Showing papers on "Cyclopropane published in 2009"

Experimental Evidence for the All-Up Reactive Conformation of Chiral Rhodium(II) Carboxylate Catalysts: Enantioselective Synthesis of cis-Cyclopropane α-Amino Acids

Abstract: Useful empirical insights onto the enantioinduction process of chiral Rh(II)-carboxylate catalysts are described in the first catalytic asymmetric cyclopropanation of alkenes with α-nitro diazoacetophenones. X-ray, solution NMR, and reactivity studies made on these complexes suggest that the level of asymmetric induction strongly depends on their active symmetry, which in turn relies on the nature of the chiral ligands’ substituents. The catalyst’s ‘All Up’ reactive conformation resulted in being necessary to obtain good stereoselectivity, and the resulting products are shown to be key intermediates in a concise synthesis of highly enantioenriched cis-cyclopropane α-amino acids.

Synthesis of [n,5]-spiroketals by ring enlargement of donor-acceptor-substituted cyclopropane derivatives.

Abstract: Exocyclic enol ethers served as starting materials for the synthesis of [n,5]-spiroketals (n = 5, 6). A metal-mediated cyclopropanation using ethyl diazoacetate afforded spiroannelated cyclopropane derivatives bearing an ester group. A reduction of the corresponding ester by LiAlH4, followed by subsequent oxidation using hypervalent iodine reagents, produced [n,5]-spiroketals in moderate to good yields. The key step within this three-step sequence is the ring enlargement of the three-membered ring with an oxygen donor and a carbonyl acceptor group into the five-membered enol ether system. Catalytic amounts of the Lewis acid Yb(OTf)3 facilitate the ring enlargement and increase the yield of the corresponding spiroketal in many cases. When Yb(OTf)3 was used, our experiments revealed an open transition state rather than a concerted mechanism because the stereochemistry of the spirocenter was not conserved during the ring enlargement. As a result, the thermodynamically more favored anomeric [n,5]-spiroketal w...

Nickel-Catalyzed Ring-Opening Hydroacylation of Methylenecyclopropanes: Synthesis of γ,δ-Unsaturated Ketones from Aldehydes

Abstract: A nickel-catalyzed intermolecular hydroacylation of methylenecyclopropanes (MCPs) has been developed. The reaction proceeds with stereospecific cleavage of the proximal C−C bond of the cyclopropane ring to give γ,δ-unsaturated ketones with high diastereoselectivities. A nickel catalyst generated in situ from Ni(cod)2 and P(n-Bu)3 with a P/Ni ratio of 1:1 is effective for the hydroacylation, in which benzaldehyde derivatives, heteroaryl aldehydes, and aliphatic aldehydes react with MCPs at 60−100 °C to afford the corresponding ketones in high yields.

Highly diastereo- and enantioselective Pd-catalyzed cyclopropanation of acyclic amides with substituted allyl carbonates.

Abstract: A highly diastereo- and enantioselective cyclopropanation reaction was realized in the reaction of acyclic amides with monosubstituted allyl carbonates via Pd-catalysis using a ferrocene ligand with H as a substituent on an oxazoline ring, providing cyclopropane products having three chiral centers in yields of 67−83%, the dr ratio being 4−23:1, and ee being 83−97%. The presence of LiCl is important for the gain of high diastereo- and enantioselectivities of the reaction.

Nickel-catalyzed, regio- and stereoselective hydroalkynylation of methylenecyclopropanes with retention of the cyclopropane ring, leading to the synthesis of 1-methyl-1-alkynylcyclopropanes.

Abstract: The C−H bond of triisopropylsilylacetylene adds to the C═C bond of substituted methylenecyclopropanes in a regio- and stereoselective manner at room temperature in the presence of nickel catalysts bearing PMePh2. The C−C bond formation takes place at the internal sp2 carbon atom with high regioselectivity from the π face opposite the substituents located in the three-membered rings.

RhI‐Catalyzed Regio‐ and Stereospecific Carbonylation of 1‐(1‐Alkynyl)cyclopropyl Ketones: A Modular Entry to Highly Substituted 5,6‐Dihydrocyclopenta[c]furan‐4‐ones

Abstract: Efficient route: A novel Rh(I)-catalyzed regio- and stereospecific carbonylation reaction of (1-alkynyl)cyclopropyl ketones by selective activation of a carbon-carbon sigma bond of the cyclopropane ring was demonstrated (see scheme). This method provides a general, efficient, stereoselective route to synthesise 1,3,5-trisubstituted and 1,3,5,6-tetrasubstituted 5,6-dihydrocyclopenta[c]furan-4-one with convertible functional groups.

