Synthesis and cationic polymerization of substituted 2,3-dihydrofurans

Cationic polymerization of substituted 2,3-dihydrofurans has been performed to investigate the substituent effect on the ring-opening polymerization. 2-Phenyl-2,3-dihydrofuran ( III ), 2-methoxy-5-phenyl-2,3-dihydrofuran ( V _a), and 2-ethoxy-5-phenyl-2,3-dihydrofuran ( V _b) were synthesized and polymerized with BF₃ etherate and AlCl₃ as acid catalysts. V _a and V _b ring-opening polymerized well to give the polymers with benzoyl as pendant group which were formed via cationic rearrangement during the ring-opening process, while III polymerized via simple opening of ethylenic double bond to form a polymer with the retention of tetrahydrofuran ring in the main chain. The nature of substituted cyclic vinyl ethers depending on substituents was also discussed.     

Lewis-acid-promoted additions of carbonyl compounds to donor-acceptor substituted cyclopropanes: a new synthesis of 2,3-dihydrofurane derivatives

Methyl 2,3-dihydrofurane-3-carboxylates are synthesized from siloxycyclopropaneesters and ketones in the presence of TiCl₄ with good yields.     

2,3-Heteroaromatic ring-fused cyclohexanones via heteroaromatic homo-Nazarov cyclization of donor-acceptor substituted cyclopropanes

Heteroaryl 2-silylmethyl-substituted cyclopropyl ketones rearrange under Lewis acid conditions via heteroaromatic homo-Nazarov cyclization to form 2,3-heteroaromatic ring fused 4-t-butyldiphenylsilylmethyl-substituted cyclohexanones.     

Copper-catalyzed one-pot N-alkenylation and N-alkylation of amides: an efficient synthesis of substituted 2,3-dihydropyrroles

A novel copper-catalyzed synthesis of substituted 2,3-dihydropyrroles via one-pot N-alkenylation and N-alkylation of amides with 1,4-diiodobut-1-ene derivatives has been developed. The reactions proceed in good to high yields using CuI as the catalyst, K₂CO₃ as the base, and rac-trans-N,N′-dimethylcyclohexane-1,2-diamine as the ligand.     

An efficient route to 4-(substituted benzyl)piperidines

A novel approach to 4-(substituted benzyl)piperidines has been developed. The key steps involve the cyclization of imines bearing an allylsilane in the side-chain followed by the palladium-catalyzed cross-coupling of the resulting 4-methylenepiperidine with organoboronic acids under an atmosphere of oxygen.     

Facile syntheses of substituted 2,3-dihydrofurans and benzofurans by palladium-catalyzed reactions of propargylic carbonates with nucleophiles

Substituted 2,3-dihydrofurans and benzofurans are synthesized by the palladium-catalyzed reaction of 5-methoxycarbonyloxy-3-pentyn-1-ols and 1-(2-hydroxyphenyl)-3-methoxycarbonyloxy-1-propyne with nucleophiles, respectively. Various substituted propargylic carbonates and nucleophiles are efficiently transformed to their corresponding products. Additionally, a reaction using substrates containing a nucleophilic phenoxy group within the same molecule also produces the corresponding dihydrofuran.     

New transformations of 2-nitro-2,3-dihydrofurans to multi-functionalized dihydrofurans

A new type of transformation of 2-nitro-3-substituted-dihydrofurans to 4-formyl-2-hydroxy-3-substituted-2,3-dihydrofurans was accomplished in 63-84% yields by use of BF₃·OEt₂ in CH₂Cl₂ at −78 to 0°C.     

Thiopyran route to polypropionates: an efficient synthesis of serricornin

The synthesis of serricornin [(4S,6S,7S)-7-hydroxy-4,6-dimethylnonan-3-one], a sex pheromone produced by the female cigarette beetle (Lasioderma serricorne F.), in seven steps from readily available racemic 1,4-dioxa-8-thiaspiro[4.5]decane-6-carboxaldehyde (6) is described. The key steps include enantioselective aldol reaction of 6 with tetrahydrothiopyran-4-one catalyzed by 5-[(2S)-pyrrolidine-2-yl]-1H-tetrazole to fabricate the tetrapropionate skeleton, stereoselective Li(s)Bu(3)BH reduction of the resulting aldol adduct, Barton-McCombie deoxygenation, and Raney nickel desulfurization.     

