A facile synthesis of dihydrobenzofuranols by photocyclization of (2‐alkoxy‐5‐methyl‐phenyl)(aryl) methanone and ethyl 2‐aroyl‐4‐methylphenyloxyacetates

Irradiation of 2‐alkoxy substituted benzophenones 2a–f and ethyl 2‐aroyl‐4‐methylphenyloxyacetates 2g–i in benzene and in acetonitrile underwent photocyclization to substituted dihydrobenzofuranols 3a–i with 3a–c in very less yield being racemate and 3d–i in good yield being mixture of cis‐trans isomers showing high stereoselectivity in benzene and decreased stereoselectivity in acetonitrile. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:212–217, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20111     

Myoglobin‐Catalyzed Olefination of Aldehydes

The olefination of aldehydes constitutes a most valuable and widely adopted strategy for constructing carbon–carbon double bonds in organic chemistry. While various synthetic methods have been made available for this purpose, no biocatalysts are known to mediate this transformation. Reported herein is that engineered myoglobin variants can catalyze the olefination of aldehydes in the presence of α‐diazoesters with high catalytic efficiency (up to 4,900 turnovers) and excellent E diastereoselectivity (92–99.9 % de). This transformation could be applied to the olefination of a variety of substituted benzaldehydes and heteroaromatic aldehydes, also in combination with different alkyl α‐diazoacetate reagents. This work provides a first example of biocatalytic aldehyde olefination and extends the spectrum of synthetically valuable chemical transformations accessible using metalloprotein‐based catalysts.     

Synthesis and Self‐Assembled Helical Supramolecular Polymer of Ethyl 7,8‐dihydro‐3‐hydroxy‐9‐methyl‐7‐(4′‐nitrophenyl)‐6H‐dibenzo[c]‐ pyran‐6‐one‐8‐carboxylate

A structurally novel compound was isolated as the main product of tandem Pechmann–dehydration between diethyl 4‐hydroxy‐4‐methyl‐2‐(4′‐nitrophenyl)‐6‐oxocyclohexane‐1,3‐dicarboxylate ( 1 ) and resorcinol in the presence of trifluoroacetic acid. The structure of the product was determined as a racemate of (7R,8R)‐ and (7S,8S)‐ethyl 7,8‐dihydro‐3‐hydroxy‐9‐methyl‐7‐(4′‐nitrophenyl)‐6H‐dibenzo[c]‐pyran‐6‐one‐8‐carboxylate ( 3a ) enantiomers by single crystal X‐ray diffraction analysis. The X‐ray crystal structure revealed that 3a possesses an extended and more stable conjugated aromatic system as a consequence of the stereoselectivity of intramolecular dehydration behavior of Pechmann condensation product 2 . In the crystal superstructure, the (7R,8R)‐ and (7S,8S)‐isomers of 3a respectively self‐assembled into left‐ and right‐handed supramolecular helical chains with a channel size of 3.70 Å × 10.20 Å in virtue of intermolecular hydrogen bonding together with dramatic twisting between carboxylate group at C8 and tricyclic ring framework of 3a , which are then arranged alternatively along the b‐axis direction with a pitch length of 7.894 Å.     

An Agostic Iridium Pincer Complex as a Highly Efficient and Selective Catalyst for Monoisomerization of 1‐Alkenes to trans‐2‐Alkenes

A unique Ir complex (^tBuNC^CP)Ir with the pyridine–phosphine pincer as the sole ligand, featuring a dual agostic interaction between the Ir and two σ C−H bonds from a t Bu substituent, has been prepared. This complex exhibits exceptionally high activity and excellent regio‐ and stereoselectivity for monoisomerization of 1‐alkenes to trans ‐2‐alkenes with wide functional‐group tolerance. Reactions can be performed in neat reactant on a more than 100 gram scale using 0.005 mol % catalyst loadings with turnover numbers up to 19000.     

