Unexpected formation of stannolanes and trigonal bipyramidal tin complexes by radical cyclization reaction

Optically active stannolanes and trigonal bipyramidal pentacoordinated tin complexes were readily prepared by radical cyclization of N-propargylated chiral aza-Morita-Baylis-Hillman adducts induced by Bu(3)SnH. A translocated radical through the cyclization attacked the Bu(3)Sn group in an S(H)2 manner.     

Thermal-Induced Dimerization Cyclization of Ethyl N-(Styrylcarbamoyl)acetates: Formation of 4-Hydroxy-2(1H)-pyridone-3- carboxamide Derivatives

Thermal-induced dimerization cyclization of ethyl N-(styrylcarbamoyl)acetate derivatives has been investigated, leading to 4-hydroxy-2(1H)-pyridone-3-carboxamide derivatives with good yields in diphenyl ether on 200–210 °C. Ethyl N-(styrylcarbamoyl)acetate derivatives readily provided the intermolecular cyclization products 4-hydroxy-2(1H)-pyridone-3-carboxylates on reflux in xylene. In addition, several related 3-acetyl-4-hydroxy-5-phenylpyridin-2(1H)-ones have been prepared. It provided an efficient preparation of 4-hydroxy-2(1H)-pyridone-3-carboxamide derivatives.     

Highly stereoselective access to 2,4- and 2,4,5-substituted tetrahydrofurans from α-silylacetic esters. A study of homoallylic stereocontrol.

Cis-2,4- and cis-cis-2,4,5-substituted tetrahydrofurans have been prepared stereoselectively using electrophile-mediated cyclization of β-hydroxyhomoallylsilanes, readily available from α-silylacetic esters.     

Novel chemical cyclization routes to prepare ladder-type conjugated molecules

Novel chemical cyclization methods using BBr₃ and fluoride have been successfully developed to produce ladder-type conjugated molecules. Both electron rich and electron poor conjugated polymers were readily prepared by BBr₃-promoted cyclization method both in solution and on film. Extension of the π-conjugated systems caused by their rigid planar structures was confirmed by the absorption or emission spectroscopy. A fluoride-induced aromatic cyclization method was also developed and an application to fluoride ion sensing scheme was suggested.     

Parallel synthesis of a multi-substituted benzo[b]furan library

The solution-phase parallel synthesis of a 121-member library of multi-substituted benzo[ b]furans is described. 2,3,5-Trisubstituted benzo[ b]furans have been prepared by the palladium-catalyzed substitution of 3-iodobenzofurans by Suzuki-Miyaura, carbonylative Suzuki, Sonogashira, Heck, and carboalkoxylation chemistry. The 3-iodobenzofurans are readily prepared in good to excellent yields by the palladium/copper-catalyzed cross-coupling of various o-iodoanisoles and terminal alkynes, followed by electrophilic cyclization with ICl.     

Synthesis of 2,3-disubstituted benzo[b]furans by the palladium-catalyzed coupling of o-iodoanisoles and terminal alkynes, followed by electrophilic cyclization

[reaction: see text] 2,3-Disubstituted benzo[b]furans are readily prepared under very mild reaction conditions by the palladium/copper-catalyzed cross-coupling of various o-iodoanisoles and terminal alkynes, followed by electrophilic cyclization with I2, PhSeCl, or p-O2NC6H4SCl. Aryl- and vinylic-substituted alkynes undergo electrophilic cyclization in excellent yields. Biologically important furopyridines can be prepared by this approach in high yields.     

