Enantiopure 4‐oxazolin‐2‐ones and 4‐methylene‐2‐oxazolidinones as chiral building blocks in a divergent asymmetric synthesis of heterocycles

Enantiopure 3‐((R)‐ and 3‐((S)‐1‐phenylethyl)‐4‐oxazoline‐2‐ones were evaluated as chiral building blocks for the divergent construction of heterocycles with stereogenic quaternary centers. The N‐(R)‐ or N‐(S)‐1‐phenylethyl group of these compounds proved to be an efficient chiral auxiliary for the asymmetric induction of the 4‐ and 5‐positions of the 4‐oxazolin‐2‐one ring through thermal and MW‐promoted nucleophilic conjugated addition to Michael acceptors and alkyl halides. The resulting adducts were transformed via a cascade process into fused six‐membered carbo‐ and heterocycles. The structure of the reaction products depended on the electrophiles and reaction conditions used. Alternative isomeric 4‐methylene‐2‐oxazolidinones served as chiral precursors for a versatile and divergent approach to highly substituted cyclic carbamates. DFT quantum calculations showed that the formation of bicyclic pyranyl compounds was generated by a diastereoselective concerted hetero‐Diels‐Alder cycloaddition.     

Mitsunobu Alkylation of Cancerostatic 5‐Fluorouridine with (2E)‐10‐Hydroxydec‐2‐enoic Acid,a Fatty Acid from Royal Jelly with Multiple Biological Activities

5‐Fluorouridine ( 1 ) – a nucleoside antimetabolite with strong cancerostatic properties – was protected i) at the 2′‐ and 3′‐OH groups with a heptan‐4‐ylidene residue and ii) at the 5′‐OH group with a (4‐methoxyphenyl)(diphenyl)methyl residue. This fully protected compound, 3 , was submitted to a Mitsunobu reaction with the N‐hydroxysuccinimide (NHS) ester, 5 , of (2E)‐10‐hydroxydec‐2‐enoic acid ( 4 ) which gave nucleolipid 6 . The latter was detritylated with Cl₂CHCOOH to yield the co‐drug 7 as NHS ester.     

G‐quadruplex DNA‐based asymmetric catalysis of michael addition: Effects of sonication,ligands, and co‐solvents

There is an escalating interest of using double stranded DNA molecules as a chiral scaffold to construct metal‐biomacromolecule hybrid catalysts for asymmetric synthesis. Several recent studies also evaluated the use of G‐quadruplex DNA‐based catalysts for asymmetric Diels‐Alder and Friedel‐Crafts reactions. However, there is still a lack of understanding of how different oligonucleotides, salts (such as NaCl and KCl), metal ligands and co‐solvents affect the catalytic performance of quadruplex DNA‐based hybrid catalysts. In this study, we aim to systematically evaluate these key factors in asymmetric Michael addition reactions, and to examine the conformational and molecular changes of DNA by circular dichroism (CD) spectroscopy and gel electrophoresis. We achieved up to 95% yield and 50% enantiomeric excess (ee) when the reaction of 2‐acylimidazole 1a and dimethylmalonate was catalyzed by 5′‐G₃(TTAG₃)₃?3′ (G4DNA1) in 20 mM MOPS (pH 6.5) containing 50 mM KCl and 40 µM [Cu(dmbipy)(NO₃)₂], and G4DNA1 was pre‐sonicated in ice bath for 10 min prior to the reaction. G‐quadruplex‐based hybrid catalysts provide a new tool for asymmetric catalysis, but future mechanistic studies should be sought to further improve the catalytic efficiency. The current work presents a systematic study of asymmetric Michael addition catalyzed by G‐quadruplex catalysts constructed via non‐covalent complexing, and an intriguing finding of the effect of pre‐sonication on catalytic efficiency. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:891–898, 2016     

A novel chiral aliphatic–aromatic diamine promoted direct,highly enantio‐ and diastereoselective Michael addition of cyclohexanone to nitroolefins under solvent‐free conditions

A series of new highly efficient chiral aliphatic–aromatic diamine catalysts have been designed and successfully applied to the asymmetric Michael addition of cyclohexanone with nitroolefins under solvent‐free conditions without any acidic additives. The desired adducts were obtained in high yields with excellent enantio‐ and diastereoselectivities of syn products (up to >99% ee, >99:1 dr). Chirality 2010. © 2010 Wiley‐Liss, Inc     

Imidazole dipeptides can quench toxic 4‐oxo‐2(E)‐nonenal: Molecular mechanism and mass spectrometric characterization of the reaction products