N‐substituted bis(tetrazol‐5‐yl)diazenes: Synthesis,spectra, X‐ray molecular and crystal structures,and quantum‐chemical DFT calculations

N‐Substituted bis(tetrazol‐5‐yl)diazenes (substituents are 1‐CH₃ ( 3a ), 1‐Ph ( 3b ), 2‐CH₃ ( 3c ), and 2‐^tBu ( 3d )) were synthesized by oxidative coupling of corresponding 5‐aminotetrazoles. All compounds were characterized with ¹H and ¹³C NMR, IR‐ and UV‐spectroscopy, and thermal analysis. Crystal and molecular structures of bis(1‐phenyltetra‐ zol‐5‐yl)diazene ( 3b ) and bis(2‐tert‐butyltetrazol‐5‐yl)diazene ( 3d ) were determined by single crystal X‐ray diffraction. Molecules of these compounds are trans‐isomers in solid. According to X‐Ray data, 3b molecule is S‐trans‐S‐trans conformer, however 3d is S‐cis‐S‐cis one. Quantum‐chemical investigation of geometry and relative stability of cis‐ and trans‐isomers and stable conformations of compounds 3a–d was carried out. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:24–35, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20574     

Structure and Photochemical Properties of r‐1, c‐2, t‐3, t‐4‐l, 3‐Bis [2‐ (5‐R‐benzoxazolyl) ] −2,4‐di (4‐R' ‐phenyl) cyclobutane

r‐1, c‐2, t‐3, t‐4‐1,3‐Bis[2‐(5‐R‐benzoxazolyl)]‐2,4‐di(4‐R'‐phenyl)cyclobutane (IIa: R=R' = H; IIb: R=Me, R'= H; IIc: R = Me, R' = OMe) was synthesized with high stereo‐selectivity by the photodimerization of trans‐l‐[2‐(5‐R‐benzoxazolyl)]‐2‐(4‐R'‐phenyl) ethene (Ia: R=R' = H; Ib: R = Me, R' = H; Ic: R = Me, R' = OMe) in sulfuric acid. The structures of IIa–IIc were identified by elemental analysis, IR, UV, ¹H NMR, ¹³C NMR and MS. The molecular and crystal structure of IIc has been determined by X‐ray diffraction method. The crystal of IIc (C₃₄H₃₀N₂O₄. 0.5C₂OH) is monoclinic, space group P2₁/n with cell dimensions of a = 1.5416(3), b =0.5625(1), c = 1.7875(4) nm, β = 91.56 (3)°, V= 1.550(1) nm³, Z = 2. The structure shows that the molecule of IIc is centrosymmetric, which indicates that the dimerization process is a head‐to‐tail fashion. The selectivity of the photodimerization of Ia–Ic has been enhanced by using acidic solvent and the reaction speed would be decreased when electron donating group was introduced in the 4‐position of the phenyl group. That the photodimerization is not affected by the presence of oxygen as well as its high stereo‐selectivity demonstrated that the reaction proceeded through an excited singlet state. It was also found that under irradiation of short wavelength UV, these dimers underwent photolysis completely to reproduce its trans‐monomers, and then the new formed species changed into their cis‐isomers through trans‐cis isomerization.     

Biocatalytic Strategy for Highly Diastereo‐ and Enantioselective Synthesis of 2,3‐Dihydrobenzofuran‐Based Tricyclic Scaffolds

2,3‐Dihydrobenzofurans are key pharmacophores in many natural and synthetic bioactive molecules. A biocatalytic strategy is reported here for the highly diastereo‐ and enantioselective construction of stereochemically rich 2,3‐dihydrobenzofurans in high enantiopurity (>99.9% de and ee), high yields, and on a preparative scale via benzofuran cyclopropanation with engineered myoglobins. Computational and structure‐reactivity studies provide insights into the mechanism of this reaction, enabling the elaboration of a stereochemical model that can rationalize the high stereoselectivity of the biocatalyst. This information was leveraged to implement a highly stereoselective route to a drug molecule and a tricyclic scaffold featuring five stereogenic centers via a single‐enzyme transformation. This work expands the biocatalytic toolbox for asymmetric C–C bond transformations and should prove useful for further development of metalloprotein catalysts for abiotic carbene transfer reactions.     

