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At least 775 records · Page 43Linked to original sources
PubMed2005

Synthesis of coumarins, 4-hydroxycoumarins, and 4-hydroxyquinolinones by tellurium-triggered cyclizations.

Coumarins, 4-hydroxycoumarins, and 4-hydroxyquinolin-2(1H)-ones can be conveniently prepared by treatment of alpha-halocarboxylic acid esters of salicylaldehyde, o-hydroxyacetophenone, methyl salicylate, and methyl N-methyl- or N-phenylanthranilates with sodium or lithium telluride. Phenylketene formation competes with cyclization of the alpha-chlorophenylacetate ester of methyl salicylate as demonstrated by a trapping experiment with benzylamine. Elemental tellurium may be recovered and reused.

Catalysis↗
PubMed2005

An N-acyliminium cyclization approach to a total synthesis of (+)-cylindricine C.

[reaction: see text] Details of problems and solutions encountered during the development of an enantioselective total synthesis of (+)-cylindricine C are described here. The total synthesis itself was accomplished in 8 steps, featuring an N-acyliminium cyclization strategy, the seldom-used Wharton rearrangement, and a key epimerization at C5.

Acylation↗
PubMed2005

Catalytic N-sulfonyliminium ion-mediated cyclizations to alpha-vinyl-substituted isoquinolines and beta-carbolines and applications in metathesis.

[reaction: see text] Catalytic Sn(OTf)2-induced cyclization of linear, aryl-containing allylic N,O-acetals produced vinyl-substituted tetrahydroisoquinolines and tetrahydro-1H-beta-carbolines. The usefulness of the vinyl moiety in the resulting products was demonstrated via the synthesis of various key building blocks for alkaloid structures. The alpha-vinyl moiety was utilized in a [2,3] sigmatropic rearrangement, in ring-closing metathesis and a cross-metathesis-based synthesis of vincantril, an antianoxia agent, and a synthetic member of the vincamine type natural products.

Alkaloids↗
PubMed2005

Oxocarbenium ion cyclizations for C-branched cyclitols: synthesis of a relay intermediate for fumagillin analogues.

A highly stereoselective oxocarbenium ion-alkene cyclization for synthesis of C-branched cylitols is described. This methodology was applied to 10S, a potentially versatile intermediate for side-chain analogues of the antiangiogenic agent fumagillin. Compound 10S was subsequently converted to diene 5. Because racemic 5 has been converted to racemic fumagillin, this synthesis of 5 constitutes a formal synthesis of the natural product.

Cyclization↗
PubMed2005

Nitrile anion cyclization with epoxysilanes followed by Brook rearrangement/ring-opening of cyclopropane nitriles/alkylation.

[reactions: see text] The reaction of delta-silyl-gamma,delta-epoxypentanenitrile derivatives 9-12 with a base and an alkylating agent affords (Z)-delta-siloxy-gamma,delta-unsaturated pentanenitrile derivatives via a tandem process that involves the formation of a cyclopropane derivative by epoxy nitrile cyclization followed by Brook rearrangement and an anion-induced cleavage of the cyclopropane ring. Exclusive formation of a (Z)-derivative from trans-epoxides is explained by the reaction pathway that involves a backside displacement of the epoxide by the alpha-nitrile carbanion and the O-Si bond formation followed by concerted processes involving Brook rearrangement and the anti-mode of eliminative ring fission of the cyclopropane from the rotamer 19. The fact that (E)-isomers are exclusively obtained from cis-epoxides and alpha-cyclopropyl-alpha-silylcarbinol derivative 26 provides experimental support for the proposed pathway.

Alkylation↗
PubMed2006

A general route toward the synthesis of the cladiellin skeleton utilizing a SmI2-mediated cyclization.

An efficient synthesis of the cladiellin skeleton is reported utilizing methods that were previously developed in this laboratory. The approach is based on a SmI(2)-mediated cyclization reaction for the construction of the oxacyclononane unit. A [4 + 3] annulation strategy was used to create the octahydroisobenzofuran moiety. This route provides the cladiellin skeleton in only 14 steps without the use of protecting groups. The present approach also enabled the synthesis of the 3,7-diastereomer of the natural product polyanthellin A.