Imidazole dipeptides, such as carnosine (β‐alanyl‐l ‐histidine) and anserine (β‐alanyl‐N^π‐methyl‐l ‐histidine), are highly localized in excitable tissues, including skeletal muscle and nervous tissue, and play important roles such as scavenging reactive oxygen species and quenching reactive aldehydes. We have demonstrated several reactions between imidazole dipeptides (namely, carnosine, and anserine) and a lipid peroxide‐derived reactive aldehyde 4‐oxo‐2(E)‐nonenal. Seven carnosine adducts and two anserine adducts were characterized using liquid chromatography/electrospray ionization‐multiple‐stage mass spectrometry. Adduct formation occurred between imidazole dipeptides and 4‐oxo‐2(E)‐nonenal mainly through Michael addition, Schiff base formation, and/or Paal‐Knorr reaction. The reactions were much more complicated than the reaction with a similar lipid peroxide‐derived reactive aldehyde, 4‐hydroxy‐2(E)‐nonenal.     

Application of NMR spectroscopy for assignment of the absolute configuration of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one

In order to assign the absolute configurations of 8‐tert‐butyl‐2‐hydroxy‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]dec‐6‐en‐10‐one ( 2a , 2b ), their esters ( 5a , 5b , 5c , 5d ) with (R)‐ or (S)‐2‐methoxyphenylacetic acid ( 4a , 4b ) have been synthesized. The absolute configurations of these compounds have been determined on the basis of NOESY correlations between the protons of the tert‐butyl group and the cyclopentane fragment of the molecules. The crucial part of this analysis was assignment of the absolute configuration at C‐5. Additionally, by calculation of the chemical shift anisotropy, δ^RS, for the relevant protons, it was also possible to confirm the absolute configurations at the C‐2 centres of compounds 2a , 2b and 5a , 5b , 5c , 5d . Chirality, 25:422–426, 2013.© 2013 Wiley Periodicals, Inc.     

2H‐Pyran‐2‐one and 2H‐Furan‐2‐one Derivatives from the Plant Endophytic Fungus Pestalotiopsis fici

Two new α‐pyrones (=2H‐pyran‐2‐ones), ficipyrones A and B ( 1 and 2 , resp.), and two new α‐furanones (=2H‐furan‐2‐ones), ficifuranones A and B ( 3 and 4 , resp.), together with three known metabolites, antibiotic F 0368 ( 5 ), hydroxyseiridin ( 6 ), and hydroxyisoseiridin ( 7 ), were isolated from solid cultures of the plant endophytic fungus Pestalotiopsis fici. Their structures were elucidated primarily by NMR spectroscopy, and the absolute configuration of 1 was deduced from the circular‐dichroism (CD) data. Compound 1 showed antifungal activity against the plant pathogen Gibberella zeae (CGMCC 3.2873) with an IC₅₀ value of 15.9 μM .     

Synthesis of 3‐Methylidene‐1‐tosyl‐2,3‐dihydroquinolin‐4(1H)‐ones as Potent Cytotoxic Agents

An efficient synthetic strategy to 3‐methylidene‐2,3‐dihydroquinolin‐4(1H)‐ones variously substituted in position 2 has been developed. The title compounds were synthesized in the reaction sequence involving reaction of diethyl methylphosphonate with methyl 2‐(tosylamino)benzoate, condensation of thus formed diethyl 2‐oxo‐2‐(2‐N‐tosylphenyl)ethylphosphonate with various aldehydes followed by successful application of the obtained 3‐(diethoxyphosphoryl)‐1,2‐dihydroquinolin‐4‐ols as Horner–Wadsworth–Emmons reagents for the olefination of formaldehyde. Also, enantioselective approach to the target compounds has been evaluated using 3‐dimenthoxyphosphoryl group as a chiral auxiliary. Single X‐ray crystal analysis of (2S)‐3‐(dimenthoxyphosphoryl)‐2‐phenyl‐1‐tosyldihydroquinolin‐4‐ol revealed the presence of strong resonance‐assisted hydrogen bond (RAHB). The obtained 3‐methylidene‐2,3‐dihydroquinolin‐4(1H)‐ones were then tested for their cytotoxic activity against two leukemia cell lines NALM‐6 and HL‐60 and a breast cancer MCF‐7 cell line. All compounds showed very high cytotoxic activity with the IC₅₀ values mostly below 1 μm in all three cancer cell lines. The selected analogs were also tested on human umbilical vein endothelial cells (HUVEC) and on human mammary gland/breast cells (MCF‐10A) to evaluate their influence on normal cells. Since one of the most serious problems in cancer chemotherapy is the development of drug resistance, the mRNA levels and activity of ABCB1 transporter considered to be the most important factor engaged in drug resistance, were evaluated in MCF‐7 cells treated with two selected analogs. Both compounds were strong ABCB1 transporter inhibitors that could prevent efflux of anticancer drugs from cancer cells.     

