A novel and highly efficient two-carbon ring expansion

[reaction: see text]. Dynamic gas-phase thermoisomerization (DGPTI) of medium- and large-ring 3-vinylcycloalkanones at 600-630 degrees C produces isomeric gamma,delta-unsaturated cycloalkanones expanded by two carbon atoms. A reaction mechanism involving an open-chain diradical intermediate, followed by intramolecular recombination under insertion of the vinyl group is proposed. Substituents on the vinyl moiety are transferred locospecifically to the ring-expanded ketones as corresponding beta- and gamma-substituents, respectively. The preparation of extraordinary cyclic allenes can be accomplished by DGPTI (540 degrees C) of 3-ethinylcyclododecanone.     

A practical method for regiocontrolled one-carbon ring contraction

A practical and efficient method for the perfluorobutanesulfonyl azide-mediated one-carbon ring contraction of cyclic enoxysilanes is described. High-yielding procedures for the elaboration of the resulting N-acyl sulfonamide products are reported.     

A highly selective one-carbon ring enlargement reaction directed by silicon

On treatment with dichloroaluminum phenoxide, 1-(trimethylsilyl)-methylcycloalkanecarbaldehydes undergo selective rearrangement of ring carbons to give the corresponding one-carbon enlarged 2-(trimethylsilyl)methylcycloalkanones in excellent yields.     

A highly diastereoselective MgI2-mediated ring expansion of methylenecyclopropanes

[reaction: see text] A highly diastereoselective MgI(2)-mediated ring expansion of methylenecyclopropane amides to functionalized pyrrolidines has been developed using chiral aromatic sulfinimines. The 2,3,4-trisubstituted pyrrolidines were isolated in generally good to excellent yields and in excellent diastereoselectivities for aromatic and heterocyclic sulfinimines.     

A highly efficient synthesis of the hemibrevetoxin B ring system

[reaction: see text] This Letter describes the synthesis of the hemibrevetoxin B tetracyclic ring system in 14 linear transformations from the Danishefsky-Kitahara diene.     

A method for synthesis of bicyclo[3.3.0]oct-1-en-3-ones from cyclobutanones with one-carbon ring expansion and its application to a formal synthesis of racemic 1-desoxyhypnophilin

A method for synthesis of 2-cyanobicyclo[3.3.0]oct-1-en-3-ones and 2-substituted bicyclo[3.3.0]oct-1-en-3-ones was developed by assembly of three components, cyclobutanones, chloromethyl p-tolyl sulfoxide, and nitriles, with one-carbon ring expansion of the cyclobutane ring. As an application of this method, a formal synthesis of 1-desoxyhypnophilin in racemic form was performed starting from 3,3-di(phenylthiomethyl)cyclobutanone.     

A highly efficient iterative approach to fused ether ring systems

An iterative synthesis of fused ether ring systems has been developed. This strategy couples a cyclic enol ether oxidation and carbon-carbon bond forming reaction in one flask with an acid catalyzed cyclic acetal formation and alkoxide elimination in another flask. The result is a general and highly efficient two flask synthesis of fused ethers as are present in a wide variety of bioactive natural products.     

A highly efficient reaction of thiobenzophenone for 1-diazoalkene

9-(Diazomethylene)fluorene formed $\text{in situ}$ in the reaction of 9-(aminomethylidene)fluorene with n-butylnitrite, was successfully trapped by thiobenzophenone to afford 1,3,4-thiadiazoline derivative which was confirmed by spectroscopic and analytical data.     

A facile ring opening reaction of furfuryl selenides. Highly efficient method for dienones

Treatment of furfuryl phenyl selenides with butyllithium or metallic lithium induces the facile ring opening reaction of their furan ring and the corresponding dienones are obtained in high yields.     

A three-carbon (n+1+2) ring expansion method for the synthesis of macrocyclic enones. Application to muscone synthesis

The three-carbon ring expansion methodology commences with the preparation of a cyclic allene (C9, C11, C13), readily available from the corresponding cycloalkene via dibromocarbene addition and subsequent treatment with methyllithium. Dichloroketene addition to the cyclic allene regioselectively provides the [2+2] cycloadduct, which is reductively dechlorinated with zinc in methanol. The resulting cyclobutanone is then catalytically hydrogenated; cyclobutanone ring opening is affected with trimethylsilyl iodide; immediate dehydroiodination of the resulting β-iodocycloalkanone with diazabicycloundecane (DBU) provides the corresponding macrocyclic enone. The 15-membered enone was converted to d,l-muscone with (CH₃)₂CuLi.     

Convenient synthesis of 1-alkoxy-1,2-dihydrophosphinine 1-oxides by ring enlargement

The addition of dichlorocarbene to the double bond of 1-alkoxy-dihydrophosphole oxides and subsequent thermolysis of the adduct so obtained affords mixtures of the two double bond isomers of alkoxy-dihydrophosphinine oxides, if the latter step is carried out in the presence of triethylamine. Experimental data support the involvement of a cationic intermediate during the opening of the cyclopropane ring. A simplified procedure for the preparation of the starting dihydrophospholes is also presented.     

Diradical-promoted (N + 2 - 1) ring expansion: an efficient reaction for the synthesis of macrocyclic ketones

[reaction: see text] A diradical-promoted (n + 2 - 1) ring expansion reaction based on vinyl side chain insertion (+2C) and decarbonylation (-1C) has been developed. Flash vacuum pyrolysis (FVP) of medium- and large-ring 2-trimethylsilyloxy-2-vinyl-cycloalkanones at 500-600 degrees C affords the one-carbon ring-expanded cycloalkanones in good yields. Methyl groups on the vinyl moiety are transformed regiospecifically as corresponding alpha- and beta-substituents, respectively. 2-Ethynyl precursor analogues react in a manner similar to give alpha,beta-unsaturated cyclic ketones.     

