Furans 3-halo

The halo-furans and -benzo[f>]furans are particularly important as precursors of the lithio derivatives (Section 3.11.3.9). Direct halogenation of furan (Section 3.11.2.2.5) is unsatisfactory, and halofurans are prepared by decarboxylation of halofurancarboxylic acids, from chloromercurio compounds, by decarboxylative halogenation of furancarboxylic acids or by partial dehalogenation of polyhalofurans. 

Halo Furans by a Consecutive SCR of Acid Chlorides, Propargyl Ethers, and Halides... 

Scheme 33 Mechanistic rationalization of the coupling-deprotection-addition-cyclocondensa-tion sequence to 3-halo furans 55...

As a consequence of the mild reaction conditions in the sequence to 3-halo furans 55 the palladium catalyst should be still intact to trigger another Pd-catalyzed coupling in the sense of a sequentially Pd-catalyzed process [31]. As a consequence, a sequential Sonogashira-deprotection-addition-cyclocondensation-Suzuki reaction, where the same catalyst system is applied in two consecutive significantly different cross-coupling reactions in the same reaction vessel should be feasible. Therefore, upon consecutive reactions of (hetero)aroyl chlorides 7 and THP-protected propargyl alcohols 54, Nal and PTSA, and finally, addition of 1.05 equiv of boronic acids 60 and sodium carbonate, the substituted 3-aryl furans 61 can be obtained in decent yields (Scheme 35). 

A carboxyl group is removed from a heterocyclic nucleus in much the same way as from an aromatic nucleus (method 13), i.e., by thermal decomposition. The pyrolysis is catalyzed by copper or copper salts and is frequently carried out in quinoline solution. The reaction is important in the synthesis of various alkyl and halo furans. Furoic acid loses carbon dioxide at its boiling point (205°) to give furan (85%). A series of halo furans have been made in 20-97% yields by pyrolysis of the corresponding halofuroic acids. The 5-iodo acid decarboxylates at a temperature of 140°, whereas the 3- and 5-chloro acids requite copper-bronze catalyst at 250°. ... 

Lithium diisopropylamide can effect C-2-deprotonation of 3-halo-furans. With furoic acid and two equivalents of lithium diisopropylamide, selective formation of the 5-Uthio lithium 2-carboxylate takes place, whereas n-butyllithium, via ort/to-assistance, produces the 3-lithio lithium 2-carboxylate. ... 

Furan also undergoes cycloadditions with allenes," " with benzyne" and with simpler dienophiles, like acrylonitrile and acrylate various Lewis acidic catalysts can assist" in some cases, zinc iodide is one such, hafnium tetrachloride another, and improved endo exo ratios are obtained in an ionic liquid as reaction solvent. Maleate and fumarate esters react if the addition is conducted under high pressure. This device can also be used to increase markedly the reactivity of 2-methoxyfuran and 2-acetoxyfuran towards dienophiles. At higher reaction temperatures alkynes and even electron-rich alkenes will add to furan. 3- or 5-Halo-furans react faster in these cycloadditions. ... 

Benzo[b]furan, 2-ethyl-3-(4-hydroxy-3,5-diiodobenzyl)-applications, 4, 708 Benzo[b]furan, halo-reactions, 4, 650 Benzo[b]furan, 2-hydroxy-tautomerism, 4, 36 aromaticity and, 4, 595 Benzo[6]furan, 3-hydroxy-tautomerism aromaticity and, 4, 595 Benzo[b]furan, 5-hydroxy-synthesis, 4, 127... 

Furan, 3-(4,8-dimethyl-3,7-nonadienyl) — see Dendrolasin Furan, 2,5-dinitro-synthesis, 4, 711 Furan, 2,5-diphenyl-bischloromethylation, 4, 607 Furan, 3-ethoxycarbonyl-2,5-dimethyl-synthesis, 4, 659 Furan, 2-ethyltetrahydro-synthesis, 3, 776, 4, 711 Furan, fluoro- F NMR, 4, 564 H NMR, 4, 564 Furan, halo-... 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

Dimerization of tetraacetylethylene (18) has led to spirofuran (19), whose structure was established by X-ray analysis (80CJC1645). A possible mechanistic pathway for its formation would involve the dimerization of an intermediate (20) which could account for the stereochemistry of the dimer (19) (Scheme 4). The same furan (21) is a possible intermediate in the formation of 3,4-diacetyl-2-halomethyl-5-methylfuran (22) from (18) with concentrated halo acids (70JCS(C)1536). The structures of the furans (22) were established by chemical and spectroscopic methods. 

Ring contraction of coumarins is used for the preparation of benzo[Z>]furans. Alkaline degradation of the 3-halo-, 4-halo- or 3,4-dihalo-coumarins gives the coumarilic acids (Scheme 104). The coumarilic acids are decarboxylated to the corresponding benzo[Z> ]f urans. Basic mercury(II) oxide oxidation of 4-phenylcoumarin (neoflavanoid) yields the 3-phenyl-benzo[/>]furan (71IJC1316). 

Photochemical reaction in acetonitrile of 2 -deoxyuridine 5 -phosphate with the halo-heteroarenes 2-iodothiophene, 2-iodofuran, l-methyl-2-iodopyrrole and 3-iodothiophene affords the C-5 heteroaryl substituted nucleotides518.6-Aryluridines have been prepared by irradiation of 6-iodouridines in benzene, anisole, thiophene, Af-methylpyrrole or 2-methyl-furan in the presence of triethylamine519. 

Photoreaction of 2-, 3- and 4-iodoquinolines with five-membered heterocycles (pyrrole, iV-methylpyrrole, furan and thiophene) affords the corresponding -(2-heteroaryl)quino-lines (n = 2, 3,4) in appreciable yields526. 3-Halo-1-methylquinolin-2-ones can be converted into 3-aryl-l-methylquinolin-2-ones by photochemical coupling with various aromatic or heteroaromatic compounds527,528. 

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