Theoretical study on the asymmetric Michael addition of cyclohexanone with trans‐β‐nitrostyrene catalyzed by a pyrrolidine‐type chiral ionic liquid

The Michael addition of cyclohexanone with trans‐β‐nitrostyrene catalyzed by a chiral ionic liquid (CIL) pyrrolidine‐imidazolium bromide, which represents a prototype of CIL‐promoted asymmetric syntheses, has been investigated by performing density functional theory calculations. We show the details of the mechanism and energetics, the influence of the acid additive on the reactivity, and the functional role of the CIL in the asymmetric addition. It is found that the reaction proceeds via two stages, i.e., the initial enamine formation, where the imine complex is first created and then isomerizes into the enamine intermediate, and the subsequent Michael addition involving a three‐step mechanism. The calculations show that the presence of the acid additive changes the imine formation mechanism and lowers the reaction barrier, as well as, more importantly, makes the reaction become highly thermodynamically favored. It is also suggested that both the anion and cation of the CIL synergically facilitate the reaction, which act as the proton acceptor in the imine‐enamine tautomerism and the stabilizer of the negative charge in the C? C bond formation process, respectively. The present theoretical study rationalizes the early experimental findings well and provides aid to some extent for the rational design of efficient CIL catalysts. Chirality 2010. © 2010 Wiley‐Liss, Inc.     

Stereoselective synthesis of fully protected (2S,4S,6S)‐2‐amino‐6‐hydroxy‐4‐methyl‐8‐oxodecanoic acid (AHMOD)

The stereocontrolled synthesis of fully protected (2S,4S,6S)‐2‐amino‐6‐hydroxy‐4‐methyl‐8‐oxodecanoic acid was accomplished using a glutamate derivative as starting material. The key steps of this stereochemical synthetic pathway involved an Evans asymmetric alkylation, a Sharpless asymmetric epoxidation, and a Grignard reaction. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

Direct asymmetric Michael addition of thioether‐based aryl sulfanyl‐propan‐2‐one to nitroalkenes catalyzed by a chiral amine‐thiourea bifunctional organocatalyst

Although the organocatalytic direct asymmetric Michael reactions of carbonyl compounds to nitroalkenes have been investigated intensely, the Michael reaction of the thioether‐based donors remains a rather undeveloped field. This work concerns the asymmetric Michael addition of aryl sulfanyl‐propan‐2‐one to nitroalkenes with benzoic acid as an additive, and chiral amine‐thiourea as a bifunctional organocatalyst. The reactions provided the highly functionalized chiral adducts with excellent enantioselectivities (up to 96% ee) and good yields. Moreover, the further transformed products exhibited excellent diastereoselectivity. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Synthesis of l‐threitol‐based crown ethers and their application as enantioselective phase transfer catalyst in Michael additions

A few new l ‐threitol‐based lariat ethers incorporating a monoaza‐15‐crown‐5 unit were synthesized starting from diethyl l ‐tartrate. These macrocycles were used as phase transfer catalysts in asymmetric Michael addition reactions under mild conditions to afford the adducts in a few cases in good to excellent enantioselectivities. The addition of 2‐nitropropane to trans ‐chalcone, and the reaction of diethyl acetamidomalonate with β‐nitrostyrene resulted in the chiral Michael adducts in good enantioselectivities (90% and 95%, respectively). The substituents of chalcone had a significant impact on the yield and enantioselectivity in the reaction of diethyl acetoxymalonate. The highest enantiomeric excess (ee ) values (99% ee ) were measured in the case of 4‐chloro‐ and 4‐methoxychalcone. The phase transfer catalyzed cyclopropanation reaction of chalcone and benzylidene‐malononitriles using diethyl bromomalonate as the nucleophile (MIRC reaction) was also developed. The corresponding chiral cyclopropane diesters were obtained in moderate to good (up to 99%) enantioselectivities in the presence of the threitol‐based crown ethers.     

