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PubMed1985

Total synthesis of (+/-)-11 alpha-hydroxyprogesterone by cyclization of a polyunsaturated epoxide.

The total synthesis of a typical 11-hydroxylated steroid, (+/-)-11 alpha-hydroxyprogesterone, was achieved by picric acid-catalyzed tricyclization of a polyunsaturated epoxide appropriately substituted with ketal, hydroxyl, and acetylenic units. This epoxide was prepared by a multistage sequence featuring two successive alkylations of intermediary, monocyclic sulfones. The first sulfone intermediate was obtained by means of a short sequence starting from levulinic acid and diethyl succinate and involving a selective cyclization reaction.

Cyclization↗
PubMed2006

Microwave-assisted rapid cyclization of lapachol, a main constituent of Heterophragma adenophyllum.

Cyclization reactions of lapachol (1) isolated from Heterophragma adenophyllum have been studied under microwave irradiation under different conditions using alumina (acidic, basic and neutral)/silica gel/montmorillonite (KSF and K-10) as solid support along with neat reaction using 2-3 drops of DMF giving naturally occurring dehydro-alpha-lapachone (2), alpha-lapachone (3), beta-lapachone (4) depending upon the nature of support and irradiation time. A novel naphthoquinone derivative adenophyllone (5) can be synthesized from lapachol using DMF under microwaves.

Bignoniaceae↗
PubMed2003

The phage T4 restriction endoribonuclease RegB: a cyclizing enzyme that requires two histidines to be fully active.

The regB gene, from the bacteriophage T4, codes for an endoribonuclease that controls the expression of a number of phage early genes. The RegB protein cleaves its mRNA substrates with an almost absolute specificity in the middle of the tertranucleotide GGAG, making it a unique well-defined restriction endoribonuclease. This striking protein has no homology to any known RNase and its catalytic mechanism has never been investigated. Here, we show, using 31P nuclear magnetic resonance (NMR), that RegB produces a cyclic 2',3'-phosphodiester product. In order to determine the residues crucial for its activity, we prepared all the histidine-to- alanine point mutants of RegB. The activity of these mutants was characterized both in vivo and in vitro. In addition, their binding capability was quantified by surface plasmon resonance and their structural integrity was probed by 1H/15N NMR correlation spectroscopy. The results obtained show that only the H48A and the H68A substitutions significantly reduce RegB activity without changing its ability to bind the substrate or affecting its overall structure. Altogether, our results define RegB as a new cyclizing RNase and present His48 and His68 as potent catalytic residues. The effect of the in vivo selected R52L mutation is also described and discussed.

Alanine↗
PubMed2004

Localized single-stranded bubble mechanism for cyclization of short double helix DNA.

Recent experiments indicate that double-stranded DNA molecules of approximately 100 base pairs in length have a probability of cyclization which is up to 10(5) times larger than that expected based on the known bending modulus of the double-helix. We argue that for short molecules, the formation of a few base pairs of single-stranded DNA can provide a "flexible hinge" that facilitates loop formation. A detailed calculation shows that this mechanism explains the experimental data.

Base Sequence↗
PubMed1991

Cyclization studies with tetra- and pentapeptide sequences corresponding to beta-casomorphins.

The tetrapeptide Boc-D-Orn-Phe-D-Pro-Gly-OH and the pentapeptide sequence Boc-Tyr(tBu)-D-Orn-Phe-D-Pro-Gly-OH were used to study the influence of different coupling reagents on the yield and purity of these model peptides. The simple structure prevented racemization and cyclodimerization and facilitated the ring formation. The most favorable effects on yield and purity were obtained in both reactions using diphenyl-phosphoryl azide (DPPA) and norborn-5-ene-2,3-dicarboximidodiphenylphosphate (NDPP), while the cyclizations with the powerful activating reagents benzotriazol-1-yl-oxy-tris(dimethylamino)-phosphonium hexafluorophosphate (BOP) and 2-(1-H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) with the exception of the cyclopentapeptide reaction with HBTU/4-dimethylaminopyridine gave unsatisfactory results.

