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PubMed2004

Construction of azacycles based on endo-mode cyclization of allenes.

A new procedure for constructing monocyclic five- and six-membered azacycles by the endo-mode ring-closing reaction of allenylazido derivatives under neutral conditions has been developed. The azabicyclo[m.n.0] compounds were prepared by applying this newly developed procedure. The seven-membered azacycle was prepared when the allene possessing an unsubstituted carboxyl amido functionality was submitted to the basic conditions. In addition, indole and quinoline skeletons were synthesized using this procedure.

Alkadienes↗
PubMed2004

First synthesis of a branched beta-C-tetrasaccharide using a triple ring closing metathesis cyclization.

The first synthesis of a branched beta-C-tetrasaccharide has been carried out through the use of an esterification-ring closing metathesis (RCM) strategy. The precursor triacid 2a was readily prepared via standard chemical methods from a known starting material, and dehydrative coupling with an excess of olefin alcohol 1a gave triester 3a in excellent yield. Methylenation of the triester 3a and subsequent triple RCM reaction was followed by an in situ hydroboration-oxidation to furnish the branched beta-C-tetrasaccharide 6a in good overall yield.

Carbohydrate Conformation↗
PubMed2005

Regio- and stereoselective synthesis of isoindolin-1-ones via electrophilic cyclization.

[reaction: see text] A variety of substituted isoindolin-1-ones are readily prepared in good to excellent yields under very mild reaction conditions by the reaction of o-(1-alkynyl)benzamides with ICl, I(2), and NBS. In a few cases, substituted isoquinolin-1-ones were obtained as the major product instead. This methodology accommodates various alkynyl amides and functional groups and has been successfully extended to heterocyclic starting materials. This chemistry has been successfully applied to the synthesis of a biologically interesting alkaloid, cepharanone B.

Aristolochic Acids↗
PubMed2005

Sequential cyclization-elimination route to carbohydrate-based oxepines.

A five-step preparation of carbohydrate-based oxepines from hept-1-enitols is presented. The hept-1-enitols were subjected to silyl protection and hydroboration/oxidation to give the protected heptan-1-itols. Swern oxidation of the homologated alcohols followed by sequential acetal formation/cyclization provided methyl 2-deoxyseptanosides that underwent elimination reactions to give the carbohydrate-based oxepines. The new sequence is an alternative to the ring-closing metathesis for the synthesis of carbohydrate-based oxepines from protected pyranose sugars. The product oxepines can serve as glycosyl donors in the synthesis of novel septanose carbohydrates. In addition, C-methylenealdehydo arabinofuranoside 16 was formed from 2-deoxyseptanoside 10 as the only product during protic acid mediated elimination reactions. This novel ring contraction complements other reported preparations of C-methylenaldehydo furanosides and underscores the acid-mediated reactivity introduced by competing eliminations between the C-1/C-2 and C-2/C-3 bonds.

Carbohydrates↗
PubMed2004

From dimerization, to cycloaddition, to atom transfer cyclization: the further chemistry of TMM diradicals.

We describe [a] the first examples of intramolecular cycloaddition of a TMM diyl to a remotely tethered aldehyde, [b] the effect of a Lewis acid upon the course of TMM chemistry, [c] examples of exclusive intramolecular cycloaddition, competitive cycloaddition and ATC, and exclusive ATC, and [d] a set of predictive guidelines with which to assess whether cycloaddition or ATC will be the preferred path, and when the two processes will be competitive. Remarkably, a wide variety of structures can be obtained simply by varying the length of the tether within the diazenes investigated. DFT calculations were used to probe the energy surfaces for both atom transfer and cycloaddition. The transition structure for atom transfer involving the captodative system indicates that it occurs earlier along the reaction coordinate than for a system having only one radical stabilizing group. This is consistent with the existence of an exothermic process leading from the initial diyl to the captodatively stabilized distonic diyl. Gratifyingly, theory agrees with observation and provides substantial insight into the chemistry.

Cyclization↗
PubMed2005

Synthesis of polycyclic aromatics and heteroaromatics via electrophilic cyclization.

[reaction: see text] A variety of substituted polycyclic aromatics are readily prepared in good to excellent yields under very mild reaction conditions by the reaction of 2-(1-alkynyl)biphenyls with ICl, I(2), NBS, and p-O(2)NC(6)H(4)SCl. This methodology readily accommodates various functional groups and has been successfully extended to systems containing a variety of polycyclic and heterocyclic rings.

Catalysis↗
PubMed2005

New regiospecific synthesis of 2-trifluoromethyl-1,5 diazapentadiene compounds and of 2-trifluoromethylquinolines, their cyclization products.

2-trifluoromethylquinolines 5 are synthesized in high yields using a perfluoroalkylated gem-iodoacetoxy derivative 3 and arylamines 4. The intermediate of this reaction, 2-trifluoromethyl-1,5-diazapentadiene compound 6, was isolated. The procedures are easy, and yields are in general high. This sequence represents a valuable new synthesis of substituted 2-trifluoromethylquinolines and of 2-trifluoromethyl-diazapentadienes (vinamidine compounds).

Aniline Compounds↗
PubMed2006

Auto-redox reaction: tin(II) chloride-mediated one-step reductive cyclization leading to the synthesis of novel biheterocyclic 5,6-dihydro-quinazolino[4,3-b]quinazolin-8-ones with three-point diversity.

