An efficient synthesis and substitution of 3-aroyl-2-bromobenzo[b]furans

A convenient method for the synthesis of 2-bromo-3-aroyl-benzo[b]furans from readily accessible precursors has been developed. The 2-bromo group has been employed as a versatile synthetic handle in both palladium-mediated couplings and direct nucleophilic substitutions to give access to a wide range of 2-substituted-3-aroyl-benzo[b]furans.     

Captodative α,β-unsaturated oxazolines as dienophiles in the asymmetric Diels-Alder reaction

Captodative oxazoline derivative 5a gave rise to four-cycloadducts 6, 7, 8 and 9 in the presence of trifluoroacetic anhydride and cyclopentadiene. Hydrolysis of 6 led to α-phenylthio carboxylic acid 14.     

Oxidation of furans II. Use of furans as masked dienophiles in the intramolecular diels-alder reaction

Chemoselective oxidation of alkadienylfurans using m-chloroperoxy-benzoic acid gave trienes which underwent intramolecular Diels-Alder cycloaddition to hydrindenones.     

Intramolecular Diels-Alder reaction of 1-aminoisobenzofurans: Application to the synthesis of benzo[c]phenanthridines

Intramolecular Diels-Alder reactions of 1-aminoisobenzofurans give benzo[c]phenanthridines. The reactive intermediates are generated from o-(diazomethyl)benzamides.     

Stereoselective synthesis of optically active cyclopenta[c]pyrans and cyclopenta[c]furans by the intramolecular Pauson–Khand reaction

An intramolecular Pauson–Khand reaction of enynes derived from homoallyl, homopropargyl, and allyl alcohols is described. 2-Heteroaryl-substituted homoallyl, homopropargyl, and allyl alcohols are easily and efficiently resolved through enzymatic resolution in high ee (91–99%) and with a known stereochemistry. Each enantiomerically enriched enyne derived from homoallyl and homopropargyl alcohols affords the conformationally most stable diastereomeric cyclopenta[c]pyran ring system as the sole product, whereas enantiomerically enriched enynes derived from allyl alcohols give a diastereomeric cis:trans mixture of the cyclopenta[c]furan ring system.     

A multi-component coupling approach to benzo[b]furans and indoles

A single step access to multiply substituted benzo[b]furans and indoles has been developed.     

Tungsten-catalyzed heterocycloisomerization approach to 4,5-dihydro-benzo[b]furans and -indoles

A W(CO)(5)·THF-catalyzed cycloisomerization of bicyclo[4.1.0] substrates to afford mono C4-substituted 4,5-dihydro-benzo[b]furans and -indoles is reported. The title compounds are versatile intermediates that lead to a range of fused bicycles including the cores of various furan-, benzofuran-, and indole-containing natural products. In many cases, the functionalization of the dihydro-benzo[b]furans and -indoles is orthogonal to that of the corresponding benzofurans and indoles and, thus, offers complementary approaches for synthesis.     

Synthesis of 6H-dibenzo[b,d]pyran-6-ones using the inverse electron demand Diels-Alder reaction

A set of coumarin-fused electron-deficient 1,3-dienes was synthesized, which differ in the nature of the electron-withdrawing group (EWG) at the terminus of the diene unit and (when EWG = CO(2)Me) the nature and position of substituents. These dienes reacted with the enamine derived from cyclopentanone and pyrrolidine to afford the corresponding cyclopenteno-fused 6H-dibenzo[b,d]pyran-6-ones, most likely via a domino inverse electron demand Diels-Alder (IEDDA)/elimination/transfer hydrogenation sequence. The parent diene (EWG = CO(2)Me, no substituents) was reacted with a range of electron-rich dienophiles (mostly enamines) to afford the corresponding 6H-dibenzo[b,d]pyran-6-ones or their nondehydrogenated precursors, which were aromatized upon treatment with a suitable oxidant. The enamines could either be synthesized prior to the reaction or generated in situ. The syntheses of 30 dibenzopyranones are reported.     

An approach to tetrahydrobenz[b,d]pyrans and hexahydrophenanthridines by reaction of dilithiated phenol and aniline with pulegone

7,8,9,10-Tetrahydro-6,6,9-trimethyldibenzo[b,d]pyran and 5,6,7,8,9,10-hexahydro-6,6,9-trimethylphenanthridine have been prepared by reaction of pulegone with lithium ortho-lithium phenolate and lithium ortho-lithium N-trimethylsilylaniline. The cyclization into the annellated systems could be performed under essentially non-acidic conditions.     

The synthesis of benzo[b]phenanthro[d]thiophenes and anthra[b]benzo[d]thiophenes

The synthesis of benzo[b]phenanthro[2, 3-d]thiophene ( 5 ), benzo[b]phenanthro[4, 3-d]thiophene ( 6 ), benzo-[b]phenanthro[2, 1-d]thiophene ( 9 ), benzo[b]phenanthro[3, 2-d]thiophene ( 14a ), anthra[1, 2-b]benzo[d]thiophene ( 24 ), anthra[2, 3-b]benzo[d]thiophene ( 29 ) and anthra[2, 1-b]benzo[d]thiophene ( 30 ) is described as well as the preparation of 13-methylbenzo[b]phenanthro[3, 2-d]thiophene ( 14b ).     