Amino Acid Sequence↗
PubMed2004

Quantitative analysis, using MALDI-TOF mass spectrometry, of the N-terminal hydrolysis and cyclization reactions of the activation process of onconase.

Onconase, a member of the ribonuclease superfamily, is a potent cytotoxic agent that is undergoing phase II/III human clinical trials as an antitumor drug. Native onconase from Rana pipiens and its amphibian homologs have an N-terminal pyroglutamyl residue that is essential for obtaining fully active enzymes with their full potential as cytotoxins. When expressed cytosolically in bacteria, Onconase is isolated with an additional methionyl (Met1) residue and glutaminyl instead of a pyroglutamyl residue at position 1 of the N-terminus and is consequently inactivated. The two reactions necessary for generating the pyroglutamyl residue have been monitored by MALDI-TOF MS. Results show that hydrolysis of Met(-1), catalyzed by Aeromonas aminopeptidase, is optimal at a concentration of >or= 3 m guanidinium-chloride, and at pH 8.0. The intramolecular cyclization of glutaminyl that renders the pyroglutamyl residue is not accelerated by increasing the concentration of denaturing agent or by strong acid or basic conditions. However, temperature clearly accelerates the formation of pyroglutamyl. Taken together, these results have allowed the characterization and optimization of the onconase activation process. This procedure may have more general applicability in optimizing the removal of undesirable N-terminal methionyl residues from recombinant proteins overexpressed in bacteria and providing them with biological and catalytic properties identical to those of the natural enzyme.

Antineoplastic Agents↗
PubMed1987

A stereospecific cyclization catalyzed by an antibody.

A monoclonal antibody elicited by a transition-state analog that is representative of an intramolecular six-membered ring cyclization reaction acted as a stereospecific, enzyme-like catalyst for the appropriate substrate. Formation of a single enantiomer of a delta-lactone from the corresponding racemic delta-hydroxyester was accelerated by the antibody by about a factor of 170, which permitted isolation of the lactone in an enantiomeric excess of about 94 percent. This finding demonstrates the feasibility of catalytic-antibody generation for chemical transformations that require stereochemical control.

Antibodies, Monoclonal↗
PubMed1997

Construction of AB-ring system of taxane framework by A-ring annulation strategy: synthesis of 1-hydroxy-8,11,11-trimethylbicyclo-[5.3.1]undec-7-en-9-one by way of intramolecular aldol cyclization to form the C1-C10 bond.

Construction of the AB-ring system of the taxane framework via an A-ring annulation strategy was demonstrated by base-mediated intramolecular aldol reaction of (Z)-2,2-dimethyl-3-(1-methyl-2-oxopropylidene)cyclooctanone, affording the title compound, 1-hydroxy-8,11,11-trimethylbicyclo[5.3.1]undec-7-en-9-one. A cyclization precursor, the tetra-substituted (Z)-alkene, was prepared from the corresponding cyclooctanone derivative, 3-[(alpha,alpha- dimethylbenzyl)dimethylsiloxy]-2,2-dimethylcyclooctanone via a bicyclic alpha,beta-unsaturated lactone intermediate.

Bridged Bicyclo Compounds↗
PubMed2001

Co2(CO)8-mediated endo mode cyclization of epoxy-alcohol: synthesis of 2-ethynyl-3-hydroxy-2-methyltetrahydropyran and 2-ethynyl-3-hydroxy-3-methyltetrahydropyran derivatives.

Successive treatment of 4,5-epoxy-5-methyl-7-trimethylsilyl-6-heptyne-1-ol with Co2(CO)8 at 0 degrees C and a catalytic amount of BF3 x OEt2 at -78 degrees C gave the tetrahydropyran derivatives with the cobalt-complexed moiety. Similarly 4,5-epoxy-4-methyl-7-trimethylsilyl-6-heptyne-1-ol underwent ring closure under the above conditions to provide the corresponding tetrahydropyran derivatives. The preferential endo mode cyclization over the exo one was observed in these experiments.

