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At least 649 records · Page 36Linked to original sources
PubMed2005

Morita-Baylis-Hillman reaction and cyclization of 1-(p-toluenesulfonyl)-1,3-butadiene with aldimines.

[reaction: see text] Aldimines 2 underwent Morita-Baylis-Hillman reaction with 1-(p-toluenesulfonyl)-1,3-butadiene (3) in the presence of 3-hydroxyquinuclidine (HQD) to afford adducts 4. The E-isomers of the products cyclized to the corresponding functionalized piperidines 8 under base-catalyzed conditions. Simultaneous equilibration of (E)-4 and (Z)-4 was effected by photoisomerization to improve the efficiency of the cyclization.

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PubMed2005

Synthesis of 2H-1,2-oxaphosphorin 2-oxides via Ag2CO3-catalyzed cyclization of (Z)-2-alken-4-ynylphosphonic monoesters.

[reaction: see text]. Six new 2-ethoxy-2H-1,2-oxaphosphorin 2-oxides were synthesized with high regioselectivity in good yields via Ag(2)CO(3)-catalyzed cyclization of (Z)-2-alken-4-ynylphosphonic monoesters in CH(2)Cl(2) at room temperature. This cyclization of P-OH to substituted alkynes is reported for the first time. The products are a class of phosphorus heterocycles with potential use and are heretofore prepared with difficulty.

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PubMed2005

Remarkable tris(trimethylsilyl)silyl group for diastereoselective [2 + 2] cyclizations.

[reaction: see text] Diastereoselective [2 + 2] cyclizations of aldehyde- and ketone-derived silyl enol ethers with acrylates is described. The use of the tris(trimethylsilyl)silyl group allows for unprecedented reactivity, yields, and selectivity for these cyclizations. The presence of silicon-silicon bonds proved to be important for this transformation, where typical silyl groups (TBS and TIPS) failed to give any desired product. The bulky bis(2,6-diphenylphenoxide) aluminum triflimide catalyst was essential for high diastereoselectivity.

Journal Article↗
PubMed2005

Facile construction of the benzofuran and chromene ring systems via PdII-catalyzed oxidative cyclization.

[reaction: see text]. We herein report the development of one-pot procedures for the conversion of allyl aryl ethers to 2-methylbenzofurans (via sequential Claisen rearrangement and oxidative cyclization) and for the conversion of aryl homoallyl ethers to chromenes (via direct oxidative cyclization). It is likely that both reactions proceed via a common Pd-catalyzed pathway involving olefin activation, nucleophilic attack, and beta-hydride elimination.

Journal Article↗
PubMed2006

Sequential Pd- and Rh-catalyzed three-component cyclization with allenylboronate platform.

[reaction: see text] A novel three-component cyclization using allenylboronate ester is described. A three-component assembly of allenylboronate ester, aryl iodides, and stabilized carbon nucleophiles took place in the presence of palladium catalyst, furnishing functionalized alkenylboronate esters with high regio- and stereoselectivity. A Rh-catalyzed cyclization of the resultant three-component products then afforded interesting carbocyclic frameworks efficiently.

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PubMed2006

The silylalkyne-Prins cyclization: stereoselective synthesis of tetra- and pentasubstituted halodihydropyrans.

[reaction: see text] A new type of Prins cyclization using silylated secondary homopropargylic alcohols and aldehydes yielding tetra- and pentasubstituted dihydropyrans is described. The presence of the trimethylsilyl group in the triple bond favors the Prins cyclization and minimizes the 2-oxonia-[3,3]-sigmatropic rearrangement as a competitive alternative pathway. Ab initio theoretical calculations of the species involved in the rearrangements support the proposed mechanism. The process is highly stereoselective, affording cis-dihydropyran as the only isomer.

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PubMed2006

Site-specific preparation of 3-fluoro-1-substituted-naphthalenes via a novel base-catalyzed cyclization reaction from (E)-monofluoroenynes.

A novel cyclization reaction for the preparation of 3-fluoro-1-substituted-naphthalenes is reported. (E)-Monofluoroenynes, which are prepared by Sonogashira coupling reaction from (Z)-1-bromo-1-fluoroalkenes, undergo cyclization to afford 3-fluoro-1-substituted-naphthalenes in good to excellent yields when treated with DABCO or DBU in refluxing N-methyl-2-pyrrolidinone (NMP). [reaction: see text]

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PubMed2006

Practical and convenient synthesis of N-heterocycles: stereoselective cyclization of N-alkenylamides with t-BuOI under neutral conditions.

[Structure: see text] tert-Butyl hypoiodite (t-BuOI) was found to be a powerful reagent for the cyclization of N-alkenylamides leading to a variety of N-heterocycles under extremely mild conditions. When N-alkenylsulfonamides were employed in the reaction, three- to six-membered saturated N-heterocycles were obtained in good to excellent yields with complete stereoselectivity. The method was applicable to the cyclization of alkenylbenzamide derivatives to afford N-, O- or N-, S-heterocycles.

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PubMed2006

Chirality multiplication and efficient chirality transfer in exo- and endo-radical cyclization reactions of 4-(4'-iodobutyl)quinolones.

[reaction: see text] Enantioselective radical cyclization reactions were performed in the presence of chiral complexing agent 1. The title compounds 3 yielded, depending on the 3'-substitution (R = H, Me), the corresponding endo- (4) or exo-product (5). The highest enantioselectivities (99% and 94% ee) were achieved with 2.5 equiv of complexing agent. The cyclization product trans-4 was obtained in 55% ee in the presence of only 0.1 equiv of complexing agent.

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PubMed2006

Iron(III)-promoted aza-Prins-cyclization: direct synthesis of six-membered azacycles.