A unique and highly efficient method for catalytic olefin aziridination

A facile, high yielding, and stereospecific method for olefin aziridination is described. This process capitalizes on the unique reactivity of sulfamate esters in combination with 1-2 mol % Rh2(tfacam)4 and PhI(OAc)2 as the terminal oxidant to promote N-atom transfer reactions. A range of structurally and electronically disparate alkenes are found to react under conditions that employ substrate as the limiting reagent and only a slight excess of H2NSO3CH2CCl3 as the nitrogen source. The product alkoxysulfonyl aziridines are useful intermediates that react smoothly with nucleophiles to generate 1,2-amine derivatives. Following aziridine ring-opening, the N-trichloroethoxysulfonyl group can be removed under mild reductive conditions (Zn(Cu)/AcOH-MeOH) to give the corresponding 1 degrees amine. The efficient and convenient performance of this chemistry should establish it as a useful tool for synthesis.     

Thermodecarbonylation of α-substituted cycloalkanones: a convenient one-carbon ring contraction method

A new and very convenient one-carbon ring contraction method is reported. Pyrolysis of α-substituted cycloalkanones at 600-650 °C under flow conditions produces the ring contracted compounds under loss of carbon monoxide. Substrates varying in ring size and nature of the α-substituent have been investigated.     

Synthesis of functionalized tetrahydro-1,3-diazepin-2-ones and 1-carbamoyl-1H-pyrroles via ring expansion and ring expansion/ring contraction of tetrahydropyrimidines

A general approach to 6-phenylthio-substituted 2,3,4,5-tetrahydro-1H-1,3-diazepin-2-ones based on the ring expansion reaction of 1,2,3,4-tetrahydropyrimidin-2-ones under the action of nucleophiles has been developed. The first step of the synthesis was preparation of N-[(2-benzoyloxy-1-tosyl)ethyl]urea by three-component condensation of 2-benzoyloxyethanal, urea and p-toluenesulfinic acid. Nucleophilic substitution of the tosyl group in the obtained sulfone with sodium enolates of α-phenylthioketones followed by cyclization-dehydration, and debenzoylation gave 4-hydroxymethyl-5-phenylthio-1,2,3,4-tetrahydropyrimidin-2-ones which were transformed into the 4-mesyloxymethyl-derivatives. Treatment of the latter with nucleophilic reagents, such as NaCN, sodium diethyl malonate, PhSNa, MeONa, NaBH(4), sodium succinimide, or potassium phthalimide, afforded the target multi-functionalized diazepinones. The obtained 6-phenylthio-diazepinones and their 6-tosyl-substituted analogues were converted into 3-substituted 1-carbamoyl-1H-pyrroles under acidic conditions as a result of ring contraction. Effective one-pot synthesis of the latter from 4-mesyloxymethyl-pyrimidines was realized using a ring expansion/ring contraction sequence.     

Palladium-catalyzed ring expansion reaction of 1-alkynylcyclobutanols with aryl iodides: an efficient route to 2-disubstituted methylenecyclopentanones

The reaction of 1-alkynylcyclobutanols with aryl iodides in the presence of Pd(OAc)₂ and Et₃N in acetonitrile at 80°C for 24 h gives 2-disubstituted methylenecyclopentan-1-ones in modest to good yields. The tandem insertion-ring expansion process proceeds via the formation of an alkynyl π-complex, followed by migration of a carbon-carbon bond of the tert-alkanol to form the cyclopentanones stereoselectively.     

Photochemical and thermal addition of alcohols to 2-cyanofuran : 1-alkoxy-2-cyanocyclopropane-3-carboxaldehydes; 2-alkoxy-3-cyano-2, 3-dihydrofuran; 1-alkoxy-1-(2-furan)-methylenimine; photochemical ring contraction; cis-trans-isomerization of dihydrofurans

The photolysis of 2-cyanofuran in alcohols yields 1-alkoxy-2-cyanocyclopropane-3-carboxaldehydes as the major products, which rearrange thermally to trans-2-alkoxy-3-cyano-2,3-dihydrofurans and small amounts of cis-isomers. Thermal mutual rearrangements between trans- and cis-2- alkoxy-3-cyano-2, 3-dihydrofurans were studied. Liquid phase thermal addition of alcohols in a temperature range between 100° and 200° yields 1-alkoxy-1-(2-furan)-methylenimines, which rearrange photochemically to 2-furyl-methylamino-ketone in case of the methoxy derivative. 3-Cyano-furan does not yield photoaddition or thermal addition products. The different behaviors of 2- and 3-substituted furans are discussed.     

A novel ring expansion of 1-carboethoxy-1,1-trimethylene-3-diazo-2-propanone

The title compound undergoes an unusual thermal cyclobutyl to cyclopentyl rearrangement initiated by the intramolecular reaction of a ketene moiety produced in situ with the pendant functionality.     

Mild and highly efficient method for the synthesis of 2-arylbenzimidazoles and 2-arylbenzothiazoles

A new, convenient method for the syntheses of 2-substituted benzimidazole and benzothizole is described. Short reaction times, large-scale synthesis, easy and quick isolation of the products, excellent chemoselectivity, and excellent yields are the main advantages of this procedure.     

A ring expansion method for the preparation of 2,3-dihydro-1,4-benzothiazines from 2-aryl-2,3-dihydrobenzothiazoles

In the presence of KF in dimethylformamide (DMF) 2-aryl-2,3-dihydrobenzothiazoles react with α-haloketones to provide in good yield 2,3-dihydro-2,3-disubstituted-1,4- benzothiazines.