Alkenes↗
PubMed2006

Synthesis of 2,3-disubstituted benzo[b]selenophenes via electrophilic cyclization.

2,3-Disubstituted benzo[b]selenophenes have been prepared by the electrophilic cyclization of various 1-(1-alkynyl)-2-(methylseleno)arenes by Br2, NBS, I2, ICl, PhSeCl, PhSeBr, and Hg(OAc)2. This method tolerates a wide variety of functional groups, including alcohol, ester, nitrile, nitro, and silyl groups, and proceeds under exceptionally mild reaction conditions.

Cyclization↗
PubMed2006

Expanding the synthetic method and structural diversity potential for the intramolecular Aza Diels-Alder cyclization.

New experimental facets have been examined to expand upon the known methods for an aromatic variant of the intramolecular Aza Diels-Alder cyclization. The specific transformation under study is one that uses functionalized anilines and an aldehyde-olefin tether to provide tetrahydroquinoline cycloadducts under mild acidic conditions. Variations investigated encompass the use of N-alkylated anilines, including one with ring-constrained nitrogen, in the context of glycine, phenylalanine, and glyoxyl ester bridging elements; bridge components with structural perturbations; modified dienophile segments; and different acid catalysts. Substituted tetrahydroquinolines obtained from many of the preceding experiments were obtained in good chemical yield, generally in excess of 80%. Designed as a platform for combinatorial chemical synthesis, this reaction manifold accommodates a range of starting materials with structurally and electronically distinct characteristics. The results of this report, in combination with the discoveries from previous work in this area, enhance the ability of the intramolecular Aza Diels-Alder transformation to generate a diverse array of quinolinic structures with multiple stereogenic centers, many of which resemble lignan and arylnaphthalene-type natural products.

Aldehydes↗
PubMed2006

Zinc- and indium-promoted conjugate addition-cyclization reactions of ethenetricarboxylates with propargylamines and alcohol: novel methylenepyrrolidine and methylenetetrahydrofuran syntheses.

A new zinc- and indium-promoted conjugate addition-cyclization reaction to afford nitrogen- and oxygen-containing five-membered heterocycles has been developed. Reaction of ethenetricarboxylates with propargylamines (1 equiv) in the presence of ZnBr2 or InBr3 afforded methylenepyrrolidines in high yields. The stoichiometric use of ZnBr2 or InBr3 at room temperature and the catalytic use of InBr3-Et3N at 80 degrees C were effective. Reaction of ethenetricarboxylates with propargyl alcohol in the presence of ZnBr2 or InBr3 afforded methylenetetrahydrofurans.

Alkynes↗
PubMed2006

A diastereoselective radical cyclization approach to substituted quinuclidines.

A new, concise, and flexible approach to novel quinuclidines has been developed, which employs a phosphorus hydride mediated radical addition/cyclization reaction in the key step. 1,7-Diene 5 reacts with diethyl thiophosphite in an efficient and diastereoselective radical addition/cyclization reaction to give trisubstituted piperidines 4ab. Piperidines 4ab are subsequently converted into 2,5-disubstituted quinuclidines using S(N)2-type cyclizations. Finally, the resulting quinuclidines are shown to undergo novel Horner-Wadsworth-Emmons-type (HWE-type) reactions to give unsaturated quinuclidines 21a and 21b, which have structures similar to that of (-)-quinine 1.

Cyclization↗
PubMed2006

Hypervalent iodine mediated intramolecular cyclization of thioformanilides: expeditious approach to 2-substituted benzothiazoles.

A new, mild, and efficient method has been developed for the synthesis of 2-substituted benzothiazoles via the intramolecular cyclization of thioformanilides by using hypervalent iodine reagents in CH2Cl2 at ambient temperature. The reaction proceeds via a thiyl radical in high yields to give the novel compound oxybis benzothiazole and is also amenable to generating combinatorial libraries of heterocyclic compounds by solid-phase synthesis.