Synthesis and application of 3‐substituted (S)‐BINOL as chiral ligands for the asymmetric ethylation of aldehydes

A series of (S)‐BINOL ligands substituted at the 3 position with some five‐membered nitrogen‐containing aromatic heterocycles were effectively prepared and their catalytic abilities were evaluated in the asymmetric addition of diethylzinc to benzaldehyde in the presence of titanium tetraisopropoxide. Under the optimized reaction conditions, titanium complex of (S)‐3‐(1H‐benzimidazol‐1‐yl)‐1,1′‐bi‐2‐naphthol was found to be the most efficient catalyst for asymmetric ethylation of various aldehydes to generate the corresponding secondary alcohols in up to 99% yield and 91% ee. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Structural studies on KijD1, a sugar C‐3′‐methyltransferase

Kijanimicin is an antitumor antibiotic isolated from Actinomadura kijaniata. It is composed of three distinct moieties: a pentacyclic core, a monosaccharide referred to as d ‐kijanose, and a tetrasaccharide chain composed of l ‐digitoxose units. d ‐Kijanose is a highly unusual nitro‐containing tetradeoxysugar, which requires at least ten enzymes for its production. Here we describe a structural analysis of one of these enzymes, namely KijD1, which functions as a C‐3′‐methyltransferase using S‐adenosylmethionine as its cofactor. For this investigation, two ternary complexes of KijD1, determined in the presence of S‐adenosylhomocysteine (SAH) and dTDP or SAH and dTDP‐3‐amino‐2,3,6‐trideoxy‐4‐keto‐3‐methyl‐d ‐glucose, were solved to 1.7 or 1.6 Å resolution, respectively. Unexpectedly, these structures, as well as additional biochemical analyses, demonstrated that the quaternary structure of KijD1 is a dimer. Indeed, this is in sharp contrast to that previously observed for the sugar C‐3′‐methyltransferase isolated from Micromonospora chalcea. By the judicious use of site‐directed mutagenesis, it was possible to convert the dimeric form of KijD1 into a monomeric version. The quaternary structure of KijD1 could not have been deduced based solely on bioinformatics approaches, and thus this investigation highlights the continuing need for experimental validation.     

Synthesis of d‐mannitol‐based crown ethers and their application as catalyst in asymmetric phase transfer reactions

A few new d ‐mannitol‐based monoaza‐15‐crown‐5 type chiral lariat ethers and 18‐crown‐6 type macrocycles were synthesized. These crown compounds were used as phase transfer catalysts in asymmetric Michael addititons and in a Darzens condensation under mild conditions to afford the corresponding products in a few cases in good to excellent enantioselectivities. In the Michael addition of diethyl acetoxymalonate to trans‐chalcone, in the addition of diethyl acetamidomalonate to ß‐nitrostyrene, in the reaction of diethyl bromomalonate with benzylidene malononitriles, in the cyclopropanation reaction of diethyl bromomalonate and 2‐benzylidene‐1,3‐indandione, and in the Darzens condensation of α‐chloroacetophenone with benzaldehyde, maximum enantioselectivities of 39%, 65%, 99%, 56%, and 62%, respectively, were obtained in the presence of the d ‐mannitol‐based macrocycles as the catalysts.     

An improved synthesis of (2S, 4S)‐ and (2S, 4R)‐2‐amino‐4‐methyldecanoic acids: assignment of the stereochemistry of culicinins

An improved synthesis of (2S, 4S)‐ and (2S, 4R)‐2‐amino‐4‐methyldecanoic acids was accomplished using a glutamate derivative as starting material and Evans' asymmetric alkylation as the decisive step. The NMR data of the two diastereomers were measured and compared with those of the natural product. As a result, the stereochemistry of this novel amino acid unit in culicinins was assigned as (2S, 4R). Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Design,Synthesis and Bioevaluation of Two Series of 3‐[(1‐Benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl]quinazolin‐4(3H)‐ones and N‐(1‐Benzylpiperidin‐4‐yl)quinazolin‐4‐amines

Two series of 3‐[(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl]quinazolin‐4(3H)‐ones and N‐(1‐benzylpiperidin‐4‐yl)quinazolin‐4‐amines were designed initially as potential acetylcholine esterase inhibitors. Biological evaluation demonstrated that N‐(1‐benzylpiperidin‐4‐yl)quinazolin‐4‐amines significantly inhibited AChE activity. Especially, two compounds of them were found to be the most potent with relative AChE inhibition percentages of 87 % in comparison to donepezil. The docking studies with AChE showed similar interactions between donepezil and four derivatives. N‐(1‐Benzylpiperidin‐4‐yl)quinazolin‐4‐amines also exhibited significant DPPH scavenging effects. The two series of compound also exerted moderate to good cytotoxicity against three human cancer cell lines, including SW620 (human colon cancer), PC‐3 (prostate cancer), and NCI?H23 (lung cancer), with 3‐[(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl]quinazolin‐4(3H)‐one being the most cytotoxic agent. 3‐[(1‐Benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl]quinazolin‐4(3H)‐one significantly induced early apoptosis and arrested the SW620 cells at G2/M phase. From this study, two compounds of N‐(1‐benzylpiperidin‐4‐yl)quinazolin‐4‐amines could serve as new leads for further design and AChE inhibitors, while 3‐[(1‐benzyl‐1H‐1,2,3‐triazol‐4‐yl)methyl]quinazolin‐4(3H)‐one could serve as a new lead for the design and development of more potent anticancer agents.     

