Fischer indole like synthesis an approach to the preparation of benzofurans and benzothiopenes

Kaminsky, Daniel; Shavel, John; Meltzer, Robert I.

doi:10.1016/s0040-4039(00)90593-8

Cited by 50 publications

(15 citation statements)

References 6 publications

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“…cm"1 (N02); nmr (CDCh) S 1.58 (d, 3, / = 7, CHCHs), 4.85 (q, 1, J = 7, CHCHs), 7.3 (m, 1, H-7), 8.5 (m, 2, H-4 and 6).…”

Section: Experimental Section18mentioning

confidence: 99%

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Antibacterial activity of nitrobenzofurans

Powers¹,

Mertes²

1970

J. Med. Chem.

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“…cm"1 (N02); nmr (CDCh) S 1.58 (d, 3, / = 7, CHCHs), 4.85 (q, 1, J = 7, CHCHs), 7.3 (m, 1, H-7), 8.5 (m, 2, H-4 and 6).…”

Section: Experimental Section18mentioning

confidence: 99%

“…Fractions 30-50 contained 1.7 g (5%) of the 7-N02 isomer 8b: mp 117.5-119°( CCh); ir (KBr) 1725 (C=0), 1520 and 1325 cm*1 (NOs); nmr (CDCh) 5 1.66 (d, 3, J = 7 Hz, CHCHs), 4.92 (q, 1, / = 7 Hz, CHCHs), 7.28 (t, 1, J = 7 Hz, aromatic H-5), 8.08 (q, 1, J = 2 and 7 Hz, aromatic H-4), 8.50 (q, J = 2 and 7…”

Section: Experimental Section18mentioning

confidence: 99%

Antibacterial activity of nitrobenzofurans

Powers¹,

Mertes²

1970

J. Med. Chem.

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“…Although the oxime ethers prepared from a,a¢-disubstituted cyclopentanone gave dihydrobenzofuran bearing a substituent in the ring juncture under acidic conditions, 4 unsymmetrically amonosubstituted oxime ethers are known to give the benzofurans as a result of rearrangement at the unsubstituted position. 5 We have now developed a new methodology for the construction of benzofurans carrying a sterically congested quaternary carbon via three sequential reactions involving acylation, regioselective [3,3]-sigmatropic rearrangement, and cyclization of oxime ethers 1 (Scheme 1). The treatment of oxime ethers 1 carrying an electron-withdrawing group (EWG) with trifluoroacetic anhydride (TFAA) gave dihydrobenzofurans 2 having an EWG in the ring juncture while the reaction of 1 with trifluoro-acetyl triflate (TFAT)-DMAP afforded dihydrobenzofurans 3 and benzofurans 4 which are substituted with EWG at the 4-position.…”

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confidence: 99%

Reagent-Controlled Regioselective [3,3]-Sigmatropic Rearrangement of N-Trifluoroacetyl Enehydroxylamine and Its Synthetic Application

Takeda¹,

Miyata²,

Kitamura³

et al. 2006

Synlett

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Regioselective [3,3]-sigmatropic rearrangement provides a novel method for the construction of hexahydrodibenzofuran with a quaternary carbon in the ring juncture. Upon treatment with TFAA, oxime ethers carrying an electron-withdrawing group smoothly underwent regioselective rearrangement to give the hexahydrodibenzofuran with a quaternary carbon. On the other hand, the reaction of oxime ethers with TFAT-DMAP gave tetrahydrodibenzofurans carrying a substituent at the 4-position. The hexahydrodibenzofuran with a quaternary carbon was effectively converted into a key intermediate for lunarine synthesis.The tricyclic benzofuran core with a sterically congested quaternary carbon is an important element in galanthamine-type, 1 morphine-type, 2 and lunarine-type 3 of alkaloids which exhibit remarkable and inherent biological activities. Therefore, synthetic access to such benzofurans has long been a challenge for synthetic chemists. In constructing the universal skeleton, many synthetic methods such as biomimetic phenolic oxidative coupling, photochemical reaction, radical cyclization, intramolecular Heck reaction, semipinacol rearrangement, intermolecular alkylation, and arylation had been reported. 1-3 However, to the best of our knowledge, there have been few papers pertaining to the construction of hexahydrodibenzofurans with a quaternary carbon in the ring juncture via [3,3]-sigmatropic rearrangement. Although the oxime ethers prepared from a,a¢-disubstituted cyclopentanone gave dihydrobenzofuran bearing a substituent in the ring juncture under acidic conditions, 4 unsymmetrically amonosubstituted oxime ethers are known to give the benzofurans as a result of rearrangement at the unsubstituted position. 5 We have now developed a new methodology for the construction of benzofurans carrying a sterically congested quaternary carbon via three sequential reactions involving acylation, regioselective [3,3]-sigmatropic rearrangement, and cyclization of oxime ethers 1 (Scheme 1). The treatment of oxime ethers 1 carrying an electron-withdrawing group (EWG) with trifluoroacetic anhydride (TFAA) gave dihydrobenzofurans 2 having an EWG in the ring juncture while the reaction of 1 with trifluoroacetyl triflate (TFAT)-DMAP afforded dihydrobenzofurans 3 and benzofurans 4 which are substituted with EWG at the 4-position. Furthermore, regioselective reaction under TFAA conditions was successively applied to the preparation of a key intermediate for (±)-lunarine synthesis. Scheme 1Recently, we have developed an efficient synthetic method for benzofurans only by changing the reagent (TFAA or TFAT-DMAP) 6 from a common starting compound. Therefore, we employed both TFAA and TFAT-DMAP conditions for the acylation of oxime ethers 5 (Scheme 2, Table 1). When the oxime ether 5a was treated with 1 equivalent of TFAA, only the substrate was recovered (entry 1). The reaction of 5a with 15 equivalents of TFAA gave a mixture of 6a, 7 7a, 8 and 9a (entry 2). The hexahydrodibenzofuran 6a was formed as a result of rearrangement of an intermed...

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“…]-sigmatropic rearrangements of oxime ethers were studied by many groups, too numerous to list completely, but among them are those of Mooradian,[122][123][124] Sheradsky,125 and Kaminsky 126. No similarly anomalous isohypsic rearrangements were reported.House studied the related rearrangement of N-vinyl hydroxylamine esters to transfer oxygenation on nitrogen to a b-carbon 127.…”

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confidence: 99%

β-Oxidation of α,β-Unsaturated Carbonyl Compounds

Sommer¹

2004

Synthesis

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The replacement of a b-hydrogen on an a,b-unsaturated carbonyl system with a non-hydrogen, non-carbon atom amounts to either a net two electron oxidation or an isohypsic substitution at the b-carbon. The resulting compounds are the functional equivalents of b-dicarbonyl derivatives. Although many methods have been developed to effect this transformation, they lack generality and the process remains an underutilized one in organic synthesis.

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Fischer indole like synthesis an approach to the preparation of benzofurans and benzothiopenes

Cited by 50 publications

References 6 publications

Antibacterial activity of nitrobenzofurans

Antibacterial activity of nitrobenzofurans

Reagent-Controlled Regioselective [3,3]-Sigmatropic Rearrangement of N-Trifluoroacetyl Enehydroxylamine and Its Synthetic Application

β-Oxidation of α,β-Unsaturated Carbonyl Compounds

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