Derivatives of α‐ and β‐S‐thiophosphates of 2‐bromo‐2‐deoxy‐D‐hexopyranose

The first examples of S‐thiophosphate derivatives of 2‐bromo‐2‐deoxy sugars 7–12 were synthesized by reacting alkyl ammonium salts 1–4 of thiophosphoric acids with α‐1,2‐cis (5) or α‐1,2‐trans dibromo sugars (6) and addition of free thiophosphoric acids 1a or 2a to 2‐bromo‐D‐glucal (13). It was observed that the solvent determines formation of either the O‐ or S‐glycosyl compound. β‐Thiophosphates can be transformed to the α‐configuration in the presence of acid in quantitative yield. The structures of the synthesized derivatives of 7–12 were confirmed by spectroscopic methods. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 465–470, 1999     

Highly Diastereoselective and Enantioselective Olefin Cyclopropanation Using Engineered Myoglobin‐Based Catalysts

Using rational design, an engineered myoglobin‐based catalyst capable of catalyzing the cyclopropanation of aryl‐substituted olefins with catalytic proficiency (up to 46 800 turnovers) and excellent diastereo‐ and enantioselectivity (98–99.9 %) was developed. This transformation could be carried out in the presence of up to 20 g L^?1 olefin substrate with no loss in diastereo‐ and/or enantioselectivity. Mutagenesis and mechanistic studies support a cyclopropanation mechanism mediated by an electrophilic, heme‐bound carbene species and a model is provided to rationalize the stereopreference of the protein catalyst. This work shows that myoglobin constitutes a promising and robust scaffold for the development of biocatalysts with carbene‐transfer reactivity.     

C3‐symmetric trialkyl phosphites as starting compounds of asymmetric synthesis

Chiral C₃‐symmetric trialkyl phosphites, derivatives, of (−)‐(1R,2S,5R)‐menthol, and (−)‐di‐O‐isopropylidene‐1,2:5,6‐α‐D ‐glucofuranose, have been studied as starting reagents for the preparation of chiral organophosphorus compounds. The reactions involve induction at the α‐carbon atom of substituted α‐alkylphosphonates. The stereoselectivity of the reaction depends on the structure of the starting compounds and the reaction conditions. The configurations of the alkylphosphonates were defined by means of NMR spectroscopy and by transformation into corresponding alkylphosphonic acids. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:138–143, 2000     

Two-step asymmetric synthesis of disubstituted N-tosyl aziridines having 98-100% ee: use of a phosphazene base

Unknown diaryl (1-3) and alkyl-phenyl (4, 5) N-tosyl aziridines have been successfully synthesized from pure (R,R,R,S(S))-(-)-sulfonium salt derived from Eliel's oxathiane, tosylimines 11a-f, and using a phosphazene base (EtP(2)) to generate the ylide. Both cis and trans aziridines have exceptionally high enantiomeric purities (98.7-99.9%). The (2R,3R)-configuration of trans-3 and the (2R,3S)-configuration of cis-4 have been determined by X-ray analysis using the Bijvoet method. The R-configuration found at C2 is consistent with the model and all previous results, therefore all trans-aziridines and cis-aziridines have been assigned the (2R,3R)- and the (2R,3S)-configurations, respectively. This two-step asymmetric synthesis can be easily used on gram quantities and involves no unstable/hazardous reagent. The chiral auxiliary is used in a stoichiometric amount but is recovered in high yield and reused.     

The 2‐N,N‐Dibenzylamino Group as a Participating Group in the Synthesis of β‐Glycosides

The novel glycosyl donor 2‐N,N‐dibenzylaminothioglycoside 1 reacts with glycopyranoside alcohols 2 , presumably via intermediate 3 , to provide 1,2‐trans‐linked disaccharides 4 in high yield (78–86 %) and with high stereoselectivity. The N‐benzyl protecting groups are readily cleaved under normal hydrogenolysis conditions, facilitating the synthesis of oligosaccharides with free amino groups.     

