minireview, the recent findings in this field are summarized, with particular emphasis on the mechanistic interpretation of the results and future aspects of this area of expertise.     

Pseudocoenzyme B12 and Adenosyl‐Factor A: Electrochemical Synthesis and Spectroscopic Analysis of Two Natural B12 Coenzymes with Predominantly ‘Base‐off' Constitution

Pseudocoenzyme B₁₂ (=Coβ‐(5′‐deoxy‐5′‐adenosyl)‐(adenin‐7‐yl)cobamide; 1 ) and adenosyl‐factor A (=Coβ‐(5′‐deoxy‐5′‐adenosyl)‐(2‐methyladenin‐7‐yl)cobamide; 3 ) are two natural analogues of coenzyme B₁₂ (=adenosylcobalamin‐Coβ‐(5′‐deoxy‐5′‐adenosyl)‐(5,6‐dimethyl‐1H‐benzimidazolyl)cobamide; 2 ), where the Co‐coordinating 5,6‐dimethyl‐1H‐benzimidazole nucleotide base of 2 is replaced by the purine bases adenine and 2‐methyladenine. In contrast to 2 , which exists solely in the ‘base‐on' form, UV/VIS spectroscopy qualitatively indicates ‘base‐off' constitution for 1 and 3 in aqueous solution. (cf. the established ‘base‐off' form as unexpected binding mode of B₁₂ cofactors in several B₁₂‐dependent enzymes, such as in methionine synthase from Escherichia coli and in glutamate mutase from Clostridium cochlearium). In the present work, pseudocoenzyme B₁₂ ( 1 ) was synthesized in 85% yield by alkylation with 5′‐O‐tosyladenosine of (adenin‐7‐yl)cob(I)amide, which was produced electrochemically from pseudovitamin B₁₂ (Coβ‐cyano‐(adenin‐7‐yl)cobamide). Likewise, adenosyl‐factor A ( 3 ) was prepared in ca. 70% yield from factor A (=Coβ‐cyano‐(2‐methyladenin‐7‐yl)cobamide; 5 ). All the spectroscopic properties of 1 and 3 in aqueous solution indicated that these two Coβ‐(5′‐deoxy‐5′‐adenosyl)‐(adenin‐7‐yl)cobamides exist predominantly in a ‘base‐off' constitution, with minor but significant contributions of the ‘base‐on' form. From the UV/VIS spectra, the temperature‐dependent equilibrium constants of the ‘base‐off'/‘base‐on' reconstitution reaction were determined as K_on ( 1 )=0.30 and K_on ( 3 )=0.48 at 25°, corresponding to a contribution of the ‘base‐on' forms of 23% for 1 and of 32% for 3 .     

Homocoenzyme B12 and Bishomocoenzyme B12: Covalent Structural Mimics for Homolyzed,Enzyme‐Bound Coenzyme B12

Efficient electrochemical syntheses of “homocoenzyme B₁₂” ( 2 , Co_β‐(5′‐deoxy‐5′‐adenosyl‐methyl)‐cob(III )alamin) and “bishomocoenzyme B₁₂” ( 3 , Co_β‐[2‐(5′‐deoxy‐5′‐adenosyl)‐ethyl]‐cob(III )alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in‐depth investigations of the structures of 2 and 3 in solution were carried out and a high‐resolution crystal structure of 2 was obtained. The two homologues of coenzyme B₁₂ ( 2 and 3 ) are suggested to function as covalent structural mimics of the hypothetical enzyme‐bound “activated” (that is, “stretched” or even homolytically cleaved) states of the B₁₂ cofactor. From crude molecular models, the crucial distances from the corrin‐bound cobalt center to the C5′ atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 Å, respectively. These values are roughly the same as those found in the two “activated” forms of coenzyme B₁₂ in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2 , the cobalt center was observed to be at a distance of 2.99 Å from the C5′ atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B₁₂. The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.     

