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At least 451 records · Page 25Linked to original sources
PubMed2002

A new tri-orthogonal strategy for peptide cyclization.

A solid phase tri-orthogonal protection/cleavage strategy that uses acidic, basic, and neutral conditions is described. Strategically protected alpha-azido-gamma-9-fluorenylmethyl-L-glutamate (1) and alpha-azido-epsilon-N-Fmoc-L-lysine (2) were incorporated into growing peptides on Wang resin using a novel azide protection strategy. These residues, separated by 1-3 monomers, were deprotected at the side chains and cyclized via lactam formation. The N-terminus was further functionalized to extend the chain. This method represents a straightforward protocol for peptide cyclization on solid support.

Amino Acids↗
PubMed2003

Acid-promoted Prins cyclizations of enol ethers to form tetrahydropyrans.

Trifluoroacetic acid efficiently catalyzes Prins cyclizations of enol ethers 8 to provide tetrahydropyrans 9 and 10. These tetrahydropyrans are isolated with combined yields of 42-85% and stereoselectivities at C(4) ranging from 95:5 to 50:50 depending on the nature of the substituent R. Unique byproducts of these cyclizations that reveal the presence of underlying equilibria have been isolated and identified. [reaction: see text]

Catalysis↗
PubMed2003

N-silyl-tethered radical cyclizations: a new synthesis of gamma-amino alcohols.

[reaction: see text] Various allylic and propargylic amines bearing a protecting group (PG) have been employed in N-silyl-tethered radical cyclizations. The resulting silapyrrolidine adducts could be smoothly oxidized, creating access to gamma-amino alcohols. The silylation, radical cyclization, and oxidation reactions could be consolidated in a one-pot process.

Allylamine↗
PubMed2003

Intramolecular cyclization reactions of 5-halo- and 5-nitro-substituted furans.

[reaction: see text] Intramolecular cyclization reactions of 5-halo- and 5-nitro-substituted furanylamides were examined. The 2-alkoxy-5-bromofuran derivative 2 produced the rearranged dihydroquinone 6 (36%), a product from the rearrangement of the intermediate oxabicycle 3. The 5-halo substituted furoyl amide 18 was converted to the polyfunctional oxabicycle 20 in 82% yield and at a much faster rate than the unsubstituted furanyl system 17. The 5-nitro-substituted furfuryl amide 33b underwent an unusual isomerization-cyclization reaction under microwave conditions to provide 1,4-dihydro-2H-benzo[4,5]furo[2,3-c]pyridin-3-one 34.

Amides↗
PubMed2003

Probing the Lewis acid-catalyzed intramolecular Diels-Alder cyclizations of allylic alkoxy-substituted (Z)-1,3-dienes.

[reaction: see text] The Lewis acid-promoted Diels-Alder reaction of (E,Z,E)-trienal 1 provides not only the expected cis-fused cycloadduct 16 but also the trans-fused products 17 and 18. Trans-fused cycloadducts 17 and 18 are also products of the Lewis acid-promoted cyclization of (E,Z,E)-trienyl acetal 2. These products presumably derive from a stepwise cyclization pathway.

Acids↗
PubMed2003

Efficient synthesis of (+/-)-erysotramidine using an NBS-promoted cyclization reaction of a hexahydroindolinone derivative.

An NBS-promoted intramolecular electrophilic aromatic substitution reaction of a hexahydroindolinone derivative was used to assemble the tetracyclic core of the erythrinane skeleton. The resulting cyclized product was transformed into (+/-)-erysotramidine in three additional steps. The cyclization reaction is also successful using variously substituted aryl and furanyl bicyclic lactams under acidic conditions. [reaction: see text]

Cyclization↗
PubMed2005

Mercuric triflate-catalyzed tandem cyclization leading to polycarbocycles.

[reaction: see text] We developed Hg(OTf)2-catalyzed cyclization of (E)-1,3-dimethoxy-5-(4-methyl-3-nonen-7-ynyl)benzene leading to the formation of (4aS,10aS)-3,4,4a,9,10,10a-hexahydro-5,7-dimethoxy-1,4a-dimethylphenanthrene in 98% yield with up to 100 catalytic turnovers. This is the first mercuric salt-catalyzed biomimetic tandem cyclization.

Catalysis↗
PubMed2004

Preparation of 1,5-methano-2,3,4,5- tetrahydro-1H-3-benzazepine via Pd-catalyzed cyclization.

[reaction: see text] A new approach to prepare 1,5-methano-2,3,4,5-tetrahydro-1H-3-benzanepine (1) is discussed. This strategy utilized a tandem Michael addition and Pd-catalyzed cyclization to afford cyanobenzofulvene acetal 13. This indene intermediate (13) was subjected to hydrogenolysis to provide an amino ester (12) and was cyclized with base to afford lactam 5. The lactam (5) was reduced with borane to afford the desired benzazepine (1).

