Showing papers on "Intramolecular reaction published in 1991"

Diels-Alder reactions in aqueous solutions. Enforced hydrophobic interactions between diene and dienophile

Abstract: Second-order rate constants and isobaric activation parameters for the intermolecular Diels-Alder reactions of cyclopentadiene with alkyl vinyl ketones (2a,b) and 5-substituted-1,4-naphthoquinones (3a-c) as well as for the intramolecular Diels-Alder reaction of N-furfuryl-N-methylmaleamic acid (4) were determined in water, monohydric alcohols, and highly aqueous solutions containing monohydric alcohols and other organic cosolvents. The intermolecular as well as the intramolecular Diels-Alder reaction are characterized by large rate accelerations upon going from an organic solvent to water as the reaction medium. A model for solvation effects on rate constants for Diels-Alder reactions in aqueous solutions is developed. The typical, huge rate enhancements for Diels-Alder reactions in water and in highly aqueous binary mixtures are significantly sensitive to substituent effects and are discussed in terms of enforced pairwise hydrophobic interactions between diene and dienophile.

A facile synthesis of bicyclo[m.n.1]alkan-1-ols. Evidence for organosamarium intermediates in the samarium(II) iodide promoted intramolecular Barbier-type reaction

Abstract: Samarium(II) iodide (SmI 2 ) has been successfully employed as a reductive coupling agent for the intramolecular Barbier-type synthesis of bicyclo[m.n.1]alkan-1-ols. Thus, a variety of 3-(ω-iodoalkyl)cycloalkanones, upon treatment with SmI 2 and a catalytic quantity of iron complex in tetrahydrofuran (THF), provide the title compounds in excellent yields. The reaction is quite general for the construction of diverse bicyclic ring systems, including the highly strained bicyclo[2.1.1]hexan-1-ol

Control of intramolecular electron transfer by a chemical reaction. The 4,4'-azopyridine/1,2-bis(4-pyridyl)hydrazine system

Abstract: The complexation of 4,4'-azopyridine by the pentaammineruthenium(II) group is described. The metal sites are coordinated to the pyridyl nitrogen atoms and interact through the conjugated azo linkage. Upon oxidation, an intervalence absorption is detected, but it is unresolved with respect to the nearby metal-to-ligand charge-transfer band. The acidification of the binuclear pentaammineruthenium(II) complex gives rise to a pH induced intramolecular redox reaction yielding a ruthenium(III) complex of the reduced form of 4,4'-azopyridine, i.e. 1,2-bis(4-pyridyl) hydrazine

Intramolecular Diels–Alder Reactions

Abstract: The Diels–Alder reaction, first described in 1928,1 has enjoyed widespread use in organic synthesis.2 This [4 + 2] cycloaddition is particularly valuable in that it forms two bonds in a cyclohexenyl system with simultaneous creation of up to four new stereocenters. Curiously, the intramolecular version in which the diene and dienophile are tethered by a connecting chain was not investigated for many years. Alder and Schumaker reported the first example of the intramolecular reaction in 1953,3 but it was not until the early 1960s that additional examples began to appear in the literature.4 Since the mid-1970s, however, a virtual explosion of interest in this reaction has occurred.5 The increased reactivity (due to favorable entropy considerations) and heightened regioselectivity (due to constraints posed by the connecting chain), as well as the potential for the synthesis of stereochemically complex polycyclic ring systems, account for the growth of applications of this reaction in organic and particularly natural products synthesis.

A novel route to biaryls via intramolecular free radical ipso substitution reactions

Abstract: A variety of usefully functionalised biaryls may be prepared under neutral conditions by an intramolecular free radical ipso substitution reaction at an sp2 sulphonyl substituted carbon atom; the overall efficiency of the process being determined both by the nature and number of atoms linking the two aromatic rings and by appropriately positioned substituents on the aromatic aceeptor ring.

Asymmetric intramolecular amidation of N-(tert-butoxycarbonyl)-3-hydroxy-4-pentenylamine. A new entry to chiral building blocks for the synthesis of biologically active nitrogen-containing compounds

Abstract: Sharpless reaction of racemic N-(tert-butoxycarbonyl)-3-hydroxy-4-pentenylamine (1) leads to both an asymmetric kinetic resolution to provide optically active 1, which was subsequently used for intramolecular amidomercuration, and asymmetric epoxidation followed by concomitant cyclization into optically active cis-3-hydroxy-2-(hydroxymethyl) pyrrolidine (3). Optically active 1 and 3 have been expediently used as chiral building blocks in the asymmetric synthesis of several biologically active natural products

Stereospecific preparation of polyfunctional olefins by the carbometalation of alkynes with polyfunctional zinc-copper organometallics. Stereospecific preparation of five-membered carbocycles by intramolecular carbocupration

