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At least 1,153 records · Page 64Linked to original sources
PubMed2002

Generation and trapping of alkene radical cations under nonoxidizing conditions: formation of six-membered rings by exo- and endo-mode cyclizations.

[reaction: see text] It is demonstrated that alkene radical cations generated by the radical ionic fragmentation of beta-(phosphatoxy)alkyl radicals undergo efficient nucleophilic capture by amines in either the 6-exo or 6-endo modes, leading to six-membered nitrogen heterocycles. Suitable placement of an alkene enables the juxtaposition of a radical cyclization resulting in the formation of both the indolizidine and 1-azabicyclo[3.2.1]octane skeleta.

Journal Article↗
PubMed2002

Intramolecular tandem Michael-type addition/aldol cyclization induced by TiCl(4)-R(4)NX combinations.

[reaction: see text] Treatment of formyl alpha,beta-enones with a TiCl(4)-R(4)NX combination induces an intramolecular aldol cyclization to furnish 2-acyl-3-halocyclohexanol with three controlled consecutive stereogenic centers. The reaction of bis-alpha,beta-enones with the combination provides cyclic diketones with high stereoselectivity via an intramolecular Michael addition reaction.

Journal Article↗
PubMed2002

A cascade cyclization approach to schweinfurthin B.

[reaction: see text] A strategy for synthesis of the hexahydroxanthene moiety of the natural products schweinfurthin A, B, and D is described. The relative stereochemistry in the key cationic cyclization step is established through the preference of the phenylselenide substituent for an equatorial orientation.

Magnetic Resonance Spectroscopy↗
PubMed2002

Synthesis of cis-solamin using a permanganate-mediated oxidative cyclization.

[reaction: see text] cis-Solamin (1) and its diastereoisomer 14 have been synthesized in 13 steps using the diastereoselective permanganate-promoted oxidative cyclization of 1,5-dienes to create the tetrahydrofuran diol core. Notably, no protecting groups are required during the stages of fragment assembly.

Benzethonium↗
PubMed2002

Continuous solid-phase synthesis and disulfide cyclization of peptide-PNA-peptide chimeras.

Chelator peptides were extended from the N-terminus of peptide nucleic acid (PNA) dodecamers, which in turn were extended from the N-termini of disulfide-bridged peptide ligand analogues, using Fmoc coupling for all residues. The cysteine thiols were cyclized on a solid support, either before or after PNA extension. This simplified synthetic approach might allow preparation of a variety of multipeptide disulfide-bridged PNA chimeras. [structure: see text]

Amino Acid Sequence↗
PubMed2002

An efficient method for cyclopentene annulation onto alpha,beta-unsaturated ketones: W(CO)(5)(L)-catalyzed 5-endo-dig cyclization of 6-siloxy-5-en-1-ynes.

[reaction: see text] A highly efficient method for the cyclopentene annulation onto alpha,beta-unsaturated ketones is described. Indium-mediated 1,4-propargylation onto alpha,beta-unsaturated ketones in the presence of tert-butyldimethylsilyl triflate and dimethyl sulfide gives the 6-siloxy-5-en-1-yne derivatives, which undergo W(CO)(5)(L)-catalyzed 5-endo-dig cyclization to give the corresponding cyclopentene derivatives in good yield.

Journal Article↗
PubMed2002

Investigation of the diastereoselective cyclization of bis-sulfonyl esters.

[reaction: see text] The diastereoselective cyclization of bissulfonyl esters was investigated by varying both the size and the placement of the substituent on the tether adjoining the reacting centers. Substitution at either the alpha or beta position relative to the ester moiety gave diastereomeric ratios of (1-3):1, while gamma substitution dramatically increased the diastereomeric ratios to (6-20):1.

Journal Article↗
PubMed2003

The first preparation of beta-lactones by radical cyclization.

beta-Lactones have, for the first time, been prepared by 4-exo-trig radical cyclization. Thus, alpha-ethenoyloxy radicals react in the presence of tributylstannane in a photothermal process to give beta-lactones. Highest yields were obtained when groups capable of stabilizing a carbon-centered radical were present at the 3-position of the alkenoate acceptor.

Journal Article↗
PubMed2003

Alkoxyamine-mediated radical cyclizations.

[reaction: see text] We present a new "conjunctive" radical cyclization process that involves the reaction of a 1,6-diene with the Tordo alkoxyamine, an agent originally developed for the radical polymerization of certain olefins through the "persistent radical effect".

Journal Article↗
PubMed2003

Synthesis of 3-aminochroman derivatives by radical cyclization.

[reaction: see text] Enantiomerically pure 5-acetyl-3-amino-3,4-dihydro-2H-1-benzopyran and methyl 3-amino-3,4-dihydro-2H-1-benzopyran-5-carboxylate were successfully synthesized starting from d- or l-serine. The formation of the benzopyran ring involved a radical cyclization step. The enantiomeric purities of the final aminochroman derivatives were determined by capillary electrophoresis using beta-cyclodextrins as a chiral selector.

Journal Article↗
PubMed2003

Enantioselective cyclization of alkene radical cations.

[reaction: see text] Enantiomerically enriched beta-(diphenylphosphatoxy)nitroalkanes undergo radical ionic fragmentation, induced by tributyltin hydride and AIBN in benzene at reflux, to give alkene radical cations in contact radical ion pairs. These contact ion pairs are trapped intramolecularly by amines to give pyrrolidines and piperidines with significant enantioselectivity ( approximately 60% ee), indicative of cyclization competing effectively with equilibration within the ion pairs. Use of an intramolecular N-propargylamine as a nucleophile provides an enantiomerically enriched pyrrolizidine skeleton via a tandem polar/radical crossover sequence.

Journal Article↗