Highly Diastereoselective Synthesis and Application of Functionalized 2,3-Dihydrofuran Derivatives from Enynones and Bis(diazo) Compounds.

A highly efficient Ag(I)-catalyzed cascade Michael addition/cyclization of enynones with 1,3-(bis)diazo compounds has been established, providing functionalized 2,3-dihydrofuran derivatives containing a diazo group and an acetylenic bond with excellent diastereoselectivity. Transformation of the diazo group and hydration of the carbon-carbon triple bond have been performed successfully in different reaction systems.

Controllable Access to Diazo-functionalized 2-Methylene-2,3-dihydrofurans and Diazo-functionalized Furans from Enynones and Diazo Carbonyl Compounds.

Using enynones and diazo carbonyl compounds as identical starting materials, methods for chemoselective and regioselective constructs of diazo-functionalized 2-methylene-2,3-dihydrofurans and diazo-functionalized trisubstituted furans have been established in a AgSbF6/DBU/DCE/0 °C system and a AgSbF6/DBU/Et2O·BF3/DCE/0 °C system, respectively. A Lewis acid and organic base cocontrolled reaction for the synthesis of diazo-functionalized trisubstituted furans is infrequent. For diazo-functionalized 2-methylene-2,3-dihydrofuran synthesis, the reaction possesses excellent diastereoselectivity and Z-selectivity. On the basis of Rh2(OAc)4-mediated unique decomposition of diazo-functionalized 2-methylene-2,3-dihydrofurans, an application to diastereoselective construction of a 5-methylene-4,7-dihydro-5H-furo[2,3-c]pyran frame has been achieved for the first time.

Silver(I)-Catalyzed Tandem Reaction of Enynones and 4-Alkenyl Isoxazoles: Synthesis of 2-(Furan-2-yl)-1,2-dihydropyridines.

Silver(I)-catalyzed tandem reaction of enynones with 4-alkenyl isoxazoles provides access to 2-(furan-2-yl)-1,2-dihydropyridines. No competitive cyclopropanation of alkenes and O-H insertion via (2-furyl)carbene complexes were observed. The cascade reaction proceeds via the formation of (2-furyl)metal carbene intermediate, the N-O bond cleavage of 4-alkenyl isoxazoles/rearrangement, subsequent 6π electrocyclic reaction, and [1,5] H-shift. The successive construction of both 1,2-dihydropyridine skeleton and furan frame has been achieved in the one-pot reaction. A broad range of readily available enynones and 4-alkenyl isoxazoles are suitable to this protocol; however, when R3 is the alkyl group such as n-Bu and Me, a complicated mixture was generated without the desired products. In addition, in the case of R4 = bulky group such as R3'SiOCH2, the reaction gave an in situ oxo-product of (2-furyl)silver carbene. An atom-economic strategy for the synthesis of 2-(furan-2-yl)-1,2-dihydropyridines has been established.

Preparation of 4-(Nitromethyl)furan Derivatives and Their Application in the Syntheses of Bis(furan-2-yl)oximes.

An efficient method for the preparation of tetrasubstituted furans, which contains a nitromethyl group at the 4-position, has been developed. The applications of 4-(nitromethyl)furans on the synthesis of highly functionalized bis(furyl)oxime were explored for the first time.

AgOTf/I2-Mediated Cyclization/Cross-Coupling/Isomerization of Enynones with Phosphorus Ylides: An Expedient Route to Stereoselective Synthesis of (E)-2-Alkenylfurans.

Ag(I)-catalyzed cascade reactions involving enynone cyclization and cross-coupling with phosphorus ylides have been achieved for the first time. Subsequent treatment of the reaction mixture with I2 afforded the corresponding (E)-α-alkenylfurans in 73-95% yields with excellent stereoselectively. A reasonable mechanism has been proposed.

Silver(I)-Catalyzed Domino Cyclization/Cyclopropanation/Ring-Cleavage/Nucleophilic Substitution Reaction of Enynones with Enamines: Synthesis of 4-(Furan-2-yl)-3,4-dihydro-2H-pyrrol-2-one.

We report the synthesis of 4-(furan-2-yl)-3,4-dihydro-2H-pyrrol-2-one derivatives. In this approach, two core structures, the furan ring and 3,4-dihydro-2H-pyrrol-2-one, are constructed via silver(I)-catalyzed cascade cyclization/cyclopropanation/ring-cleavage/nucleophilic substitution reaction of enynones with enamines. A reasonable mechanism has been proposed. This method possesses some advantages such as high chemoselectivity, mild reaction conditions, simple operation, and short reaction time.

Regioselective Reversal Cyclization To Access either Eight-Membered Sulfur-Containing Heterocycle-Fused γ-Pyrones or 2-(1,4-Dithianyl)-4-pyrones from the Same Precursors.

