Rearrangements of cyclobutenones. Electrocyclic ring closure and thermal ring expansions of 3-allenyl- and 3-alkynyl-2-dienyl-4,4-dimethoxycyclobutenones.
TL;DR: Thermal rearrangements of 2-allenyl- and 2-alkynyl-3-(2-ethenylphenyl)-4,4-dimethoxycyclobutenones were studied and showed to be viable synthetic precursors to benzo[a]anthracene-7,12-diones, compounds representing the framework of the angucycline group of naturally occurring antibiotics.
Abstract: Thermal rearrangements of 2-allenyl- and 2-alkynyl-3-(2-ethenylphenyl)-4,4-dimethoxycyclobutenones were studied. At ambient temperature, the allenyl compounds undergo an electrocyclic cascade to give bicyclo[4.2.0]octadienyl-fused cyclobutenones. These unusual tetracyclic cyclobutenones were shown to be viable synthetic precursors to benzo[a]anthracene-7,12-diones, compounds representing the framework of the angucycline group of naturally occurring antibiotics. In contrast, the 2-alkynylcyclobutenones are stable at ambient temperature but undergo a facile rearrangement at 110 degrees C (toluene) to give the previously unknown naphthalene derivatives, 1,2-dihydro-2,2-dimethoxy-1-(3-alkenylidene)naphtho[2,1-b]furans.
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TL;DR: Transformation of Cyclobutane Derivatives inNatural Product Syntheses: A Review of the Transformations in Organic Syntheses.
Abstract: I. Introduction 1485II. Scope of This Review 1485III. Transformations of Cyclobutane Derivatives inOrganic Syntheses1486A. Ring-Opening Reactions 1486B. Ring-Contraction Reactions 1493C. Ring-Expansion Reactions 14951. Five-Membered Rings 14952. Six-Membered Rings 15093. Seven-Membered Rings 15174. Eight-Membered Rings 15215. Nine-Membered Rings 1523IV. Transformations of Cyclobutane Derivatives inNatural Product Syntheses1524A. Ring-Opening Reactions 1524B. Ring-Expansion Reactions 15261. Five-Membered Rings 15262. Six-Membered Rings 15293. Seven-Membered Rings 15324. Eight-Membered Rings 1533V. Conclusion 1534VI. Acknowledgments 1534VII. References 1534
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166 citations
TL;DR: In this article, a novel sequential reaction sequence consisting of a series of processes including Sonogashira coupling, propargyl-allenyl isomerization, cycloaddition, denitrogenation, and highly selective biradical coupling reaction, providing an efficient synthesis of fused polycyclic pyrrole derivatives.
Abstract: In this paper, we report a novel sequential reaction sequence consisting of a series of processes including Sonogashira coupling, propargyl-allenyl isomerization, [3 + 2] cycloaddition, denitrogenation, and highly selective biradical coupling reaction, providing an efficient synthesis of fused polycyclic pyrrole derivatives. The reaction features one C-C bond and two C-N bonds formed to construct the pyrrole unit, together with two rings efficiently assembled in one stroke.
40 citations
TL;DR: A stepwise process involving Sonogashira coupling, propargyl allenyl isomerization, and consecutive [4 + 2] cyclization has been realized, leading to an efficient synthesis of polycyclic compounds containing a 2,3-dihydrofuran unit.
Abstract: A stepwise process involving Sonogashira coupling, propargyl allenyl isomerization, and consecutive [4 + 2] cyclization has been realized, leading to an efficient synthesis of polycyclic compounds containing a 2,3-dihydrofuran unit. Most attractive for synthetic interest is the finding that up to four stereogenic centers could be generated in one step with high stereoselectivity.
35 citations
TL;DR: The efficiency of the one-pot thermal ring expansion/4π-electrocyclization process has been demonstrated through its application in the total synthesis of taiwaniaquinone H, which has been accomplished in three steps and 14% overall yield in a protecting-group-free manner starting from commercially available materials.
Abstract: A strategy to the 6-5-6 tricyclic scaffold of taiwaniaquinoids was established on the basis of a one-pot thermal ring expansion/4π-electrocyclization process. The efficiency of this methodology has been demonstrated through its application in the total synthesis of taiwaniaquinone H, which has been accomplished in three steps and 14% overall yield in a protecting-group-free manner starting from commercially available materials.
24 citations
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Abstract: (11) Potassium ferricyanide has previously been used to convert w'c-1,2-dicarboxylate groups to double bonds. See, for example, L. F. Fieser and M. J. Haddadln, J. Am. Chem. Soc., 86, 2392 (1964). The oxidative dldecarboxylation of 1,2-dlcarboxyllc acids is, of course, a well-known process. See Inter alia (a) C. A. Grob, M. Ohta, and A. Weiss, Helv. Chim. Acta, 41, 1911 (1958); and (b) E. N. Cain, R. Vukov, and S. Masamune, J. Chem. Soc. D, 98 (1969).
4,641 citations
TL;DR: Propargyl ethers HCCCH2OR [R = alkyl or-CH(CH8)(OC2H5)] have been isomerized with good yields into the corresponding allenyl ether's CH2CCHOR by warming with potassium tert-butoxide at 70°.
Abstract: Propargyl ethers HCCCH2OR [R = alkyl or-CH(CH8)(OC2H5)] have been isomerized with good yields into the corresponding allenyl ethers CH2CCHOR by warming with potassium tert.-butoxide at 70°.
These allenyl ethers can be metallated with butyllithium in ether or alkali amides in liquid ammonia. In ether, subsequent alkylation with alkyl halides R′Hal affords α-substituted allenyl ethers CH2CC(R′)OR. Alkylation in liquid ammonia produces a mixture of this same compound and the γ-substituted product R′CHCCHOR. In both cases reasonable yields are obtained. Sodamide and potassium amide quickly convert allenyl ethers CH2CCHOR into metallated propargyl ethers MCC-CH2OR (M = Na or K). If alkylation is not performed almost simultaneously with the metallation with sodamide or potassium amide, the only alkylation product obtained is R′CCCH2OR.
943 citations
TL;DR: In this article, a potentially general regiospecific synthesis of benzo- and naphthoquinones is described, which starts with dimethyl squarate (1), which is converted to the cyclobutenone ketal 9 upon sequential treatment with an organolithium reagent and then BF 3 etherate or TFAA in THF/methanol.
Abstract: A potentially general regiospecific synthesis of benzo- and naphthoquinones is described. This method starts with dimethyl squarate (1), which is converted to the cyclobutenone ketal 9 upon sequential treatment with an organolithium reagent and then BF 3 etherate or TFAA in THF/methanol. Treatment of these with a second lithium reagent followed by hydrolysis gives the cyclobutenones 5.Addition of an alkynyl-,alkenyl- or aryllithium agent to 5 followed by hydrolysis of the ketal linkage gives the corresponding 4-alkynyl-4-alkenyl- or 4-aryl-4-hydroxycyclobutenones 7-9 and these readly rearrange to the respective quinones or hydroquinones upon thermolysis in refluxing benzene.In a similar fashion,15 was employed as a reagent to prepare mono- and disubstituted hydroquinones and quinones
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