Efficient synthesis of furan-2-ylacetates, 7-(alkoxycarbonyl)benzofurans, and 7-(alkoxycarbonyl)-2,3-dihydrobenzofurans based on cyclization reactions of free and masked dianions: a "cyclization/dehydrogenation" strategy

[reaction: see text] A variety of furan-2-ylacetates have been prepared by dehydrogenation of monocyclic 2-alkylidenetetrahydrofurans, which are readily available by cyclizations of open-chained 1,3-dicarbonyl dianions with 1-bromo-2-chloroethane. 5'H-[2,3']Bifuranyl-2'-ones are available based on sequential "cyclization/dehydrogenation" reactions of alpha-acetyl-gamma-butyrolactones. A variety of 7-(alkoxycarbonyl)benzofurans and 7-(alkoxycarbonyl)-2,3-dihydrobenzofurans were prepared by a cyclization/dehydrogenation strategy. These reactions rely on cyclizations of 2-oxocycloalkane-1-carboxylate-derived 1,3-dicarbonyl dianions ("free dianions") or 1,3-bis-silyl enol ethers ("masked dianions") with various 1,2-dielectrophiles.     

2-Acylamino-3-chloroacrylonitriles,Promising Reagents for Heterocyclization

2-Acylamino-3-chloroacrylonitriles prepared by treating available N-(1,2,2-trichloroethyl)amides of carboxylic acids with sodium cyanide readily undergo cyclization in the presence of excess hydrazine hydrate. The cyclization products, 5-amino-4-acylaminopyrazoles, were applied to the synthesis of new imidazo[4,5-c]pyrazole and pyrazolo[1,5-a]pyrimidine derivatives.     

Novel synthesis of flavonoids of Scutellaria baicalensis Georgi

A concise and efficient total synthesis of the flavonoids baicalein, oroxylin A and wogonin was described. Intramolecular oxidative cyclization followed by demethylation of chalcone 1, readily prepared from trimethoxyphenol, afforded, depending upon the controlled conditions, baicalein or oroxylin A in excellent yields. Demethylation of 1 yielded 3, which, by oxidation with I(2)/dimethyl sulfoxide (DMSO), was readily converted to oroxylin A and wogonin after column chromatography.     

Synthesis of functionalized acetophenones by [3 + 3] cyclizations of 1,3-bis-silyl enol ethers with 2-acetyl-3-silyloxyalk-2-en-1-ones

The TiCl₄ mediated cyclization of 1,3-bis-silyl enol ethers with 2-acetyl-1-silyloxybut-1-en-3-one and 3-acetyl-4-silyloxypent-3-en-2-one, readily prepared from 3-formyl(acetylacetone) and triacetylmethane, afforded a variety of functionalized acetophenones.     

A facile synthesis of 2-methylquinolines via Pd-catalyzed aza-wacker oxidative cyclization

A novel Pd-catalyzed Wacker-type oxidative cyclization under air is described. By using this cyclization, a series of 2-methylquinolines are readily prepared with good yields under mild conditions.     

Synthesis of 3-iodoindoles by electrophilic cyclization of N,N-dialkyl-2-(1-alkynyl)anilines

[reaction: see text] A wide variety of N-alkyl-3-iodoindoles are readily prepared under very mild reaction conditions by the palladium/copper-catalyzed cross-coupling of N,N-dialkyl-o-iodoanilines and terminal alkynes, followed by electrophilic cyclization by I(2)(). Alkyl-, aryl-, and vinylic-substituted alkynes all undergo iodocyclization in excellent yields.     

Synthesis of indoles via 6pi-electrocyclic ring closures of trienecarbamates

A new method for the preparation of indoles from readily available alpha-haloenones and alpha-(trialkylstannyl)enecarbamates is described. Following a Stille coupling, trienecarbamate 2 is electronically activated to undergo a facile 6pi-electrocyclic ring closure and subsequent oxidation to afford protected aniline 4. Upon deprotection and reductive amination, acid 5 underwent clean cyclization to N-acetylindole 6 (Ac2O, NEt3, 130 degrees C). This method has been used to construct a variety of substituted indoles that are not easily prepared by conventional indole annelation methods.     