A regioselective reverse strategy for the construction of eight-membered sulfur-containing heterocycle-fused γ-pyrones and 2-(1,4-dithianyl)-4-pyrones starting from 2-diazo-γ-pyrones and dithioacetals was achieved for the first time. The process combines C-S bond formation via sulfur ylides and C-C bond formation via electron transfer to afford the target molecules in a facile manner with 100% regioselectivity and in excellent isolated yields (up to 90%).

Synthesis and Rhodium(II)-Mediated Cascade Cyclopropanation/Rearrangement/Isomerization of Diazo 2,3,5-Trisubstituted Furans: The Construction of Penta-substituted Aromatic Compounds.

Ag(I)-catalyzed synthesis of diazo-trisubstituted furans starting from diazo-cumulated allenyl ketones has been investigated. The Rh2(OAc)4-catalyzed reaction of the diazo 2,3,5-trisubstituted furans provided penta-substituted aromatics via cascade intermolecular cyclopropanation/rearrangement/isomerization. The cyclopropanation on the furan ring/rearrangement of cyclopropane moiety has been reported. A reasonable mechanism is proposed.

Synthesis of 2-alkenylfurans via a Ag(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides.

A silver(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides to construct carbon-carbon double bonds has been developed. The strategy provides a novel method for the synthesis of 2-alkenylfurans. This is the first cross-coupling reaction between metal-carbene complexes and iodonium ylides.

Synthesis of Trifunctionalized Naphtho[1,2- b]furans Based on the Strategy for the Construction of Both Furan and Naphthalene Cycle.

Pd(PPh3)2Cl2-catalyzed selective tandem cyclization/oxidation of available conjugated diazo ene-yne-ketones under O2 atmosphere led to the formation of diazo trisubstituted furans. The Rh2(OAc)4-mediated selective C(sp2)-H insertion at the ortho-position of 2-aryl group (R1) of the furan moiety under N2 atmosphere occurred to construct naphthalene cycle, affording trifunctionalized naphtho[1,2- b]furans. C(sp2)-H insertion at the 4-position of the furan ring, and Wolf rearrangement of diazo moiety have not been observed.

Ag(I)-Catalyzed Tandem Reaction of Conjugated Ene-yne-ketones in the Presence of PhI(OAc)2 and Triethylamine: Synthesis of 2-Alkenylfurans.

Silver-catalyzed tandem cyclization-elimination reactions of conjugated ene-yne-ketones in PhI(OAc)2/triethylamine system lead to the formation of 2-alkenylfurans. 2-Furylsilver carbene and phenyliodonium ylide are proposed as the key intermediates in these transformations.

Regioselective Reversal in the Cyclization of 2-Diazo-3,5-dioxo-6-ynoates (Ynones, Ynamide): Construction of γ-Pyrones and 3(2H)-Furanones Starting from Identical Materials.

The AgSbF6-catalyzed cyclization of 2-diazo-3,5-dioxo-6-ynoates (ynones, ynamide) in alcoholic solvents affords γ-pyrones, whereas the AgOAc-catalyzed cyclization in 1,2-dichloroethane (DCE) produces 3(2H)-furanones. The cyclization reactions proceeded cleanly under mild reaction conditions, and the desired γ-pyrones or 3(2H)-furanones were obtained in excellent yield. It was observed for the first time that both the catalyst and solvent play key roles in the selective formation. This unique method for the reversal of regioselectivity proved to be highly efficient except for substrates with aliphatic and Me3Si groups at the triple bond position.

Synthesis of Pyrano[3,2-c]pyrazol-7(1H)-one Derivatives by Tandem Cyclization of 2-Diazo-3,5-dioxo-6-ynoates (Ynones).

The construct of the core of pyrano[3,2-c]pyrazol-7(1H)-one derivatives is realized. The key step includes a tandem cyclization, namely, a metal-free cascade 6-π electrocyclic ring closure-Michael reaction of 2-diazo-3,5-dioxo-6-ynoates (ynones). The cascade reaction cleanly generated the desired products in excellent yields under mild conditions.

Comparative studies of Acyl-CoA dehydrogenases for monomethyl branched chain substrates in amino acid metabolism.

Short/branched chain acyl-CoA dehydrogenase (SBCAD), isovaleryl-CoA dehydrogenase (IVD), and isobutyryl-CoA dehydrogenase (IBD) are involved in metabolism of isoleucine, leucine, and valine, respectively. These three enzymes all belong to acyl-CoA dehydrogenase (ACD) family, and catalyze the dehydrogenation of monomethyl branched-chain fatty acid (mmBCFA) thioester derivatives. In the present work, the catalytic properties of rat SBCAD, IVD, and IBD, including their substrate specificity, isomerase activity, and enzyme inhibition, were comparatively studied. Our results indicated that SBCAD has its catalytic properties relatively similar to those of straight-chain acyl-CoA dehydrogenases in terms of their isomerase activity and enzyme inhibition, while IVD and IBD are different. IVD has relatively broader substrate specificity than those of the other two enzymes in accommodating various substrate analogs. The present study increased our understanding for the metabolism of monomethyl branched-chain fatty acids (mmBCFAs) and branched-chain amino acids (BCAAs), which should also be useful for selective control of a particular reaction through the design of specific inhibitors.