Handbook of synthetic photochemistry

Abstract: PHOTOCHEMICAL METHODS Photochemistry and Organic Synthesis Irradiation Apparatus Further Experimental Parameters Photochemical Steps in Synthesis CARBON-CARBON BOND FORMATION BY PHOTOELIMINATION OF SMALL MOLECULES IN SOLUTION AND IN CRYSTALS Introduction Photochemical C-C Bond Formation in Solution Reactions in the Solid State Conclusions INTERMOLECULAR ADDITION REACTIONS ONTO C-C MULTIPLE BONDS Introduction Addition to C-C Double Bonds Addition to C-C Triple Bonds Conclusions FORMATION OF A 3-MEMBERED RING Introduction Di-Pi-Methane Rearrangement Oxa-Di-Pi-Methane Rearrangement and Related Rearrangements [2+1] Cycloaddition of Alkenes with Carbenes Formation of a Cyclopropane Via Intramolecular Hydrogen Abstraction [3+2] Cycloaddition of Arenes with Alkenes Photochemical Synthesis of Three-Membered Heterocycles FORMATION OF A 4-MEMBERED RING I Introduction [2+2]-Photocycloaddition of Non Conjugated Alkenes [2+2]-Photocycloaddition of Aromatic Compounds Photochemical Electrocyclic Reactions Intramolecular Gamma-Hydrogen Abstraction (Yang Reaction) Metal Catalyzed Reactions Various Methods Conclusions FORMATION OF A 4-MEMBERED RING II Introduction [2+2]-Photocycloaddition of Enones (Substrate Type A1) [2+2]-Photocycloaddition of Vinylogous Amides and Esters (Substrate Clases A2 and A3) [2+2]-Photocycloaddition of Alpha, Beta-Unsaturated Carboxylic Acid Derivatives (Substrate Classes A4, A5 and A6) Conclusions and Perspectives FORMATION OF A 4-MEMBERED RING III (OXETANES) Introduction Generally Accepted Mechanisms of the Paterno-Buchi Reaction Regio- and Siteselective Synthesis of Oxetanes Stereoselective Syntheses of Oxetanes Conclusive Remarks FORMATION OF A 5-MEMBERED RING Formation of Five-Membered Ring: Intramolecular Delta-H Abstraction Formation of Five-Membered Rings Via [3+2]-Cycloadditions Photochemical Electrocyclization Reactions: Synthesis of Fused Five-Membered Ring Compounds Photoinduced Electron Transfer Radical Cation Mediated Cyclizations: Synthesis of Five-Membered Carbocyclic as well as Heterocyclic Ring Systems FORMATION OF 6-MEMBERED RING (AND LARGER RINGS) Introduction Photoelectron Transfer Initiated Cyclizations Photoinduced 6Pi-Electrocyclization Photocycloaddition Reactions Remote Intramolecular Hydrogen Abstraction Ring Contraction and Ring Enlargement Other Reactions Summary AROMATIC AND HETEROAROMATIC SUBSTITUTION BY SRN1 AND SN1 REACTIONS Introduction General Mechanistic Features Carbon-Carbon Bond Formation Carbon-Heteroatom Bond Formation Synthesis of Bi, Tri, and Poliaryls Synthesis of Carbocycles and Heterocycles SINGLET OXYGEN AS A REAGENT IN ORGANIC SYNTHESIS Introduction Dioxetanes Endoperoxides Allylic Hydroperoxides Tandem Singlet Oxygen Reactions Conclusion SYNTHESIS OF HETEROAROMATICS VIA REARRANGEMENT REACTIONS Introduction Synthesis of Five-Membered Rings with One Heteroatom Synthesis of Five-Membered Rings with Two Heteroatoms Synthesis of Five-Membered Rings with Three Heteroatoms Synthesis of Six-Membered Rings Synthesis of Seven-Membered Rings Concluding Remarks PHOTOLABILE PROTECTING GROUPS IN ORGANIC SYNTHESIS Introduction Photolabile Protecting Groups Chromatic Orthogonality Two-Photons Absorption Perspectives and Conclusion

In situ generation of zinc carbenoids from diazo compounds and zinc salts: asymmetric synthesis of 1,2,3-substituted cyclopropanes.

Abstract: The first enantioselective cyclopropanation of alkenes using zinc carbenoids generated in situ from diazo compounds and zinc salts is reported. This new method allows the highly enantio- and diastereoselective synthesis of 1,2,3-substituted cyclopropanes via aryl-substituted carbenoids. The first Simmons-Smith reaction using a catalytic amount of zinc to generate enantioenriched cyclopropane is also reported.

Enantioselective synthesis of 1,2,3-trisubstituted cyclopropanes using gem-dizinc reagents.

