A versatile palladium-mediated three-component reaction for the one-pot synthesis of stereodefined 3-arylidene-(or 3-alkenylidene-)tetrahydrofurans

Enantioselective synthesis of 2,3-dihydrofurans using a bifunctional quinine/squaramide organocatalyst

Enantioselective synthesis of 2,3-dihydrofurans is carried out via domino type Michael/SN2 reactions using α-bromonitroalkenes and 1,3-dicarbonyl compounds catalyzed by a quinine/squaramide organocatalyst.

Synthesis and anti-tumor activity of novel ethyl 3-aryl-4-oxo-3,3a,4,6-tetrahydro-1H-furo[3,4-c]pyran-3a-carboxylates

A series of ethyl 3-aryl-4-oxo-3,3a,4,6-tetrahydro-1H-furo[3,4-c]pyran-3a-carboxylates were prepared through the metal-catalyzed domino reaction of alkylidene malonates and 1,4-butynediol under a one-pot reaction condition at room temperature. Their in vitro anti-proliferative activities were subsequently evaluated in A549, QGY and HeLa cells. The majority of the compounds showed potent anti-tumor activity against HeLa cells. In particular, compound 3l was the most potent compound with IC(50) value of 5.4 μM. For the first time, the X-ray structure of the anti-tumor ethyl 3-aryl-4-oxo-3,3a,4,6-tetrahydro-1H-furo[3,4-c]pyran-3a-carboxylates is determined.

Palladium-catalyzed cyclization of propargylic compounds

Many groups have explored the scope of the palladium-based cyclization of propargylic compounds since Tsuji's first report in 1985. Through the proper positioning of an internal nucleophilic center and the judicious selection of an appropriate external nucleophile, the synthetic chemist can effectively assert control over the course of the reaction and its products. However, initial investigations were very limited: only heterocyclic compounds were originally synthesized. We have found the palladium-catalyzed cyclization of propargylic compounds to be a very efficient method for producing both carbocyclic and heterocyclic compounds. In this Account, we discuss the cyclization reactions of functionalized propargylic compounds with a variety of nucleophiles that we have developed over the past few years. We also review similar reactions reported by other groups. We focus here on the cyclization of functionalized propargylic compounds containing a carbon nucleophilic center that is in close proximity to the propargylic moiety. We conducted a detailed investigation of their cyclizations with carbon nucleophiles, with nitrogen nucleophiles, with oxygen nucleophiles, and without nucleophiles. We have developed several efficient and useful methods for the synthesis of indenes, naphthalenes, polycycles, and spirocyclic compounds. All of these reactions proceed satisfactorily under very mild conditions; high regio- and stereoselectivity have been observed as well. In the course of our studies, we provided the first demonstration of a novel tandem C-H activation/bis-cyclization reaction of propargylic compounds with terminal alkynes. In addition, we used external nucleophiles to investigate the cyclization of functionalized propargylic compounds that bear an unsaturated carbon-carbon or carbon-heteroatom bond. We presented the first report of the use of external nucleophiles to initiate a novel cyclization of functionalized propargylic compounds containing an electrophile. This revelation provided a fresh perspective through the discovery of a new type of domino cyclization of propargylic compounds. Metal-catalyzed cyclization of propargylic compounds can provide indenes, cyclopentanones, cyclic carbonates, benzofurans, and a range of other cyclic molecules. A thorough understanding of the mechanisms involved in this class of reaction affords exceptional synthetic control, as shown here by our development of efficient procedures and reagents for palladium-catalyzed propargylic cyclizations.

An Improved Catalyst System for the Iron-Catalyzed Intermolecular Ring-Expansion Reactions of Epoxides

A considerable improvement is reported in the iron-catalyzed ring-expansion reactions of epoxides generating tetrahydrofuran derivatives by formal insertion of an alkene. Optimization of the catalyst system revealed that a preformed [Fe(salen)] complex minimizes the formation of polymerization side-products so that increased yields of intermolecular reactions were obtained. However, more importantly, the scope of the reaction could also be enlarged considerably. The iron-catalyzed ring-expansion reaction can now be applied to some styrene oxide derivatives, acting as radical donors, as well as to a wide variety of acceptor-substituted acyclic alkenes and cyclic dienes that act as radical acceptors. The use of unsymmetrical radical acceptors led to interesting questions concerning the regiochemistry of the reactions. The conservation of the stereochemistry of the starting materials in the products was investigated through a study of the reactions of E- and Z-configured acceptor-substituted double bonds. The reactions of fumaric and maleic esters were performed and the ratios of diastereomeric and regioisomeric products were determined.

Palladium-Catalyzed Decarboxylative Aza-Michael Addition−Allylation Reactions between Allyl Carbamates and Activated Olefins. Generation of Quaternary Carbon Adjacent to Secondary Amine Carbon Center

The reaction of allyl carbamates with activated olefins in the presence of Pd(PPh3)4 catalyst in THF proceeded smoothly at room temperature to give the corresponding beta,alpha-bisadducts, beta-amino-alpha-allylated products, in high yields. Not only highly activated olefins containing two cyano groups but also 2-cyano enones underwent facile aza-Michael addition--allylation with various allylic carbamates giving the corresponding products in high yields and with high diastereoselectivities. The stereochemistry of the singly formed product was confirmed with the help of X-ray crystallographic technique. It is an excellent method for creating beta-amino alpha-allyl ketones having two contiguous stereocenters: quaternary carbon adjacent to secondary amine carbon center.