Superior reactivity of thiosemicarbazides in the synthesis of 2-amino-1,3,4-oxadiazoles

A facile and general protocol for the preparation of 2-amino-1,3,4-oxadiazoles is reported. This method relies on a tosyl chloride/pyridine-mediated cyclization of a thiosemicarbazide, which is readily prepared by acylation of a given hydrazide with the appropriate isothiocyanate. Cyclization of the thiosemicarbazide consistently outperforms the analogous semicarbazide cyclization under these conditions, for 18 distinct examples. Utilizing this protocol, we have prepared 5-alkyl- and 5-aryl-2-amino-1,3,4-oxadiazoles in 78-99% yield.     

Synthesis of 2(3H)-furanones via electrophilic cyclization

A variety of highly substituted 2(3H)-furanones are readily prepared from 3-alkynoate esters and the corresponding acids via electrophilic cyclization. Successful electrophiles in this process include I2, ICl, and PhSeCl. This highly efficient process proceeds under mild conditions, tolerates various functional groups, and generally provides substituted 2(3H)-furanones in good to excellent yields.     

Electrochemical Tri- and Difluoromethylation-Triggered Cyclization Accompanied by the Oxidative Cleavage of Indole Derivatives

Considering their unique roles in organic synthesis, and pharmaceutical and agrochemical applications, the development of fluoroalkylation, cyclization, and indole oxidative cleavage are important topics. Herein, an unprecedented electrochemical tri- and difluoromethylation/cyclization/indole oxidative cleavage process occurring in an undivided cell is presented. The protocol employs a readily prepared Langlois reagent as the fluoroalkyl source, affording a series of tri- or difluoromethylated 2-(2-acetylphenyl)isoquinoline-1,3-diones in good yields with excellent stereoselectivity. It is worth noting that this new methodology merges the fluoroalkylation/cyclization of N-substituted acrylamide alkenes with the oxidative cleavage of an indole C(2)=C(3) bond under external oxidant-free conditions.     

Rhodium-catalyzed tandem heterocyclization and carbonylative [(3+2)+1] cyclization of diyne-enones

A simple and highly efficient rhodium-catalyzed tandem heterocyclization and carbonylative [(3+2)+1] cyclization reaction of readily available diyne-enone and carbon monoxide was developed, leading to a novel polycyclic furan scaffold. Synthetically useful highly substituted phenols can also be easily prepared via a straightforward one-pot synthetic strategy.     

Formal radical cyclization onto benzene rings: a general method and its use in the synthesis of ent-nocardione A

An indirect method is described for effecting radical cyclization onto a benzene ring. Cross-conjugated dienones 6, which are readily prepared from phenols, undergo radical cyclization (6 --> 7 --> 8), and the products (8) are easily aromatized. The method has been applied to the synthesis of ent-nocardione A (21).     

Synthesis of some N-alkylated 1,2,4-triazoles, 1,3,4-oxadiazoles,and 1,3,4-thiadiazoles based on N-(furan-2-yl-methylidene)-4,6-dimethyl-1H-pyrazolo-[3,4-<Emphasis Type="Italic">b</Emphasis>]pyridine-3-amine

N-(Furan-2-ylmethylidene)-4,6-dimethyl-1H-pyrazolo[3,4-b]pyridine-3-amine was prepared and alkyla- ted with the corresponding halo compounds to afford N-alkylated products. 2-[3-(Furan-2-ylmethy- lideneamino)-4,6-dimethyl-1H-pyrazolo[3,4-b]pyridin-1-yl]acetohydrazide was converted into the key intermediate thiosemicarbazide, which undergoes cyclization reactions under acidic and basic conditi- ons to give 1,2,4-triazole, 1,3,4-oxadiazole, and 1,3,4-thiadiazole derivatives. Condensation of the hyd- razide with monosaccharide aldoses gave the corresponding sugar hydrazones, which on treatment with acetic anhydride readily undergo cyclization reaction to afford oxadiazoline derivatives.     

Palladium(0)-catalyzed cyclization of 2-butenylene dicarbamates forming 4-vinyl-2-oxazolidones

2-Butenylene dicarbamates prepared from but-2-en-1,4-diol and isocyanates readily undergo the cyclization in the presence of a phosphine-palladium(0) catalyst to produce 4-vinyl-2-oxazolidones in high yields.