Rh(II)-catalyzed [2,3]-sigmatropic rearrangement of sulfur ylides derived from N-tosylhydrazones and sulfides

In this paper, Rh₂(OAc)₄-catalyzed [2,3]-sigmatropic rearrangement of sulfur ylides derived from N-tosylhydrazones and sulfides is reported. A series of tosylhydrazones derived from aldehydes were successfully used for [2,3]-sigmatropic rearrangement by reaction with either allylic phenyl sulfides or propargyl phenyl sulfides. The reaction conditions were optimized and afforded the products in moderate to good yields. In addition, a novel and convenient approach for the synthesis of cyclobutenones and cyclopropanes has been developed through direct oxidation of the rearrangement products.     

Synthesis of functionalized CF3-containing heterocycles via [2,3]-sigmatropic rearrangement and sequential catalytic carbocyclization

A new efficient access to functionalized CF₃-substituted and nitrogen or sulfur-containing heterocycles has been developed directly from diazocompounds CF₃C(N₂)Z (Z=CO₂Me, P(O)(OEt)₂). The method is based on the direct selective synthesis of doubly unsaturated substrates followed by metal-mediated carbocylization. The first step has been performed by Cu(II)-catalyzed [2,3]-sigmatropic rearrangement of propargyl- or/and allyl-containing sulfur and nitrogen ylides leading to fluorinated enynes, diolefins, and especially allenynes derivatives. The second step involves their carbocyclization via ring closing metathesis and Pauson-Khand reaction.     

Rh(I)-catalyzed allenic Pauson-Khand reaction: first construction of the bicyclo[6.3.0]undecadienone ring system

The Rh(I)-catalyzed Pauson-Khand reaction of allenynes afforded the bicyclo[6.3.0]undecadienones as well as their benzo and furo derivatives. In addition, a novel [RhCl(CO)₂]₂-catalyzed [2,3]-sigmatropic rearrangement of the sulfinic ester species of propargyl alcohols was developed.     

Bi(OTf)3 catalyzed synthesis of acyclic β-sulfanyl ketones via a tandem Meyer-Schuster rearrangement/conjugate addition reaction

A new strategy to prepare acyclic β-carbonyl thioethers from propargyl alcohols and sulfur nucleophiles is reported. The investigation of the reaction substrates scope indicated that primary 3-aryl propargyl alcohols and thiols underwent the transformation smoothly. The reaction probably proceeded a Bi(OTf)₃-catalyzed tandem Meyer-Schuster rearrangement of 3-aryl propargyl alcohol, followed by a thiol Michael conjugate addition of thiols to in situ generated α, β-unsaturated ketones. The method was 100% atom economic, high-yielding, and easy to handle, making it a valuable method for the construction of β-carbonyl sulfides.     

Synthesis of oxygen-containing heterocyclic compounds based on the intramolecular O-H insertion and Wolff rearrangement of α-diazocarbonyl compounds

The addition products of Ti(IV)-enolate derived from β-keto α-diazo carbonyl compound to ketones or α,β-unsaturated compounds were subjected to Rh₂(OAc)₄-catalyzed and photo-induced diazo decomposition. The Rh₂(OAc)₄-catalyzed reaction afforded intramolecular O-H insertion products, while the photo-induced reaction gave Wolff rearrangement/intramolecular nucleophilic addition products. The transformations represent new approaches to tetrahydrofuran and γ-butyrolactone derivatives.     

Palladium(II)-catalyzed ring expansion of a 1-alkenyl cyclopentanol

7,7-Dimethyl-2-methylenenorbornan-1-ol, a strained bicyclic 1-alkenyl cyclopentanol, undergoes Wagner-Meerwein rearrangement to fenchone under treatment with a catalytic amount of PdCl₂(PPh₃)₂ in refluxing N-methylpyrrolidin-2-one. The described reaction constitutes the first example of the palladium(II)-catalyzed ring expansion of 1-alkenyl cyclopentanols to the corresponding cyclohexanones.     

Au(I)-catalyzed efficient synthesis of α-acyloxy-α′-silyl ketones from α-acyloxy-α-alkynylsilanes

The Au(I)-catalyzed novel conversion of α-acyloxy-α-alkynylsilanes to α-acyloxy-α′-silyl ketones is reported. Ph₃PAuOTf in dioxane in the presence of 1 equiv of H₂O efficiently catalyzed both the [3,3] sigmatropic rearrangement and allenic transformation of the resulting ester to give the α-acyloxy-α′-silyl ketones in a one-pot procedure.     