Benzazepines↗
PubMed2004

Concerted nature of AB ring formation in the enzymatic cyclization of squalene to hopenes.

The cyclization of the A-B rings of squalene to hopene is studied computationally (DFT). A transition structure is found for a concerted, asynchronous pathway for the formation of chair-chair decalin carbocation. The computationally derived conformer leading to this asynchronous transition structure is remarkably similar to the analogous region of 2-azasqualene encapsulated by squalene-hopene cyclase recently reported by Schulz. A concerted A-B ring closure is likely to occur in the cyclization of squalene to hopene. [structure--see text]

Crystallography, X-Ray↗
PubMed2005

Ruthenium-catalyzed diyne hydrative cyclization: synthesis of substituted 1,3-diene synthons.

[reaction: see text]. A novel and versatile strategy for the synthesis of highly functionalized substituted 3-sulfolenes based on [CpRu(CH3CN)3]PF6-catalyzed hydrative cyclization has been developed. A marked ketone directing effect in ruthenium-catalyzed cyclization was observed for the first time. This provides complementary chemoselectivity for the synthesis of 3-sulfolenes and other cyclic enones. The utility of this method has been demonstrated by SO2 extrusion of 3-sulfolenes to afford 1,3-dienes and the subsequent inter- and intramolecular Diels-Alder reaction.

Alkenes↗
PubMed2005

Stereoselective Nazarov cyclizations of bridged bicyclic dienones.

[reaction: see text] Bridged bicyclic dienones underwent silyl-directed Nazarov cyclization with generally very high diastereoselectivity. In most cases, a strong preference for cyclization to the product with an exo-disposed cyclopentenone was seen. However, the presence of additional unsaturation in the three-carbon bridge of a bicyclo[3.2.1]octadiene system caused complete reversal in selectivity with exclusive formation of the endo-cyclopentenone. The observed selectivities are believed to result from a combination of steric and electronic effects.

Bridged Bicyclo Compounds↗
PubMed2005

Oxidative cyclization reactions: amide trapping groups and the synthesis of furanones.

Intramolecular anodic coupling reactions involving ketene dithioacetal radical cations and amide trapping groups have been examined. The reactions generate furanone products and benefit greatly from the addition of water to the reaction medium. The cyclization reactions lead to products having stereochemistry that is directly analogous to oxidative cyclization reactions utilizing ketene dithioiacetal radical cations and alcohol trapping groups. [reaction: see text]

Amides↗
PubMed2005

Prins cyclizations in silyl additives with suppression of epimerization: versatile tool in the synthesis of the tetrahydropyran backbone of natural products.

[reactions: see text] A catalytic Prins cyclization reaction has been developed. The involvement of trimethylsilyl halides offers a versatile route to the formation of cis-4-halo-2,6-disubstituted tetrahydropyran rings. The problem of epimerization in Prins cyclization has also been addressed in the total synthesis of (-)-centrolobine using this methodology.

Biological Products↗
PubMed2005

N,O-isopropylidenated threonines as tools for peptide cyclization: application to the synthesis of mahafacyclin B.

[structure: see text] The influence of a single N,O-isopropylidenated threonine turn-inducer on the cyclization of a linear heptapeptide precursor to mahafacyclin B has been investigated. Incorporation of an N,O-isopropylidenated threonine more than doubles the head-to-tail cyclization yield. The N,O-isopropylidene grouping is then readily disassembled to give the antimalarial cyclic peptide in high yield.

Amino Acid Sequence↗
PubMed2006

Diastereoselective cationic tandem cyclizations to N-heterocyclic scaffolds: total synthesis of (-)-dysibetaine PP.

[reaction: see text] Herein, we report a short and diastereoselective synthesis of the natural product (-)-dysibetaine PP. The key step in the synthetic sequence is a novel highly diastereoselective tandem-cyclization reaction of an enantiomerically pure dipeptide. This cyclization methodology is applied in the synthesis of a broader range of N-heterocyclic scaffolds.

Animals↗
PubMed2006

Atom transfer cyclization catalyzed by InCl3 via halogen activation.

[reaction: see text] Indium trichloride was found to be an efficient catalyst for the cyclization of allylic halides and alkynes with atom transfer in methylene chloride. Mechanistic evidence supports a cationic reaction pathway with Lewis acid activation of the allylic halogen. Concomitant nucleophilic attack by the alkyne and trapping with halide led to atom transfer cyclization products. Depending on alkyne substitution, a bromine atom was transferred from the substrate or a chlorine atom was transferred from the solvent.

Alkynes↗