[reaction: see text] A new iron(III) halide-promoted aza-Prins cyclization between gamma,delta-unsaturated tosylamines and aldehydes provides six-membered azacycles in good to excellent yields. The process is based on the consecutive generation of gamma-unsaturated-iminium ion and further nucleophilic attack by the unsaturated carbon-carbon bond. Homoallyl tosylamine leads to trans-2-alkyl-4-halo-1-tosylpiperidine as the major isomer. In addition, the alkyne aza-Prins cyclization between homopropargyl tosylamine and aldehydes gives 2-alkyl-4-halo-1-tosyl-1,2,5,6-tetrahydropyridines as the only cyclic products.

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PubMed2006

5-Exocyclic products, 2,3,5-trisubstituted tetrahydrofurans via Prins-type cyclization.

[reaction: see text] 5-Exocyclic products, 2,3,5-trisubstituted tetrahydrofurans, were synthesized from homopropargylic alcohols with terminally substituted alkynes and various aldehydes via Prins-type cyclization. It is of interest that the exocyclic vinyl cation generated as a result of Prins-type cyclization could be trapped as a vinyl triflate when CH2Cl2 was used as a solvent, whereas in ethereal solution the vinyl cation underwent hydrolysis to give the corresponding ketone product.

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PubMed2000

Stereochemistry of the cyclization-rearrangement of (+)-copalyl diphosphate to (-)-abietadiene catalyzed by recombinant abietadiene synthase from Abies grandis.

[reaction: see text] Syntheses and enzymatic cyclizations of 8alpha-hydroxy-17-nor copalyl diphosphate (8a), (15R)-[15-2H1] 8b, and (15R,17E)-[15-3H1,17-2H1] copalyl diphosphate ([2H,3H] 2) catalyzed by recombinant abietadiene synthase (rAS) gave 17-nor manoyl oxide (9a), (16E)-[16-2H1] 9b, and (15S,16R)-[16-2H1,16-3H1] abietadiene ([2H1,3H1] 4), respectively. These and other results indicate that conversion of CPP (2) to abietadiene (4) occurs by anti S(N)' cyclization to a sandaracopimar-15-en-8-yl carbocation intermediate (13+, 13beta-methyl) followed by hydrogen transfer and methyl migration suprafacially on the si face of the vinyl group.

Abietanes↗
PubMedDate not supplied

Cyclization of N-terminal S-carbamoylmethylcysteine causing loss of 17 Da from peptides and extra peaks in peptide maps.

Enzymatic digests of proteins S-alkylated with iodoacetamide may contain peptides with N-terminal S-carbamoylmethylcysteine. These can be partly converted to a form with 17 Da lower mass and increased HPLC retention. Proof by synthesis supported by MS/MS and NMR spectroscopy was used to show that N-terminal S-carbamoylmethyl-L-cysteine can cyclize, losing NH3 to form an N-terminal residue of (R)-5-oxoperhydro-1,4-thiazine-3-carboxylic acid. The abbreviation Otc is proposed for the (R)-5-oxoperhydro-1,4-thiazine-3-carbonyl residue. The rate of cyclization is significant in 0.1 M NH4HCO3 at 37 degrees C, with the half-life of the acyclic form being 10-12 h for several peptides tested. This is similar to the rate at which N-terminal pyroglutamate forms from N-terminal glutamine.

Chromatography, High Pressure Liquid↗
PubMed2001

Electrocatalytic cyclization of dithiothreitol on a chemically modified electrode by analogy with protein action.

Electrocatalytic oxidative cyclization of dithiothreitol (DTT(SH)2) to a disulfide product was demonstrated on a Nafion/lead-ruthenium oxide pyrochlore chemically modified electrode (NPyCME). The process at the NPyCME with DTT(SH)2 is similar to the behaviour of protein in a disulfide linkage, which can be demonstrated by product analysis using HPLC coupled with UV spectroscopy. A possible electrocatalytic mechanism for DTT(SH)2 oxidation to dihydroxydithiane [i.e. cyclized DTT(S-S)] on the NPyCME was proposed in terms of Py-Ru(IV)/Py-Ru(VI) redox active sites. This physical aspect was further utilized for high precision analytical assays using flow injection analysis (FIA), with a linearity up to 50 microM and a detection limit (S/N = 3) of 28 nM (8.64 pg) in a 20 microL sample loop. This is the most sensitive method ever reported for DTT(SH)2 detection assays. The interference from dissolved oxygen, disulfide and glucose is almost negligible. The present method offers an easy route for extension to redox-related protein studies.

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PubMed2002

Nucleophilic transition metal based cyclization of allenes.

Allenes bearing a pro-nucleophile can be cyclized on treatment with a wide variety of transition metal catalysts and reagents: palladium, cobalt, ruthenium, silver, rhodium, lanthanides, gold. The nucleophilic groups can be nitrogen, oxygen or carbon based and can form rings of various sizes, often with good control of stereochemistry. A variety of mechanisms can be proposed for these reactions and the metal complex can be used to introduce a variety of functional groups during cyclization. Several heterocyclic natural products have been prepared using a selection of these reactions.

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PubMed2002

Synthesis and application of chiral bisphosphines through lithiation-conjugate addition tandem cyclization of chiral alpha,beta,psi,omega-unsaturated bisphosphine oxide.

Upon treatment with lithium diisopropylamide achiral and chiral alpha,beta,psi,omega-unsaturated bisphosphine oxides underwent lithiation-conjugate addition tandem cyclization to afford the corresponding endo-alpha,beta-unsaturated cyclic bisphosphine oxides; sequential stereoselective reduction of the cyclized bisphosphine oxide gave the corresponding trans- and cis-bisphosphines that were successfully applicable in a catalytic asymmetric hydrogenation as chiral bisphosphine ligands.

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