Functional characterization of rat glutaryl-CoA dehydrogenase and its comparison with straight-chain acyl-CoA dehydrogenase.

Glutaryl-CoA dehydrogenase catalyzes the oxidative decarboxylation of the γ-carboxylate of the substrate, glutaryl-CoA, to yield crotonyl-CoA and CO(2). The enzyme is a member of the acyl-CoA dehydrogenase (ACD) family of flavoproteins. In the present study, the catalytic properties of this enzyme, including its substrate specificity, isomerase activity, and interactions with inhibitors, were systematically studied. Our results indicated that the enzyme has its catalytic properties very similar to those of short-chain and medium-chain acyl-CoA dehydrogenase except its additional decarboxylation reaction. Therefore, the inhibitors of fatty acid oxidation targeting straight chain acyl-CoA dehydrogenase could also function as inhibitors for amino acid metabolism of lysine, hydroxylysine, and tryptophan.

Oct-2-en-4-ynoyl-CoA as a specific inhibitor of acyl-CoA oxidase.

Oct-2-en-4-ynoyl-CoA was found to be a specific inhibitor of acyl-CoA oxidase in fatty acid oxidation in peroxisomes that has no inhibitory effect on acyl-CoA dehydrogenase in mitochondria. The inhibition reaction involves a nucleophilic attack of Glu421 to the delta carbon of the inhibitor. The result indicates that acyl-CoA oxidase and acyl-CoA dehydrogenase have certain differences in active-site structure, which makes it possible to control fatty acid oxidation selectively in either mitochondria or peroxisomes with different enzyme inhibitors.

Characterization of mitochondrial trifunctional protein and its inactivation study for medicine development.

Mitochondrial trifunctional protein (MTP) catalyzes three consecutive step reactions in the beta-oxidation of long-chain fatty acids, and plays important roles in control and regulation of the beta-oxidation. We overexpressed in E. coli, and purified the MTP as a Mistic fusion protein, which was found to be an alpha(2)beta(2) protein complex and characterized with kinetic studies. Trimetazidine, used for treating chronic stable angina, has been proposed to be an inhibitor of the beta-subunit. We found that a catalytic cysteine residue C105 was labeled by trimetazidine through MS/MS analysis of a trimetazidine-labeled peptide fragment obtained from pepsin digested beta-subunit inactivated by trimetazidine. The MTP beta-subunit was then comparatively studied with monofunctional 3-ketoacyl-CoA thiolase through sequence alignment, site-directed mutagenesis, characterization of variant enzymes with kinetic studies, and homology modeling. The results indicate that the catalytic residues of the MTP beta-subunit are positioned in the active site similarly to those of monofunctional 3-ketoacyl-CoA thiolase.

Inactivation of thiolase by 2-alkynoyl-CoA via its intrinsic isomerase activity.

Selective inactivation of cytosolic thiolase by 2-alkynoyl-CoA via its intrinsic isomerase activity was studied, which provides an example for rationally developing mechanism-based inhibitors based on a side activity of the enzyme, and may become a supplemental method for better treatment of cardiovascular disease and cancer.

Formation of an enolate intermediate is required for the reaction catalyzed by 3-hydroxyacyl-CoA dehydrogenase.

Fluorinated substrate analogs were synthesized and incubated with rat liver 3-hydroxyacyl-CoA dehydrogenase, which reveals that the formation of an enolate intermediate is required for the reaction catalyzed by the enzyme.

Mutation of Lys242 allows Delta3-Delta2-enoyl-CoA isomerase to acquire enoyl-CoA hydratase activity.

We report here a novel example of generating hydratase activity through site-directed mutagenesis of a single residue Lys242 of rat liver mitochondrial Delta3-Delta2-enoyl-CoA isomerase, which is one of the key enzymes involved in fatty acid oxidation and a member of the crotonase superfamily. Lys242 is at the C-terminal of the enzyme, which is far from the active site in the crotonase superfamily and forms a salt bridge with Asp149. A variety of mutant expression plasmids were constructed, and it was observed that mutation of Lys242 to nonbasic residues allowed the mutants to have enoyl-CoA hydratase activity confirmed by HPLC analysis of the incubation mixture. Kinetic studies of these mutants were carried out for both isomerase and hydratase activities. Mutant K242C showed a k(cat) value of 1.0 s(-1) for hydration reaction. This activity constitutes about 10% of the total enzyme activity, and the remaining 90% is its natural isomerase activity. To the best of our knowledge, this is the first report on the generation of functional promiscuity through single amino acid mutation far from the active site. This may be a simple and efficient approach to designing a new enzyme based on an existing template. It could perhaps become a general methodology for facilitating an enzyme to acquire a type enzymatic activity that belongs to another member of the same superfamily, by interrupting a key structural element in order to introduce ambiguity, using site-directed mutagenesis.