Synthesis and Characterization of (5′-Deoxyadenosin-5′-yl)cobalamin ( = ‘Adenosylcobalamin’) Analogues Mimicking the Transition-State Geometry of Coenzyme-B12-Dependent Rearrangements

A convergent synthesis of the five novel analogues 1a–e of (5′-deoxyadenosin-5′-yl)cobalamin ( = ‘adenosylcobalamin’) is described. The analogues 1a–e carry oligomethylene chains (C₃–C₇) inserted between the central Co-atom and the 5′-O-atom of the adenosine moiety and are thought to mimick the transition-state geometry in coenzyme- B ₁₂-catalyzed rearrangement. All five analogues were characterized by NMR, UV, and FAB mass spectrometry.     

Bromodimethylsulfonium bromide (BDMS) in ionic liquid: a mild and efficient catalyst for Beckmann rearrangement

Bromodimethylsulfonium bromide (BDMS)-catalyzed Beckmann rearrangement of a variety of ketoximes has been carried out in the imidazolium-based ionic liquid [bmim]PF₆ under mild conditions without using any additional cocatalyst or solvent to afford excellent conversion and selectivity. The ionic liquid is recovered and reused for up to three runs without any loss of efficiency.     

Molecular rearrangement of epoxide derived from sesquiterpenes by Lewis acid catalysis

The molecular rearrangement under mild conditions of two epoxides derived from the sesquiterpenic himachalenes has been studied using BF₃·Et₂O as the Lewis acid catalyst. Both reactions are efficient and selective affording in each case one new enantiomerically pure ketone isolated in good yield. Their structures have been fully characterized by spectroscopic methods. Mechanisms for the BF₃-catalyzed reactions are proposed.     

Synthesis and transformations of metallacycles 46. Catalytic cycloalumination reaction in the synthesis of bis(phospholanes)

An efficient one-pot synthesis of bis(phospholanes) was elaborated, which included a sequential Cp₂ZrCl₂-catalyzed cycloalumination of α,ω-diolefins with AlEt₃, giving the corresponding bis(aluminacyclopentanes), and their in situ reaction with dihalophosphines to furnish the target α,ω-bis(phospholanes). The reaction of these compounds with H₂O₂ or elementary sulfur gave the corresponding bis(phospholane 1-oxides) and bis(phospholane 1-sulfides).

     

I2-Catalyzed Cycloisomerization of Ynamides: Chemoselective and Divergent Access to Indole Derivatives

Herein, an I₂-catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom-economy. This protocol not only represents the first molecular-iodine-catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine-ynamides involving distinctively different C(sp³)−C(sp³) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I₂. Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5-exo-dig cyclization/rearrangement process.     

Highly efficient KF/Al2O3-catalyzed versatile hetero-Michael addition of nitrogen, oxygen, and sulfur nucleophiles to α,β-ethylenic compounds

The first example of KF/Al₂O₃-catalyzed versatile hetero-Michael addition reaction of nitrogen, oxygen, and sulfur nucleophiles was developed for facile preparation of organic compounds of widely different structures. In contrast with the existing methods using many acidic catalysts, this method is very general, simple, high-yielding, environmentally friendly, and oxygen and moisture tolerant.     

The methoxycarbonylcarbene insertion into 1,3-dithiolane and 1,3-oxathiolane rings

Treatment of substituted 1,3-dithiolanes and 1,3-oxathiolanes with methyl diazoacetate in the presence of Rh₂(OAc)₄ effects ring expansion to the corresponding substituted 1,4-dithiane-2-carboxylates and 1,4-oxathiane-3-carboxylates. The sulfur ylides initially generated in these reactions undergo Stevens rearrangement in competition with both [2,3]-C-C-sigmatropic rearrangement and intramolecular fragmentation. In the case of 2-styryl-substituted 1,3-oxathiolane and 1,3-dithiolane, ring expansion on one-, three- and four-carbons subsequently takes place.     