Toluene dioxygenase-catalyzed cis-dihydroxylation of benzo[b]thiophenes and benzo[b]furans: synthesis of benzo[b]thiophene 2,3-oxide

Enzymatic cis-dihydroxylation of benzo[b]thiophene, benzo[b]furan and several methyl substituted derivatives was found to occur in both the carbocyclic and heterocyclic rings. Relative and absolute configurations and enantiopurities of the resulting dihydrodiols were determined. Hydrogenation of the alkene bond in carbocyclic cis-dihydrodiols and ring-opening epimerization/reduction reactions of heterocyclic cis/trans-dihydrodiols were also studied. The relatively stable heterocyclic dihydrodiols of benzo[b]thiophene and benzo[b]furan showed a strong preference for the trans configuration in aqueous solutions. The 2,3-dihydrodiol metabolite of benzo[b]thiophene was utilized as a precursor in the chemoenzymatic synthesis of the unstable arene oxide, benzo[b]thiophene 2,3-oxide.     

Iodocyclization of ethoxyethyl ethers to alkynes: a broadly applicable synthesis of 3-iodobenzo[b]furans

A wide variety of 3-iodobenzo[b]furans were readily prepared by iodocyclization of 2-alkynyl-1-(1-ethoxyethoxy)benzenes with the I(coll)2PF6-BF3 x OEt2 combination. Aryl-, vinylic-, and alkyl-substituted alkynes undergo iodocyclization in good to excellent yields. The mechanism of the reaction is also discussed.     

Diastereoselective synthesis of 2-aryl-3-vinyl-2,3-dihydrobenzo[b]furans through a Sakurai reaction: a mechanistic proposal

The condensation of 2,3-dihydrobenzoxasilepins with aromatic aldehydes in the presence of boron trifluoride to form 2,3-dihydrobenzofurans shows a level of diastereoselection which is a function of the electronic nature of the aldehyde and the polarity of the solvent. The study of the mechanism of the reaction demonstrated that it proceeds through a ring-opened allylfluorosilane, which is stable enough to be isolated and characterized.     

Alpha,beta-unsaturated 2-acyl imidazoles as a practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in water

Alpha,beta-unsaturated 2-acyl imidazoles are a novel and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in water. The Diels-Alder products are obtained with very high diastereoselectivities and enantioselectivities in the range of 83-98%. The catalytic reaction was performed on a 1.0 mmol scale, and the imidazole auxiliary was removed readily.     

Studies on the Diels-Alder reaction of annulated furans: application to the synthesis of substituted phenanthrenes

In an attempt to utilize the synthetic power of the furan nucleus in the synthesis of complex natural products, we have developed an entry into annulated furan intermediates that involves an electrochemical annulation protocol. An approach to the eunicellin diterpenes based on this methodology required the use of an intermolecular Diels-Alder reaction on an annulated furan. Although well known for simple furans, there is a paucity of related examples on annulated furans. To examine the feasibility of such an approach to these diterpenes, we have studied this key cycloaddition reaction. Our studies indicate that the process can be hampered by a facile retro-Diels-Alder that is highly dependent on the dienophile employed.     

Diels-Alder reactions of 3-substituted coumarins in water and under high-pressure condition. An uncatalyzed route to tetrahydro-6H-benzo[c]chromen-6-ones

[reaction: see text] Diels-Alder reactions of 3-substituted coumarins 1a-g with methyl-1,3-butadienes 2a-c carried out in water alone and in CH2Cl2 under 9 kbar pressure are reported. In aqueous medium satisfactory results were obtained by operating at 150 degrees C, whereas under high pressure the cycloadditions were complete at 60-70 degrees C with excellent yields (85-95%). The reactions with isoprene (2b) always resulted in the exclusive formation of para cycloadducts, whereas with (E)-piperylene (2c) only ortho products were detected. The cycloaddition of 3-phenylsulfonylcoumarin (1a) with (E)-piperylene (2c) allowed the endo adduct to be obtained exclusively, whereas 3-carboxycoumarin (1b) reacted with 2c to give a mixture of the corresponding endo/exo adducts in a 58:42 ratio in water and in a 45:55 ratio under high-pressure condition.     

One‐pot Synthesis of Benzamidonaphtho[2,1‐b]furans and Benzamidobenzo[b]furans as Novel Polycyclic Heteroaromatic Compounds

An operationally simple, green and efficient procedure for one‐pot synthesis of novel polycyclic heteroaromatic compounds such as benzamidonaphtho[2,1‐b]furans and benzamidobenzo[b]furans has been developed from the reaction of arylglyoxals, benzamide, and phenols. The reactions were mediated with low amounts of yttrium nitrate hexahydrate as a suitable Lewis acid catalyst without using solvent.     

Acetylation of indolobenzo[b]furans,pyrrolocarbazole, pyrrolophenothiazine dioxide and pyrroloacridine under Vilsmeier reaction conditions

The course of the Vilsmeier acetylation of new heterocyclic systems, namely, indolo[4,5-d]-, indolo[6,5-d]-, indolo[5,6-d]-, indolo[5,4-d]benzo[b]furans, 3H-pyrrolo[2,3-c]carbazole, 3H-pyrrolo[2,3-c]pheno-thiazine 11,11-dioxide, and 3H-pyrrolo[2,3-c]acridine depends on the type of fusion of the pyrrole ring. Angular heterocycles are acetylated at the -position of the pyrrole ring, while linear heterocycles under analogues conditions give dimerization products with a substituent at the nitrogen atom of the hydrogenated part of the dimer molecule. 3H-Pyrrolo[2,3-c]phenothiazine 11,11-dioxide and 3H-pyrrolo[2,3-c]acridine are not acetylated under Vilsmeier reaction conditions.Georgian Technical University, 380075 Tbilisi, Georgia. D. E. Mendeleev Russian Chemical Engineering University, 125047 Moscow. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1331–1336, October, 1996. Original article submitted August 19, 1996.