Abstract: The addition of copper reagents prepared by transmetalation from polyfunctional organozinc iodides to reactive alkynes such as 1-(methylthio)-1-hexyne, phenylacetylene and acetylene, leads to stereochemically well-defined (syn addition), highly functionalized alkenylcopper reagents that after protonation, allylation, iodolysis, or stannylation afford stereochemically pure di-, tri-, and tetrasubstituted olefins. A similar intramolecular carbocupration has allowed the preparation of highly substituted five-membered carbocycles. Compared to lithium-or magnesium-copper reagents, the highly functionalized copper reagents can contain an ester, nitrile, or chloride function. However, they display a lower reactivity than the lithium- or magnesium-copper reagents and do not react with unactivated terminal alkynes

Synthesis of Some Carbazolequinone Alkaloids and Their Analogues. Facile Palladium-Assisted Intramolecular Ring Closure of Arylamino-1, 4-benzoquinones to Carbazole-1, 4-quinones

Abstract: Carbazolequinone alkaloids, murrayaquinone-A (1a), and pyrayaquinone-A (2) and -B (3), as well as pyrano[3, 2-b]carbazolequinone 5, an isomer of 2 and 3, were prepared conveniently by the palladium-assisted intramolecular ring closure of the corresponding 2-arylamino-methyl-1, 4-benzoquinones 9a and 9i-k. A series of their analogues, 6-, 7-, and 8-substituted 3-methylcarbazole-1, 4-quinone derivatives 1b-g and 2-methylcarbazolequinones 6a-g corresponding to 1a-g, was also prepared in the same manner. Comparative analysis of the carbon-13 nuclear magnetic resonance spectra was found to provide useful information for the structural assignment of either 2- or 3-methylcarbazolequinones.

Syntheses of (±)-α- and (±)- γ- Lycorane via a Stereocontrolled Organopalladium Route

Abstract: Total syntheses of (+/-)-alpha-and (+/-)-gamma-lycorane are described. The key steps in the syntheses are the stereocontrolled palladium-catalyzed intramolecular 1,4-chloroamidation of 12 to 13 and the subsequent anti-stereoselective copper-catalyzed S(N)2' reaction of allylic chloride 13 with [3,4-(methylenedioxy)phenyl]magnesium bromide to give 14. Hexahydroindole 14 has the required relative stereochemistry between carbons 3a, 7, and 7a for alpha-lycorane (1a) and was transformed to the latter via 15 and 16. The epimeric gamma-lycorane (2) was obtained by performing the Bischler-Napieralski cyclization on 14, which led to a highly stereoselective isomerization to give exclusively 17. Compound 17 was subsequently transformed to 2. The overall yield from ester 8 to (+/-)-alpha- and (+/-)-gamma-lycorane was 40 and 36%, respectively.

Intramolecular bis-silylation of carbon-carbon double bonds leading to stereoselective synthesis of 1,2,4-triols

Abstract: Intramolecular Bis-Silylation of Carbon -Carbon Double Bonds Leading to Stereoselective Synthesis of 1,2,4-Triols

Enantioselective synthesis of the bottom half of chlorothricolide. 3. Studies of the steric directing group strategy for stereocontrol in intramolecular Diels-Alder reactions

Abstract: The intramolecular Diels-Alder reactions of a series or C(7)-alkoxy-substituted 2(E),8(Z),10(E)-undecatrienoates and trienals containing removable C(9)-Br or C(9)-SiMe 3 substituents were studied as part of a program directed toward the total synthesis of the bottom half of chlorothricolide

Cycloadditions. 46. Thermally induced intramolecular oxime olefin cycloadditions leading to N-bridgehead systems. Stereochemistry and molecular mechanics calculations

Abstract: The intramolecular oxime olefin cycloaddition (IOOC) of proline and pipecolinic acid derivatives proceeds thermally with a high degree of stereoselectivity to provide a new route to functionalized pyrrolizidines, indolizidines, or quinolizidines. The ring closure proceeds with simultaneous stereoselective introduction of three or four stereocenters. Molecular mechanics calculations have been refined to accurately predict not only which stereoisomer is preferred but also the syn and anti coupling constants in these tricyclic molecules

Organic reactions catalyzed by solid superacids. 5. Perfluorinated sulfonic acid resin (Nafion-H) catalyzed intramolecular Friedel-Crafts acylation

Abstract: Nafion-H, a perfluorinated sulfonic acid resin, catalyzed the intramolecular Friedel-Crafts acylation of diarylalkane-2-carboxylic acids and arylalkanoic acids in refluxing p-xylene to provide cyclic ketones. The reactions were clean, and the water that was formed as a byproduct did not deactivate the catalyst. It was also found that the intramolecular acylation of the corresponding acid chlorides occurred under much milder reaction conditions. The mechanism of the reaction is discussed

Total synthesis of deoxybouvardin and RA-VII: macrocyclization via an intramolecular Ullmann reaction

Abstract: We detail successful studies on the implementation of an intramolecular Ullmann condensation reaction for direct closure to the 14-membered diaryl ethers that have proven inaccessible or less accessible by alternative routes and the use of this key macrocyclization reaction in the total synthesis of RA-VII and deoxybouvardin