A Convergent Synthesis of a Twelve‐Membered Macrolide Natural Product, (6R,12S)‐6‐Hydroxy‐12‐methyl‐1‐oxacyclododecane‐2,5‐dione

A new polyketide metabolite, the twelve‐membered macrolide 1 , isolated from the endophytic fungal strain Cladosporium tenuissimum LR 463 of Maytenus hookeri, whose structure had been determined as (6R,12S)‐6‐hydroxy‐12‐methyl‐1‐oxacyclododecane‐2,5‐dione, was synthesized for the first time by a convergent strategy via Yamaguchi esterification of 2 with 3 and ring‐closing metathesis (RCM) to afford the cyclic ester 1 that was eventually transformed to the target molecule. However, the total synthesis revealed that the assigned structure of the natural product is not correct.     

Total Synthesis of Hapalindole‐Type Natural Products

A unified and bioinspired oxidative cyclization strategy was used in the first total syntheses of naturally occurring 12‐epi‐hapalindole Q isonitrile, hapalonamide H, deschloro 12‐epi‐fischerindole I nitrile, and deschloro 12‐epi‐fischerindole W nitrile, as well as the structural revision of the latter. Hapalindoles H and Q were also synthesized.     

4,10‐Dibromoanthanthrone as a New Building Block for p‐Type,n‐Type,and Ambipolar π‐Conjugated Materials

New p‐type, n‐type, and ambipolar molecules were synthesized from commercially available 4,10‐dibromoanthanthrone dye. Substitution at the 4,10‐ and 6,12‐positions with different electron‐rich and electron‐poor units allowed the modulation of the optoelectronic properties of the molecules. A bis(dicyanovinylene)‐functionalized compound was also prepared with a reduction potential as low as ?50 mV versus Ag⁺ with a crystalline two‐dimensional lamellar packing arrangement. These characteristics are important prerequisites for air‐stable n‐type organic field‐effect transistor applications.     

Synthesis of Natural Atisane‐Type Diterpenoids by retro‐Biomimetic Transformations

An efficient one step, retro‐biomimetic procedure for the synthesis of natural products having the atisane structure is described (Scheme 2), natural products which are components of medicinal plants and possess relevant biological activity. Their structures were confirmed by chemical transformations and spectral data. The starting materials were the known ent‐kaur‐16‐en‐19‐oic acid ( 1 ) and ent‐trachyloban‐19‐oic acid ( 2 ), diterpenoids readily available from the waste of sunflower.     

Stereoselective Total Synthesis of Iso‐Cladospolide B and the 12 Membered‐Macrolactone (6S,12R)‐6‐Hydroxy‐12‐methyloxacyclododecane‐2,5‐dione

Total syntheses of iso‐cladospolide B ( 1 ) and the 12‐membered macrolactone (6S,12R)‐6‐hydroxy‐12‐methyloxacyclododecane‐2,5‐dione ( 2 ), a non‐natural product, were achieved from a common intermediate starting from commercially available 1,9‐nonane diol.     

The Crystal Structure of Solvent‐Free Silver Dodecachloro‐closo‐dodecaborate Ag2[B12Cl12] from Aqueous Solution

Colourless octahedral single crystals of solvent‐free Ag₂[B₁₂Cl₁₂] (cubic, Pa3¯; a = 1238.32(7) pm, Z = 4) are obtained by the metathesis reaction of Cs₂[B₁₂Cl₁₂] with an aqueous solution of silver nitrate (AgNO₃) and recrystallization of the crude product from water. The crystal structure is best described as a distorted anti‐CaF₂‐type arrangement in which the quasi‐icosahedral [B₁₂Cl₁₂]^2— anions (d(B—B) = d(B—Cl) = 177—180 pm) are arranged in a cubic closest‐packed fashion. The tetrahedral interstices are filled with Ag⁺ cations which are strongly displaced from their ideal positions. Thereby each silver atom gets coordinated by six chlorine atoms from the edges of three [B₁₂Cl₁₂]^2— anions providing a distorted octahedral coordination sphere to the Ag⁺ cations (d(Ag—Cl) = 283—285 pm, CN = 6).