Abstract: The first asymmetric cyclopropanation of allylic alcohols using gem-dizinc carbenoids, which allows the synthesis of 1,2,3-substituted cyclopropane derivatives in high yields and excellent enantio- and diastereoselectivities, is reported. The initially formed cyclopropylzinc undergoes an in situ B/Zn exchange with the stoichiometric chiral ligand to generate a cyclopropyl borinate that can be directly engaged in a Suzuki−Miyaura cross-coupling reaction.

Reaction of allylic phosphoranes with iron porphyrin carbenoids: efficient, selective, and catalytic intermolecular formal carbenoid insertion into olefinic C-H bonds.

Abstract: A highly efficient, selective, and catalytic intermolecular formal carbenoid insertion reaction into olefinic C−H bonds of allylic phosphoranes is described. This novel insertion reaction proved to proceed via a catalytic cyclopropanation of allylic phosphoranes with diazoacetate, followed by ring opening of the resulting cyclopropane ylide. On the basis of this observation, a facile synthetic method yielding 1,1,4-trisubstituted 1,3-butadienes with high stereoselectivity by a one-pot reaction of tributylphosphine-derived salt 11 with MDA and aldehydes under mild conditions has been developed.

Cyclization and ring-expansion processes involving samarium diiodide promoted reductive formation and subsequent oxidative ring opening of cyclopropanol derivatives.

Abstract: Samarium diiodide promoted reaction of various alpha-bromomethyl cycloalkanones, followed by subsequent treatment with trimethylsilyl chloride, leads to the production of cyclopropyl silyl ethers embedded in bicyclo[m.1.0]alkane frameworks. Treatment of the ethers with oxidative electron-transfer reagents, such as Fe(III), Ce(IV), and Mn(III) salts, generates ring-expanded ketones that convert to cyclic conjugated enones in moderate to good yields. In addition, the reduction-oxidation reaction sequences can be successfully performed in one pot. The regioselectivities of cyclopropane ring opening in the bicyclic substrates depend on the oxidizing agents used. For example, reactions promoted by FeCl3 with pyridine lead to the expected ring-expansion process involving internal-bond cleavage of bicycloalkane and yielding cyclic enones as final products. In contrast, reactions with Ce(NH4)2(NO3)6 or Mn(OAc)3 as oxidizing agents proceed by way of external-bond cleavage to give alpha-iodomethyl cycloalkanones.

Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

Abstract: This study undertakes a theoretical investigation into uncommon hydrogen bonds between the ethyl cation (C2H5 +) and π hydrocarbons. Firstly, it considers the hyperconjugation effect of the ethyl cation, in which the non-localized hydrogen (H+) is taken to be a pseudoatom bound to the carbons of the methyl groups. The goal of the research is to use this electronic phenomenon to gain a better understanding of the (H+···π) and (H+···p-π) hydrogen bonds, which are considered uncommon because they are formed through the interaction of the H+ of the ethyl cation with the π bonds of the acetylene (C2H2) and ethene (C2H4), as well as with the pseudo-π bond of the cyclopropane (C3H6). In view of this, B3LYP/6-311++G(d,p) calculations were used to determine the geometries of the C2H5 +···C2H2, C2H5 +···C2H4, and C2H5 +···C3H6 hydrogen-bonded complexes. Deformations of the bond lengths and bond angles of these systems were analyzed geometrically. Examination of the stretch frequencies and absorption intensities of the (H+···π) and (H+···p-π) hydrogen bonds has revealed red-shifts in π and p-π bonds. After structural modeling and vibrational characterization, analysis of the charge transfer following the ChelpG approach and subsequently quantification of the hydrogen bond energies (basis sets superpostition error and zero point vibrational energies being considered) were used to predict the strength of the (H+···π) and (H+···p-π) hydrogen bonds. In addition, the molecular topography was estimated using the quantum theory of atoms in molecules (QTAIM). QTAIM was chosen because of a desire to understand the (H+···π) and (H+···p-π) hydrogen bonds chemically on the basis of the quantity of charge density and interpretation of Laplacian fields.

Preparation of both enantiomers of cyclopropane derivatives from the reaction of vinyl selenones with di-(−)-bornyl malonate

Abstract: The reaction of vinyl selenones with di-(−)-bornyl malonate and sodium hydride occurred with low diastereoselectivity and afforded a mixture of two diastereomeric cyclopropane derivatives in comparable yields. These, however, could be easily separated by chromatography. Removal of the bornyl group afforded highly enantiomerically enriched cyclopropanes. An example of the simple conversions of these cyclopropanes into useful cyclopropane α-amino acids is also illustrated. The syntheses of several vinyl selenides and selenones are also described.

Metal-free activation in the anionic ring-opening polymerization of cyclopropane derivatives.

