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PubMed2001

Troublesome alkoxythiophenes. A highly efficient synthesis via cyclization of gamma-keto esters.

A variety of alkoxythiophenes have been synthesized that represent ideal subunits for the synthesis of a new class of thermotropic liquid crystals via palladium-catalyzed cross-coupling reactions and organometallic derivatization. The methodology used represents the first highly efficient synthesis of alkoxythiophenes unlike previous pathways that have presented serious synthetic difficulties when the alkoxy chain consisted of more than four carbon atoms. The scope of the new procedure (relative to liquid crystalline intermediates) is presented and is compared and contrasted with the current literature.

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PubMed2002

Domino carbocationic cyclization of functionalized cyclopropyl ketones: facile one-pot access to peri- and angularly fused polycyclic aromatic and heteroaromatic frameworks.

Conjugate adducts obtained by base-induced 1,4-addition-elimination of various aryl/heteroaryl acetonitriles with 1-(2-arylcyclopropyl)-3,3-(bismethylthio)-2-propen-1-ones have been shown to undergo facile acid-induced domino carbocationic rearrangement yielding a variety of substituted tricyclic aromatic and heteroaromatic frameworks in high yields in a one-pot operation. The methodology provides efficient, high-yield routes for synthesis of novel substituted dihydrophenalenes, dihydrobenzo[d,e]anthracene, cyclopenta[a]naphthalene, and fused heteroaromatics such as substituted 4,5-dihydrobenzo[c,d]indole, dihydronaphtho[1,8-b,c]thiophene, dihydroindeno[5,4-b]- and -[4,5-b]-thiophenes, cyclopenta[a]carbazole, and dihydrocyclopenta[e]indazol-3-one derivatives. The probable mechanism of this interesting domino process appears to involve stepwise or concomitant acid-induced ring opening and intramolecular cyclocondensation of cyclopropyl ketones to give benzo-fused arene (or heteroarene) intermediates bearing a reactive benzylic carbocation that is captured intramolecularly either by a preexisting aromatic (or heteroaromatic) ring or by a newly formed benzene ring to give either peri-fused or angularly fused products, respectively. Thus, the overall domino process entails formation of two C-C bonds, a substituted benzene ring along with a peri-fused cyclohexane or angularly fused cyclopentane ring in a single operation.

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PubMed2003

Synthesis of seven-membered ring diazepin-2-ones via palladium-catalyzed highly regioselective cyclization of 2-vinylpyrrolidines with aryl isocyanates.

The first palladium-catalyzed ring-expansion reaction of 2-vinylpyrrolidines with aryl isocyanates to form seven-membered ring heterocycles is described. This regioselective reaction requires 5 mol % of Pd(2)(dba)(3).CHCl(3) and 10 mol % of dppp at 40-60 degrees C in THF and results in the formation of 1,3-diazepin-2-ones in good isolated yields. When Pd(OAc)(2) and PPh(3) were utilized in the reaction, an intramolecular hydrogen migration occurs resulting in the formation of conjugated diene derivatives of urea.

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PubMed2003

A novel synthesis of functionalized tetrahydrofurans by an Oxa-Michael/Michael cyclization of gamma-hydroxyenones.

An approach to highly functionalized tetrahydrofuran derivatives based upon a novel Oxa-Michael/Michael dimerization of cis-gamma-hydroxyenones is presented. The reaction begins with either 1,2-dioxines or trans-gamma-hydroxyenones and proceeds by addition of one molecule of trans-gamma-hydroxyenone to another molecule of cis- or trans-gamma-hydroxyenone catalyzed by an alkoxide or hydroxide base. Subsequent intramolecular Michael addition of the keto-enolate gives the observed tetrahydrofurans. Substitution at both the 2- and 4-positions of the gamma-hydroxyenone is tolerated; however, for 4-substituted gamma-hydroxyenones, selectivity issues arise due to the possibility of heterochiral or homochiral dimerizations. The major products were those with all contiguous groups trans.

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PubMed2003

Cascade cyclizations via N,4-didehydro-2-(phenylamino)pyridine biradicals/zwitterions generated from enyne-carbodiimides.

Thermolysis of the enyne-carbodiimides 7 having the central carbon-carbon double bond incorporated as part of the cyclopentene ring favors the formation of the corresponding N,4-didehydro-2-(phenylamino)pyridine intermediates, either as the sigma,pi-biradicals 8 or as the zwitterions 8', for subsequent synthetic elaborations. By placing appropriate substituents at the acetylenic terminus, a variety of the intramolecular decay routes are available for the initially formed sigma,pi-biradicals/zwitterions, leading to the 5,6-dihydrobenzo[c][1,8]naphthyridine 21, the 1,2,3,4-tetrahydro[1,8]naphthyridine 24 and related compounds 25 and 26, the 5,6-dihydrobenz[f]isoquinoline 28, and the benzofuro[3,2-c]pyridine 30. Surprisingly, the use of the dimethylamino group of the 2-(dimethylamino)phenyl substituent to capture the carbocationic center in the zwitterion 8e' furnished the 5H-pyrido[4,3-b]indole 32 in only 14% yield. The majority of the products were the 1H-pyrrolo[2,3-b]quinolines 34 and 35, isolated in 48 and 7% yields, respectively. However, it was possible to redirect the reaction toward 32 by conducting thermolysis of the enyne-carbodiimide 7e in the presence of 5 equiv of dimethylphenylsilyl chloride. Under this reaction condition, the 2-pyridone imine 37 was isolated in 86% yield, which on exposure to silica gel was converted to 32 in essentially quantitative yield. Thermolysis of the enyne-carbodiimide 42 having a methoxymethyl substituent at the acetylenic terminus led to the formation of 46' as a pyridine analogue of ortho-quinone methide imine. An intramolecular hetero-Diels-Alder reaction of 46' then furnished the tetrahydro[1,8]naphthyridino[2,1-c][1,4]benzoxazine 47.

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PubMed2002

Steric hindrance-controlled Pd(0)-catalyzed coupling-cyclization of 2,3-allenamides and organic iodides. An efficient synthesis of iminolactones and gamma-hydroxy-gamma-lactams.

Under the catalysis of 1 mol % Pd(PPh3)4, the reaction of 4,4-disubstituted 2,3-allenamides and organic iodides in toluene afforded iminolactones stereospecifically in >90% yields using K2CO3 (2 equiv)-5 mol % TBAB as the base. A similar reaction with 4-monosubstituted 2,3-allenamides afforded gamma-hydroxy-gamma-lactams in relatively lower yields. The N/O-attack selectivity may be determined by the steric effect at the 4-position of 2,3-allenamides.

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PubMed2003

Temperature- and time-dependent stereochemical control in thermally induced keto-ene cyclizations.

Reaction conditions determine the stereoselection in the intramolecular keto-ene reaction. The thermolysis of chiral 2-acyl-3-allyl-substituted 1,3-perhydrobenzoxazines derived from (-)-8-aminomenthol gives a mixture of only two cis-3-hydroxy-3,4-disubstituted pyrrolidine nuclei. The stereochemistry of the major diastereoisomer depends on both the temperature and the reaction time.

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PubMed2003

Cyclization of alpha-Oxo-oximes to 2-substituted benzoxazoles.

Reactions of oximes 9, 17, and 19 with electrophiles 15a-f and 24 in the presence of anhydrous potassium carbonate or triethylamine give 2-substituted condensed ring oxazoles 10, 16a-c, 18a-d, 20a-c, and 25 in a new general route to these compounds.

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