[reaction: see text] A tin(II) chloride-mediated short, efficient, and practical regioselective synthesis of biheterocyclic 5,6-dihydro-quinazolino[4,3-b]quinazolin-8-ones with three-point diversity is reported. A one-step reductive transformation of 2-(2-nitrophenyl)-3H-quinazolin-4-one in various alcohols furnished the desired tetracyclic product in good yields with high purity.

Chlorides↗
PubMed2006

Syntheses of isochromenes and naphthalenes by electrophilic cyclization of acetylenic arenecarboxaldehydes.

Highly substituted 1H-isochromenes, isobenzofurans, and pyranopyridines can be prepared by allowing o-(1-alkynyl)arenecarboxaldehydes and ketones to react with I2, ICl, NIS, Br2, NBS, p-O2NC6H4SCl, or PhSeBr and various alcohols or carbon-based nucleophiles at room temperature. Naphthyl ketones and iodides are also readily prepared by the reaction of 2-(1-alkynyl)arenecarboxaldehydes with I2 and simple olefins or alkynes.

Acetylene↗
PubMed2006

Acid-catalyzed cyclization of anthracenol derivatives to homotriptycenes.

10-Benzyl-9,10-dihydroanthracen-9-ols, having high electron densities in the benzene ring, exhibit in the presence of acid a transannular ring closure to the corresponding homotriptycenes in almost quantitative yields. Since the starting compounds are easily accessible from 9(10H)-anthracenone, this process represents the most facile route to such pentacyclic systems. An electron-releasing methoxy group enables the intramolecular electrophilic substitution in its para position. In the absence of such an activation, a number of alternative processes can occur, namely the acid-catalyzed dehydration to anthracene derivatives with (R not = H) or without (R = H) rearrangement or a disproportionation reaction of the secondary alcohol to the corresponding ketone and hydrocarbon.

Acids↗
PubMed2006

A new synthesis of 2,3-dihydrobenzo[1,4]dioxine and 3,4-dihydro-2H-benzo[1,4]oxazine derivatives by tandem palladium-catalyzed oxidative aminocarbonylation-cyclization of 2-prop-2-ynyloxyphenols and 2-prop-2-ynyloxyanilines.

2-[(Dialkylcarbamoyl)methylene]-2,3-dihydrobenzo[1,4]dioxine and 3-[(dialkylcarbamoyl)methylene]-3,4-dihydro-2H-benzo[1,4]oxazine derivatives (3 and 5, respectively) were synthesized for the first time starting from readily available 2-prop-2-ynyloxyphenols 1 and 2-prop-2-ynyloxyanilines 4, respectively, through tandem oxidative aminocarbonylation of the triple bond-intramolecular conjugate addition. Reactions were carried out in the presence of catalytic amounts of PdI2 in conjunction with an excess of KI in N,N-dimethylacetamide (DMA) as the solvent at 80-100 degrees C and under 20 atm (at 25 degrees C) of a 4:1 mixture of CO-air. The reaction showed a significant degree of stereoselectivity, the Z isomers being formed preferentially or exclusively. The configuration around the double bond of the major stereoisomers was unequivocally established by X-ray diffraction analysis.

Aniline Compounds↗
PubMed2006

Divergent chemical synthesis of prolines bearing fluorinated one-carbon units at the 4-position via nucleophilic 5-endo-trig cyclizations.

N-[3-(Trifluoromethyl)homoallyl]sulfonamides, prepared via ring opening of (S)-glycidyl ethers or 2-aryloxiranes with 1-(trifluoromethyl)vinyllithium, underwent intramolecular addition or S(N)2'-type reaction in the normally disfavored 5-endo-trig fashion, leading to 2-substituted 4-(trifluoromethyl)- or 4-(difluoromethylene)pyrrolidines. Both alpha- and beta-face-selective hydrogenation of the 4-difluoromethylene group afforded syn- and anti-4-(difluoromethyl)pyrrolidines, respectively. These sequences, followed by the oxidation of a 2-hydroxymethyl or 2-aryl group, successfully provided prolines with a trifluoromethyl, difluoromethylene, or difluoromethyl group at the 4-position, including optically active prolines.

Carbon↗
PubMed2006

Ortho effect in the Bergman cyclization: electronic and steric effects in hydrogen abstraction by 1-substituted naphthalene 5,8-diradicals.

We present a detailed theoretical study of geometries, electronic structure, and energies of transition states and intermediates completing the full Bergman cycloaromatization pathway of ortho-substituted enediynes with a focus on polar and steric contributions to the kinetics and thermodynamics of hydrogen abstraction. This study provides a rare unambiguous example of remote substitution that affects reactivity of a neutral reactive intermediate through an sigma framework.

Cyclization↗
PubMed2006

Oxocyclostylidol, an intramolecular cyclized oroidin derivative from the marine sponge Stylissa caribica.

Pyrrole-imidazole alkaloids are widely distributed in marine sponges of the orders Halichondrida and Agelasida. Chemical investigation of the Caribbean sponge Stylissa caribica has led to the isolation of the first brominated pyrrole-imidazole alkaloid containing an oxidized pyrrole moiety. The isolation and structure elucidation of oxocyclostylidol (1) are discussed in detail.

Alkaloids↗