Alkynes↗
PubMed2004

Synthesis of nitrogen-functionalized cyclohexanes using chemoselective conjugate addition of phenyllithium to linear omega-nitro-alpha,beta,psi,omega-unsaturated ester and subsequent stereoselective intramolecular nitro-Michael cyclization.

Nitrogen-functionalized cyclohexane derivatives with three contiguous chiral centers were synthesized by nitroalkene-selective conjugate addition of phenyllithium to a omega-nitro-alpha,beta,psi,omega-unsaturated ester and subsequent stereocontrolled intramolecular nitro-Michael cyclization with cesium fluoride and a quaternary ammonium bromide. The cyclohexanes were applicable to the total synthesis of alpha-, beta- and gamma-lycoranes.

Cyclization↗
PubMed2004

Mechanistic considerations for the consecutive cyclization of 2,3-dibromopropylamine hydrobromide giving a strained molecule, 1-azabicyclo[1.1.0]butane.

The effective formation of 1-azabicyclo[1.1.0]butane (2) by treatment of 2,3-dibromopropylamine hydrobromide (1) with n-BuLi could be understood considering a rational reaction pathway via both transition states 10 and 19 based on the intramolecular Br...Li(+) coordination. A similar cyclization pathway starting from N-benzyl-3-bromopropylamine hydrochloride (17) to afford N-benzylazetidine (18) could also be postulated on the basis of a transition state 20 involving the intramolecular Br...Li(+) coordination.

Aza Compounds↗
PubMed2005

Development of useful reactions involving tandem cyclizations based on the novel reactivities of allenic compounds.

This review highlights author's recent study on allenic compounds. In the first section, organocopper-mediated ring-opening reaction of ethynylaziridines and palladium-catalyzed reductive synthesis of allenes including those which are not accessible by other means, are described. In the second section, palladium-catalyzed stereoselective cyclization of allenes and tandem reaction leading to aziridines, pyrrolidines, benzoisoindoles, and cyclopropanes are presented. The final section presents aziridination and medium-ring formation by intramolecular reaction of bromoallenes, which are scarcely investigated until quite recently. The latter reaction is based on our recent discovery that bromoallenes can act as allylic dication equivalents in the presence of a palladium catalyst and alcohol.

Alkadienes↗
PubMed2006

Cyclization of 1-benzyl-1,2-dihydro-2-(substituted methylene)quinolines to pyrrolo[1,2-a]quinoline derivatives.

1-Alkyl-2-alkylthioquinolinium salts were prepared from 1-alkyl-2(1H)-quinolones via 1-alkyl-2(1H)-thioquinolones in two steps. Under mild conditions, the reaction of 1-alkyl-2-alkylthioquinolinium iodides with active methylene compounds in the presence of sodium hydride afforded 1-alkyl-1,2-dihydro-2-(substituted methylene)quinolines in good yields. The cyclization of 1-benzylquinolines using acetic anhydride produced the corresponding pyrrolo[1,2-a]quinoline derivatives.

Acetic Anhydrides↗
PubMed2003

[6-endo-trig mode cyclization to a hydrindanone using samarium (II) iodide].

Samarium (II) iodide has been employed to promote the vinylogous pinacol coupling reaction of aldehyde to alpha, beta-unsaturated ketones. The diastereoselectivity of 6-endo-trig mode products was changed by the addition of a proton source and/or HMPA and by the reaction temperature. The stereochemistry of the hydrindanone was controlled by the coordinated samarium species, resulting in the cis-orientation in respect of the hydroxyl group at C-4 and the juncture proton at C-3a under mild reaction conditions. Coronafacic acid has been synthesized from a hydrindanone prepared by the cyclization reaction of the enone-aldehyde with samarium (II) iodide.

Cyclization↗