Pd(II)-catalyzed aliphatic Claisen rearrangements of acyclic allyl vinyl ethers

Palladium (II)-catalyzed [3,3] sigmatropic rearrangement of acyclic allyl vinyl ethers delivers 2,3-anti disubstituted pentenal Claisen adducts with high diastereoselectivity. Reaction conditions for circumventing allyl vinyl ether cleavage that had previously plagued catalyzed rearrangement of α-unsubstituted vinyl ether substrates are described. Merging Pd(II) catalysis with the facile access to the Claisen substrates afforded by Ir(I)-catalyzed olefin isomerization provides an expedient procedure for realizing asymmetric anti-selective Claisen rearrangements.     

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide

Stereoselective synthesis of the C94-C104 fragment of symbiodinolide which is a polyol marine natural product with a molecular weight of 2860 has been accomplished. The synthetic route features Achmatowicz rearrangement and RuO₄-catalyzed dihydroxylation for the construction of the tetrahydropyran moiety and the dithiane addition to the aldehyde for the introduction of the side chain.     

Synthesis of N-aminopyrazoles by Fe(II)-catalyzed rearrangement of 4-hydrazonomethyl-substituted isoxazoles

A novel effective method is reported for the preparation of 1-amino-1H-pyrazole-4-carboxylic acid derivatives by Fe(II)-catalyzed rearrangement of isoxazoles having (2,4-dinitrophenylhydrazono)methyl substituent at C4. The reaction proceeds smoothly for both E and Z isomers of 4-(hydrazonomethyl)isoxazoles, and this means it is not necessary to separate mixtures of E/Z-isomers of the hydrazones prepared by reaction of 5-methoxy/pirrolidino-4-carbonylisoxazoles and 2,4-dinitrophenylhydrazine. The rearrangement proceeds via the formation of an aziridine intermediate which can be isolated in certain cases. The 2-nitro group in the synthesized 1-[(2,4-dinitrophenyl)amino]-1H-pyrazole-4-carboxylic esters can be selectively reduced in two steps via acylation of the amino group followed by hydrogenation-deacylation using H₂-Pd/C.     

Chemo- and stereoselective six-membered oxonium ylide formation–[2,3]-sigmatropic rearrangement of 2-diazo-3-ketoesters with dirhodium(II) catalyst and its application to the synthesis of (+)-tanikolide

Dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement of 6-allyloxy-2-diazo-3-ketoesters possessing a C-6 substituent is described. The reaction of 6-alkyl- or 6-aryl-substituted 6-allyloxy-2-diazo-3-ketoesters with a catalytic amount of Rh₂(S-PTTL)₄ proceeded in a chemoselective and stereoselective manner to provide 6-substituted 2-allyl-3-oxotetrahydropyran-2-carboxylates in good yields and with high diastereoselectivities. To demonstrate the utility of this sequential reaction, we conducted the total synthesis of (+)-tanikolide, in which the construction of the δ-lactone skeleton was achieved by employing a 2-iodobenzamide-catalyzed oxidative cleavage of tetrahydropyran-2-methanol.     

Reactions of siliranes with elemental sulfur and with t-butyl mercaptan. Preparation of the 2,3-dithia-1-silacyclopentane ring system

The reaction of 1,1-dimethyl-trans-2,3,-bis(2′,2′-dimethylcyclopropylidene)-1-silirane with S₈ gives a mixture of four isomeric products derived from incorporation of one sulfur atom and a cyclopropylcarbinyl-to-butenyl type rearrangement. This silirane reacts with t-butyl mercaptan to give a product of mercaptan addition in which a cyclopropylcarbinyl-to-butenyl rearrangement also has occurred. Hexamethylsilirane reacts with S₈ to give 1,1,4,4,5,5-hexamethyl-2,3-dithia-1-silacyclopentane in high yield. These reactions are discussed in terms of free radical mechanisms.     

Synthesis of intramolecularly cyclized poly(phenylene sulfide)s of the thianthrene-type

Oligomers containing thianthrene units in the chain are synthesized by AlCl₃-catalyzed reaction of diphenylsulfide, thianthrene, poly(p-phenylene sulfide), and poly(m-phenylene disulfide) with sulfur at 80°C. The products are compared with that obtained by the reaction of diphenylsulfide with AlCl₃ at 225°C. IR spectra and elemental analyses are consistent with cyclic chain structures and show a higher cyclization for CHCl₃-insoluble fractions. X-ray diffraction analysis indicates the same structure for the samples obtained with the different methods of synthesis. A possible structural model is tentatively proposed.