β-Alkyl-α-allylation of Michael Acceptors through the Palladium-Catalyzed Three-Component Coupling between Allylic Substrates, Trialkylboranes, and Activated Olefins

The palladium-catalyzed three-component beta-alkyl-alpha-allylation reaction of activated olefins has been achieved. For example, in the presence of 5 mol % of Pd(PPh3)4, the reaction of benzylidenemalononitrile 1a with Et3B and allyl acetate 2a in THF proceeded smoothly at 40 degrees C to give the corresponding beta-ethyl-alpha-allylated product 3a in 81% yield.

Structures, Biological Activities, and Total Syntheses of 13-Hydroxy- and 13-Acetoxy-14-nordehydrocacalohastine, Novel Modified Furanoeremophilane-Type Sesquiterpenes from Trichilia cuneata

[reaction: see text] 13-Hydroxy-14-nordehydrocacalohastine (2) and 13-acetoxy-14-nordehydrocacalohastine (3), two novel modified furanoeremophilane-type sesquiterpenes isolated from Trichilia cuneata, showed inhibitory activities for membrane lipid peroxidation in mitochondria and microsomes. The first, highly convergent total syntheses of new compounds 2 and 3 have also been achieved via a palladium-mediated three-component coupling reaction between 2-iodotoluene (7), 1-penten-4-yn-3-ol (8), and diethyl ethoxymethylenemalonate (9).

Asymmetric Synthesis of 2,3-Dihydrofurans and of Unsaturated Bicyclic Tetrahydrofurans through α-Elimination and Migratory Cyclization of Silyloxy Alkenyl Aminosulfoxonium Salts. Generation and Intramolecular O,Si-Bond Insertion of Chiral Disubstituted β-Silyloxy Alkylidene Carbenes

Treatment of the bis(allylsulfoximine)titanium complexes derived from the beta-methyl-substituted acyclic allylic sulfoximines 13a and 13b with aldehydes gave with high selectivities the corresponding sulfoximine-substituted homoallylic alcohols which were isolated as the silyl ethers 15a-h. Methylation of sulfoximines 15a-h afforded the aminosulfoxonium salts 5a-h which upon treatment with LiN(H)tBu gave in high yields the enantio- and diastereomerically pure silyl-substituted 2,3-dihydrofurans 4a-h. Treatment of the titanium complexes derived from the cyclic allylic sulfoximines 17a, 17b, and ent-17c with p-MeOC(6)H(4)CHO delivered with high selectivities the corresponding sulfoximine-substituted cyclic homoallylic alcohols which were isolated as the silyl ethers 18a, 18b, and ent-18c, respectively. Methylation of sulfoximines 18a, 18b, and ent-18c furnished the aminosulfoxonium salts 8a, 8b, and ent-8c, respectively, whose treatment with LiN(H)t-Bu gave the enantio- and diastereomerically pure fused bicyclic 2,3-dihydrofurans 6a, 6b, and ent-6c, respectively, in good yields. It is proposed that the 1-alkenyl aminosulfoxonium salts 5a-h, 8a, 8b, and ent-8c react with the base under alpha-elimination and formation of the acyclic and cyclic beta-silyloxy alkylidene carbenes 2a-h, 7a, 7b, and ent-7c, respectively, which then undergo a 1,5-O,Si-bond insertion and 1,2-silyl migration. The cyclic aminosulfoxonium salts 8a, 8b, and ent-8c upon treatment with 1,8-diazabicyclo[5.4.0]-7-undecene did not undergo an alpha-elimination but suffered a novel migratory cyclization with formation of the enantio- and diastereomerically pure bicyclic tetrahydrofurans 9a, 9b, and ent-9c, respectively. It is proposed that the 1-alkenyl sulfoxonium salts 8a, 8b, and ent-8c are isomerized to the allylic aminosulfoxonium salts 10a, 10b, and ent-10c, respectively, which then suffer an intramolecular substitution of the (dimethylamino)sulfoxonium group by the silyloxy group followed by a desilylation. The syntheses of the 2,3-dihydrofurans 4a-h, 6a, and 6b and of the tetrahydrofurans 9a and 9b are accompanied by the formation of sulfinamide 16 of >or=98% ee, which can be converted via sulfoxide 28 of >or=98% to the starting sulfoximine 11 of >or=98% ee.

Palladium-mediated three-component synthesis of furo[2,3-b]pyridones by one-pot coupling of 3-iodopyridones, alkynes, and organic halides

[reaction: see text] The one-pot assembly of 4-alkoxy-3-iodo-2-pyridones, terminal alkynes, and organic halides has been achieved by integration of two sequential palladium-mediated cross-coupling reactions--Sonogashira and Wacker-type heteroannulation processes--and subsequent deprotection of the alkoxy group to afford furo[2,3-b]pyridones.

Palladium-catalyzed aminoallylation of activated olefins with allylic halides and phthalimide

The three-component aminoallylation reaction of the activated olefins 2 with the phthalimide 1a and allyl chloride proceeded very smoothly in the presence of Pd(2)dba(3).CHCl(3) (5 mol %)/P(4-FC(6)H(4))(3) (40 mol %) and Cs(2)CO(3) (3 equiv against 2) in dichloromethane at room temperature to give the corresponding aminoallylated products, N-pent-4-enylphthalimides 3, in 58-99% yields. The reaction of oxazolidinone 1b also proceeded very smoothly to give N-(2,2-dicyano-1-phenylpent-4-enyl)oxazolidinone in a quantitative yield; however, the Tsuji-Trost-type allylation products 4 were obtained in the case of dibenzylamine, N-tosylaniline, and pyrrolidin-2-one. Further, 2 underwent cycloaddition with N-tosylvinylaziridine 9a in the presence of Pd(2)dba(3).CHCl(3) (5 mol %)/P(4-FC(6)H(4))(3) (40 mol %) in THF at room temperature, giving the corresponding pyrrolidines 11 in 69-99% yields.