A general method for preparation of N‐Boc‐protected or N‐Fmoc‐protected α,β‐didehydropeptide building blocks and their use in the solid‐phase peptide synthesis

N‐(tert‐butyloxycarbonyl) or N‐(9‐fluorenylmethoxycarbonyl) dipeptides with C‐terminal (Z)‐α,β‐didehydrophenylalanine (?^ZPhe), (Z)‐α,β‐didehydrotyrosine (?^ZTyr), (Z)‐α,β‐didehydrotryptophan (?^ZTrp), (Z)‐α,β‐didehydromethionine (?^ZMet), (Z)‐α,β‐didehydroleucine (?^ZLeu), and (Z/E)‐α,β‐didehydroisoleucine (?^Z/EIle) were synthesised from their saturated analogues via oxidation of intermediate 2,5‐disubstituted‐oxazol‐5‐(4H)‐ones (also known as azlactones) with pyridinium tribromide followed by opening of the produced unsaturated oxazol‐5‐(4H)‐one derivatives in organic‐aqueous solution with a catalytic amount of trifluoroacetic acid or by a basic hydrolysis. In all cases, a very strong preference for Z isomers of α,β‐didehydro‐α‐amino acid residues was observed except of the ΔIle, which was obtained as the equimolar mixture of Z and E isomers. Reasons for the (Z)‐stereoselectivity and the increased stability of the aromatic α,β‐didehydro‐α‐amino acid residue oxazol‐5‐(4H)‐ones over the corresponding aliphatic ones are also discussed. It is the first use of such a procedure to synthesise peptides with the C‐terminal unsaturated residues and a peptide with 2 consecutive ΔPhe residues. This approach is very effective especially in the synthesis of peptides with aliphatic α,β‐didehydro‐α‐amino acid residues that are difficult to obtain by other methods. It allowed the first synthesis of the ?Met residue. It is also more cost‐effective and less laborious than other synthesis protocols. The dipeptide building blocks obtained were used in the solid‐phase synthesis of model peptides on a polystyrene‐based solid support. Peptides containing aromatic α,β‐didehydro‐α‐amino acid residues were obtained with PyBOP or TBTU as a coupling agent with good yields and purities. In the case of aliphatic α,β‐didehydro‐α‐amino acid residues, a good efficiency was achieved only with DPPA as a coupling agent.     

Regulation of the 5‐HT3A Receptor‐Mediated Current by Alkyl 4‐Hydroxybenzoates Isolated from the Seeds of Nelumbo nucifera

Four known alkyl 4‐hydroxybenzoates, i.e., methyl 4‐hydroxybenzoate ( 1 ), ethyl 4‐hydroxybenzoate ( 2 ), propyl 4‐hydroxybenzoate ( 3 ), and butyl 4‐hydroxybenzoate ( 4 ), were isolated from the seeds of Nelumbo nucifera Gaertner (Nymphaeaceae) for the first time. The structures of the isolates were identified by 1D‐ and 2D‐NMR spectroscopy and comparison with published values. The compounds were evaluated for their effects on the 5‐HT‐stimulated inward current (I_5‐HT) mediated by the human 5‐HT₃A receptors expressed in Xenopus oocytes. Compounds 1 and 2 enhanced the I_5‐HT, but 4 reduced it. These results indicate that 4 is an inhibitor of the 5‐HT₃A receptors expressed in Xenopus oocytes.     

Lewis acid catalysed methylation of N‐(9H‐fluoren‐9‐yl)methanesulfonyl (Fms) protected lipophilic α‐amino acid methyl esters

This work reports an efficient Lewis acid catalysed N‐methylation procedure of lipophilic α‐amino acid methyl esters in solution phase. The developed methodology involves the use of the reagent system AlCl₃/diazomethane as methylating agent and α‐amino acid methyl esters protected on the amino function with the (9H‐fluoren‐9‐yl)methanesulfonyl (Fms) group. The removal of Fms protecting group is achieved under the same conditions to those used for Fmoc removal. Thus the Fms group can be interchangeable with the Fmoc group in the synthesis of N‐methylated peptides using standard Fmoc‐based strategies. Finally, the absence of racemization during the methylation reaction and the removal of Fms group were demonstrated by synthesising a pair of diastereomeric dipeptides. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.