Reaction of Grignard Reagents with Diethyl Perfluoroacyl (1-Cyanoethyl)phosphonates. Synthesis of Perfluoroalkylated α,β-Unsaturated Nitriles with Predominant Z-Selectivity

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Gold(I)‐Catalyzed Highly Diastereo‐ and Enantioselective Cyclization–[4+3] Annulation Cascades between 2‐(1‐Alkynyl)‐2‐alken‐1‐ones and Anthranils

This work reports gold‐catalyzed [4+3]‐annulations of 2‐(1‐alkynyl)‐2‐alken‐1‐ones with anthranils to yield epoxybenzoazepine products with excellent exo‐diastereoselectivity (dr>25:1). The utility of this new gold catalysis is manifested by applicable substrates over a broad scope. More importantly, the enantioselective versions of these [4+3]‐cycloadditions have been developed satisfactorily with chiral gold catalysts under ambient conditions (DCM, 0 °C); the ee levels range from 88.0–99.9 %. With DFT calculations, we postulate a stepwise pathway to rationalize the preferable exo‐stereoselection.     

Squaramide‐Catalyzed Asymmetric aza‐Friedel–Crafts/N,O‐Acetalization Domino Reactions Between 2‐Naphthols and Pyrazolinone Ketimines

N‐Boc ketimines derived from pyrazolin‐5‐ones were explored to develop an unprecedented domino aza‐Friedel–Crafts/N,O‐acetalization reaction with 2‐naphthols. The novel method requires a catalyst loading of only 0.5 mol % of a bifunctional squaramide catalyst, is scalable to gram amounts, and provides a new series of furanonaphthopyrazolidinone derivatives bearing two vicinal tetra‐substituted stereogenic centers in excellent yields (95–98 %) and stereoselectivity (>99:1 d.r. and 97–98 % ee ). A different reactivity was observed in the case of 1‐naphthols and other electron‐rich phenols, which led to the aza‐Friedel–Crafts adducts in 70–98 % yield and 47–98 % ee .     

Development of Chiral Spiro P‐N‐S Ligands for Iridium‐Catalyzed Asymmetric Hydrogenation of β‐Alkyl‐β‐Ketoesters

The chiral tridentate spiro P‐N‐S ligands (SpiroSAP) were developed, and their iridium complexes were prepared. Introduction of a 1,3‐dithiane moiety into the ligand resulted in a highly efficient chiral iridium catalyst for asymmetric hydrogenation of β‐alkyl‐β‐ketoesters, producing chiral β‐alkyl‐β‐hydroxyesters with excellent enantioselectivities (95–99.9 % ee) and turnover numbers of up to 355 000.     

Synthesis of trans‐Disubstituted Alkenes by Cobalt‐Catalyzed Reductive Coupling of Terminal Alkynes with Activated Alkenes

A cobalt‐catalyzed reductive coupling of terminal alkynes, RC?CH, with activated alkenes, R′CH?CH₂, in the presence of zinc and water to give functionalized trans‐disubstituted alkenes, RCH?CHCH₂CH₂R′, is described. A variety of aromatic terminal alkynes underwent reductive coupling with activated alkenes including enones, acrylates, acrylonitrile, and vinyl sulfones in the presence of a CoCl₂/P(OMe)₃/Zn catalyst system to afford 1,2‐trans‐disubstituted alkenes with high regio‐ and stereoselectivity. Similarly, aliphatic terminal alkynes also efficiently participated in the coupling reaction with acrylates, enones, and vinyl sulfone, in the presence of the CoCl₂/P(OPh)₃/Zn system providing a mixture of 1,2‐trans‐ and 1,1‐disubstituted functionalized terminal alkene products in high yields. The scope of the reaction was also extended by the coupling of 1,3‐enynes and acetylene gas with alkenes. Furthermore, a phosphine‐free cobalt‐catalyzed reductive coupling of terminal alkynes with enones, affording 1,2‐trans‐disubstituted alkenes as the major products in a high regioisomeric ratio, is demonstrated. In the reactions, less expensive and air‐stable cobalt complexes, a mild reducing agent (Zn) and a simple hydrogen source (water) were used. A possible reaction mechanism involving a cobaltacyclopentene as the key intermediate is proposed.     