Anilides↗
PubMed2006

cis-Tetrahydrofuran-3,4-diol structure as a key skeleton of new protecting groups removable by self-cyclization under oxidative conditions.

A variety of carbonate-type acyl groups having a cis-tetrahydrofuran-3,4-diol (1,4-anhydroerythritol) backbone structure and a TrS or MMTrS group have been examined as new "protected" protecting groups of the 5'-hydroxyl group of nucleosides. These acyl groups were designed in a manner where they could be deprotected by I(2)-promoted removal of the TrS or MMTrS group followed by self-cyclization involving an intramolecular attack of the once-generated neighboring hydroxyl group on the acyl carbon. It turned out that these acyl groups could be introduced into the 5'-hydroxyl group of a 3'-O-protected thymidine derivative by use of the corresponding acyl imidazolides or 4-nitrophenyl esters as well as by reaction with carbonyldiimidazole or 4-nitrophenyl chloroformate. Among the acyl groups tested, it was found that the CTFOC group could be easily introduced into the 5'-hydroxyl group of 3'-masked deoxyribonucleoside derivatives and rapidly removed under mild conditions using iodine.

Acylation↗
PubMed2006

Synthesis of phosphaisoquinolin-1-ones by Pd(II)-catalyzed cyclization of o-(1-alkynyl)phenylphosphonamide monoesters.

A Pd(II)-catalyzed intramolecular cyclization of o-(1-alkynyl)phenylphosphonamide monoethyl esters was examined, and a new class of six-membered phosphorus heterocycles (phosphaisoquinolin-1-ones) were formed with high regioselectivity and good yields. The present reaction is the first example of intramolecular addition of P-NH to substituted alkynes, which provides a valuable way to synthesize novel phosphorus heterocycles with potential bioactivities.

Amides↗
PubMed2006

Tandem Michael addition/amino-nitrile cyclization from 2-formyl-1,4-DHP in the synthesis of novel dihydroindolizine-based compounds.

A simple and efficient methodology for the synthesis of a small library of substituted indolizines with different degrees of saturation starting from the racemic 2-formyl-1,4-DHP reagent was described. The large synthetic possibilities of this reagent as well as of its Knoevenagel corresponding 2-dicyanovinyl-1,4-DHP reagent were investigated using four kinds of activated methylenes as nucleophiles. The key step of the sequential reaction was based on the highly diastereoselective tandem Michael addition/intramolecular amino-nitrile cyclization catalyzed by an organic base, which resulted in the formation of 1,7-dihydroindolizines in a diastereoselective manner. The process seems to be a straightforward one and can be extended to numerous active methylenes such as malononitrile, 1,3-diketones, and alkyl acetoacetates. The 1,3-hydrogen shift of partially hydrogenated indolizines was accomplished easily with a base at room temperature, giving rise to the corresponding 7,8-dihydroindolizines in very good yields. Interestingly, when the active methylene bears a leaving group, the latter process could not be accomplished because a rare cis-elimination of phenylsulfinic acid and nitrous acid preceded the hydrogen shift. The resulting 1,7-dihydroindolizines bearing an exo-methylene group at C1 were not isolated in all cases, as they turned rapidly to indolizines as the thermodynamically more stable products. During these investigations, oxidization of 1,7-dihydroindolizines with CuCl(2) resulted in the formation of polysubstituted pyridines. Also, the epimerization of certain 1,7-dihydroindolizines was evidenced in the solution studied by NMR spectroscopy, whereas in the solid state, they existed only in a unique form as shown by X-ray diffraction analysis of a representative structure. Finally, all products reported herein bear a primary amine and a nitrile function crucial for further transformations. These include the introduction of various pharmacophore groups at either NH(2) or CN groups as well as at both groups at the same time to access the more elaborated indolizines fused to N- or N,N-heterocycles.

Amines↗