Palladium-catalyzed allylalkynylation of benzynes: a highly efficient route to substituted 1-allyl-2-alkynylbenzenes

Benzynes, generated in situ from 2-(trimethylsilyl)aryl triflates and cesium fluoride, undergo allylalkynylation with allylic chlorides and alkynylstannanes in the presence of palladium catalyst to give 1-allyl-2-alkynylbenzenes in good to excellent yields. [reaction: see text]     

Palladium-catalyzed cyclization reactions of propargylic carbonates with nucleophiles: a methodology for the syntheses of substituted 2,3-dihydrofurans and benzofurans

Phenoxy-substituted 2,3-dihydrofurans were synthesized by the palladium-catalyzed reaction of 5-methoxycarbonyloxy-3-pentyn-1-ol with phenols. The propargylic carbonate containing a nucleophilic phenoxy group also reacted in the presence of palladium to produce the product. The reaction of 1-(2-hydroxyphenyl)-3-methoxycarbonyloxy-1-propyne with 2-methyl-1,3-cyclohexanedione or 2-methyl-1,3-cycohexanedione yielded the substituted benzofurans. The propargylic compound having a acetoxy group as a leaving group exhibited similar reactivity.     

Liquid-phase hydroformylation of 2,3- and 2,5-dihydrofurans

The synthesis of 3-formyl derivatives of the tetrahydrofuran series was carried out by hydroformylation of 2,3-dihydrofuran and 2,5-dimethoxy-2,5-dihydrofuran in the presence of HRh(CO)(PPh₃)₃. The influence of the temperature, pressure, catalyst concentration, and the nature of the solvent on the conversion of dihydrofuran, the composition of aldehydes obtained and the selectivity of their formation was investigated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2569–2571, November, 1989.     

Regioselective synthesis of trisubstituted 2,3-dihydrofurans from donor-acceptor cyclopropanes or from reaction of the Corey ylide with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones

[reactions: see text] Regioselective synthesis of 2,4,5- or 3,4,5-trisubstituted 2,3-dihydrofurans has been realized by using donor-acceptor cyclopropanes or by a Corey ylide reaction with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones. The method allowed a straightforward synthesis of the natural product calyxolane B.     

Silicon-assisted ring opening of donor-acceptor substituted cyclopropanes. An expedient entry to substituted dihydrofurans

[reaction: see text]. (tert-butyldiphenylsilyl)methylcyclopropanes undergo ring opening to furnish substituted dihydrofurans in good to excellent yields on treatment with TiCl4 in dichoromethane. The silicon that assists the regioselective ring opening is retained in the product to allow further functional group manipulations.     

Simple route to substituted tetralones

A two step route to highly substituted tetralones via Mn(III) oxidation and cyclization has been demonstrated.     

Preparation and photochemical rearrangements of 2-phenyl-2,5-cyclohexadien-1-ones. an efficient route to highly substituted phenols

[reaction: see text]. The synthesis of 2-phenyl-2,5-cyclohexadien-1-ones 1a-c and 2a-b from methyl 3-phenylbenzoate 4 and methyl 2-methoxy-5-phenylbenzoate 8 by the Birch reduction alkylation methodology is described. 1a-c and 2a-b undergo regiospecific photorearrangements at 300 nm to give tetrasubstituted phenols 14a-c and pentasubstituted phenols 18a-b, respectively. The type A photoproducts 17a-b resulting from irradiation of 2a-b at 366 nm have been isolated as approximately 1:1 diastereomer mixtures. When an optimized condition is applied, a single diastereomer of 17a is obtained.     

Electrophilic activation and cycloisomerization of enynes: a new route to functional cyclopropanes

Transformations of enynes in the presence of transition-metal catalysts have played an important role in the preparation of a variety of cyclic compounds. Recent developments in the activation of triple carbon-carbon bonds by electrophilic metal centers have provided a new entry to the selective synthesis of cyclopropane derivatives from enynes. The mechanisms of these reactions involve catalytic species with both ionic and cyclopropylcarbenoid character. This type of activation will undoubtedly be further developed for application to other unsaturated hydrocarbons and inspire new catalytic cascade reaction sequences. This Minireview discusses the recent developments in electrophilic activation of enynes and shows that simple catalysts such as [Ru(3)(CO)(12)], PtCl(2), and cationic gold complexes are efficient precursors to promote the formation of functional polyclic compounds.