Abstract: The successful activation observed when using ButP4 phosphazene base and thiophenol or bisthiols for the anionic ring opening polymerization (ROP) of di-n-propyl cyclopropane-1,1- dicarboxylate is described. Well-defined monofunctional or difunctional polymers with a very narrow molecular weight distribution were obtained through a living process. Quantitative end-capping of the propagating malonate carba- nion was accessible by using either an electro- philic reagent such as allyl bromide or a strong acid such as HCl. Kinetics studies demonstrated a much higher reactivity compared to the conven- tional route using alkali metal thiophenolates.

Synthesis of (+)-Coronafacic Acid

Abstract: An enantioselective synthesis of (+)-coronafacic acid has been achieved. Rhodium-catalyzed cyclization of an α-diazoester provided the intermediate cyclopentanone in high enantiomeric purity. Subsequent Fe-mediated cyclocarbonylation of a derived alkenyl cyclopropane gave a bicyclic enone that then was hydrogenated and carried on to the natural product.

Asymmetric synthesis of trans-disubstituted cyclopropanes using phosphine oxides and phosphine boranes.

Abstract: The stereocontrolled synthesis of trans-disubstituted cyclopropylketones has been achieved from beta-alkyl, gamma-benzoyl phosphine oxides via a three-step cascade reaction incorporating an acyl transfer, phosphinoyl transfer and cyclisation to form the cyclopropane. Using Evans' chiral oxazolidinone auxiliary and by masking the phosphine oxide moiety as a phosphine borane we have extended the method to the synthesis of enantiomerically-enriched trans-disubstituted cyclopropyl ketones.

Addition of carbenes derived from aryldiazoacetates to arenes using chloro(tetraphenylporphyrinato)iron as catalyst

Abstract: Chloro(tetraphenylporphyrinato)iron, Fe(TPP)Cl, is an active catalyst for the Buchner addition of para-substituted methyl 2-phenyldiazoacetates, 1a–d, to substituted benzenes Yields greater than 70% have been achieved at temperatures ranging from 60–100°C Reactions of substituted methyl 2-phenyldiazoacetates with benzene gave rapidly equilibrating mixtures of norcaradienecycloheptatriene valence isomers, 2a–d/2′a–d, in yields over 70% Treatment of chlorobenzene with methyl 2-phenyldiazoacetate produced a regio-isomeric mixture of 7-carbomethoxy-2-chloro-7-phenylnorcaradiene/7-carbomethoxy-2-chloro-7-phenylcycloheptatriene, 3a/3′a, and 7-carbomethoxy-3-chloro-7-phenylnorcaradiene/7-carbomethoxy-3-chloro-7-phenylcycloheptatriene, 4a/4′a When p-methylanisole was treated with methyl 2-phenyldiazoacetate at 80°C, a product that largely favored a fused cyclopropane structure, 7-carbomethoxy-2-methoxy-5-methyl-7-phenylnorcaradiene, 12a, was obtained along with the benzylic C–H insertion product methyl 3-(p-methoxyphenyl)-2-phenylpropionate, 13a Heating the norcaradiene product 12a at 110°C yielded the ring-opened diarylacetate, 14a The diene forms of the fluxional norcaradiene-cycloheptatriene systems were trapped with benzyne to give one stereoisomer of 3,3-disubstituted benzhomobarralenes, 18a–d The norcaradiene-cycloheptatriene valence isomers were quantitatively converted into ring-opened diaryl acetate products upon acidification in acetonitrile Rates for the addition of methyl (p-chlorophenyl)diazoacetate to benzene were first order with respect to the diazo reagent A concerted mechanism involing an iron carbene complex is proposed for these iron porphyrin-catalyzed Buchner reactions

A temporary stereocentre approach for the asymmetric synthesis of chiral cyclopropane-carboxaldehydes.

Abstract: A novel way of combining chiral auxiliaries and substrate directable reactions is described that employs a three-step sequence of aldol/cyclopropanation/retro-aldol reactions for the asymmetric synthesis of enantiopure cyclopropane-carboxaldehydes. In the first step, reaction of the boron enolate of (S)-N-propionyl-5,5-dimethyl-oxazolidin-2-one with a series of α,β-unsaturated aldehydes affords their corresponding syn-aldol products in high de. In the second step, directed cyclopropanation of the alkene functionalities of these syn-aldols occurs under the stereodirecting effect of their ‘temporary’β-hydroxyl stereocentres to give a series of cyclopropyl-aldols in high de. Finally, retro-aldol cleavage of the lithium alkoxide of these cyclopropyl-aldols results in destruction of their temporary β-hydroxy stereocentres to afford the parent chiral auxiliary and chiral cyclopropane-carboxaldehydes in >95% ee. The potential of this methodology has been demonstrated for the asymmetric synthesis of the cyclopropane containing natural product cascarillic acid in good yield.