Enantioselective Addition of Cyclic Enol Silyl Ethers to 2‐Alkenoyl‐Pyridine‐N‐Oxides Catalysed by CuII–Bis(oxazoline) Complexes

Asymmetric reactions involving (E)‐3‐aryl‐1‐(pyridin‐2‐yl‐N‐oxide)prop‐2‐en‐1‐ones and cyclic enol silyl ethers show good yields and excellent enantioselectivities (up to 99.9 % ee) when catalysed by bis(oxazoline)–Cu^II complexes. Different reaction pathways can be followed by different enol silyl ethers: with 2‐(trimethylsilyloxy)furan, a Mukaiyama–Michael adduct is obtained, whereas a hetero Diels–Alder cycloadduct was formed by using (1,2‐dihydronaphthalen‐4‐yloxy)trimethylsilane. In the latter reaction, the absolute configuration of the product is consistent with a reagent approach to the less hindered Re face of the coordinated substrate in the reactive complex.     

Platinum and Palladium Complexes Bearing New (1R,2R)‐(–)‐1,2‐Diaminocyclohexane (DACH)‐Based Nitrogen Ligands: Evaluation of the Complexes Against L1210 Leukemia

The reaction of (1R,2R)‐(–)‐1,2‐diaminocyclohexane ( 1 ) [DACH] with the aldehyde (1R)‐(–)‐myrtenal ( 2 ) in MeOH afforded the bidentate diimine ligand, (1R,2R)‐(–)‐N¹,N²‐bis{(1R)‐(–)myrtenylidene}‐1,2‐diaminocyclohexane ( 3 ) in a high yield. Reduction of 3 using LiAlH₄ led to the formation of the desired ligand ( 4 ) (1R,2R)‐(–)‐N¹,N²‐bis{(1R)‐(–)myrtenyl}‐1,2‐diaminocyclohexane. Treatment of compound 4 with K₂PtCl₄ or K₂PdCl₄ yielded the corresponding platinum(II) and palladium(II) complexes, Pt‐5 and Pd‐6 , respectively. The reaction of compound 3 with K₂PtCl₄ gave the diimine complex Pt‐7 . The cytotoxic activity of the complexes Pt‐5 , Pd‐6 and Pt‐7 was tested and compared to the approved drugs, cisplatin ( Cis ‐Pt ) and oxaliplatin ( Ox‐Pt ). The complexes ( Pt‐5 , Pd‐6 and Pt‐7 ) inhibit L1210 cell line proliferation with an IC₅₀ of 0.6, 4.2, and 0.7 μL, respectively as evidenced by measuring thymidine incorporation.     

Decomposition of model alkoxyamines in simple and polymerizing systems. II. Diastereomeric N‐(2‐methylpropyl)‐N‐(1‐diethyl‐phosphono‐2,2‐dimethyl‐propyl)‐aminoxyl‐based compounds

Thermal reactions of the alkoxyamine diastereomers DEPN‐R′ [DEPN: N‐(2‐methylpropyl)‐N‐(1‐diethylphosphophono‐2,2‐dimethyl‐propyl)‐aminoxyl; R′: methoxy‐carbonylethyl and phenylethyl] with (R,R) + (S,S) and (R,S) + (S,R) configurations have been investigated by ¹H NMR at 100 °C. During the overall decay the diastereomers interconvert, and an analytical treatment of the combined processes is presented. Rate constants are obtained for the cleavage and reformation of DEPN‐R′ from NMR, electron spin resonance, and chemically induced dynamic nuclear polarization experiments also using 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl (TEMPO) as a radical scavenger. The rate constants depend on the diastereomer configuration and the residues R′. Simulations of the kinetics observed with styrene and methyl methacrylate containing solutions yielded rate constants for unimeric and polymeric alkoxyamines DEPN‐(M)_n‐R′. The results were compatible with the known DEPN mediation of living styrene and acrylate polymerizations. For methyl methacrylate the equilibrium constant of the reversible cleavage of the dormant chains DEPN‐(M)_n‐R′ is very large and renders successful living polymerizations unlikely. Mechanistic and kinetic differences of DEPN‐ and TEMPO‐mediated polymerizations are discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3264–3283, 2002