Furan, dihydro cycloaddition reactions

Benzo[c]furan, 1-cyan 0-3-phenyl-synthesis, 4, 683 Benzo[c]furan, 1,3-diatyl-cycloaddition reactions, 4, 635 reduction, 4, 614 synthesis, 4, 701 Benzo[c]furan, 1,3-di-t-butyl-synthesis, 4, 703 Benzo[c]furan, 1,3-dihydro-NMR, 4, 572-574, structure, 4, 549 synthesis, 4, 683 Benzo[c]furan, 4,7-dihydro-synthesis, 4, 682... 

Benzo[c]furan, 1,3-dihydro-1,3-diphenyl-mass spectrometry, 4, 585 Benzo[c]furan, 1,3-dfmethyl-synthesis, 4, 699, 701 Benzo[c]furan, 1,3-diphenyl-cycloaddition reactions, 4, 67 electrophilic substitution, 4, 604 history, 4, 533... 

Furan, 2,3-dihydro-5-methyl-polymers, 1, 276 Furan, 2,3-dihydro-3-methylene- H NMR, 4, 577 Furan, 2,5-dihydro-2-methylene- H NMR, 4, 577 tautomerism aromaticity and, 4, 595 Furan, 2,5-dihydro-2-nitro-structure, 4, 550 Furan, 2,3-dihydroxy-tautomerism, 4, 37 Furan, 2,4-dihydroxy-tautomerism, 4, 37 Furan, 3,4-dihydroxy-tautomerism, 4, 37 Furan, 2,5-diiodo-nitration, 4, 602 synthesis, 4, 712 Furan, 3,4-diiodo-reactions, 4, 650 Furan, 2,3-dimethoxy-synthesis, 4, 625, 648 Furan, 2,5-dimethoxy-synthesis, 4, 648 Furan, 3,4-dimethoxy-cycloaddition reactions, 4, 64, 625 lithiation, 4, 651 reactions... 

A Furan enters into a cycloaddition reaction with the dienophile diethyl acetylcnedicarboxylate to form an adduct. Catalytic reduction occurs preferentially at the more electron-rich double bond of the adduct, and the dihydro product, upon heating, undergoes a reverse Diels-Alder reaction to release ethene and the diesier (Scheme 6.29),... 

Two approaches, based on furan, have found wide application in carbohydrate synthesis. Cycloaddition reactions of furan with 2-substituted acrylonitrile or acrolein lead to oxabicycloheptanes which, in tnm, can be transformed to monosaccharides. On the other hand, furfuryl alcohols can be converted—either by the Clauson-Kaas reaction or by mild oxidation—into 5,6-dihydro-4-pyrones, suitable for easy functionalization to sugars. 

Furan, 2-(p-hydroxyphenyI)-2,5-dihydro-synthesis, 4, 677 Furan, 3-hydroxytriphenyl-tautomerism, aromaticity and, 4, 595 Furan, iodo-synthesis, 4, 712 Furan, 2-iodo-Grignard reagents, 4, 79 Furan, 5-iodo-2-nitro-synthesis, 4, 711 Furan, 2-isopropyl-cycloaddition reactions with ff, ,a, ff -tetrabromoacetone, 1, 418 Furan, 2-lithio-5-methyl-synthesis, 1, 419 Furan, 2-mercapto-tautomerism, 4, 38 Furan, 3-mercapto-tautomerism, 4, 38 Furan, methoxy-planarity, 4, 544 synthesis, 4, 664 Furan, 2-methoxy-synthesis, 4, 648 Furan, 3-methoxy-Diels-Alder reaction, 1, 416 synthesis, 4, 649... 

Heterocycles Both non-aromatic unsaturated heterocycles and heteroaromatic compounds are able to play the role of ethene dipolarophiles in reactions with nitrile oxides. 1,3-Dipolar cycloadditions of various unsaturated oxygen heterocycles are well documented. Thus, 2-furonitrile oxide and its 5-substituted derivatives give isoxazoline adducts, for example, 90, with 2,3- and 2,5-dihydro-furan, 2,3-dihydropyran, l,3-dioxep-5-ene, its 2-methyl- and 2-phenyl-substituted derivatives, 5,6-bis(methoxycarbonyl)-7-oxabicyclo[2.2.1]hept-2-ene, and 1,4-epoxy-l,4-dihydronaphthalene. Regio- and endo-exo stereoselectivities have also been determined (259). 

That deprotonation of enol radical cations is a plausible reaction can be deduced from Yoshida s work [107,113] on the electrooxidative addition of 1,3-dicarbonyl compounds 14 in acetonitrile to substituted olefins yielding dihydro-furans 18 as [3 + 2] cycloaddition products in good to excellent yields. Al-... 

Methoxy- and 2-acetoxy-furans are available from 2,5-dimethoxy- and 2,5-diacetoxy-2,5-dihydro-furans (18.1.1.4) via acid-catalysed elimination. They undergo Diels-Alder cycloadditions the adducts can be further transformed into benzenoid compounds by acid-catalysed opening. 3,4-Dihydroxyfuran is undetectable in tautomeric equilibria between mono-enol and dicarbonyl forms the dimethyl ether behaves as a normal furan, undergoing easy a-electrophilic substitution, mono- or dilithiation at the a-position(s), and Diels-Alder cycloadditions. 2,5-Bis(trimethylsilyloxy)furan is synthesised from succinic anhydride it too undergoes Diels-Alder additions readily. Both furan-2- and -3-thiols can be obtained by reaction of lithiated furans with sulfur in each case the predominant tautomer is the thiol form. ... 

Reasonable yields of the adducts (103) are obtained when acetone is irradiated in the presence of 2,3-dihydro- and 2-methyl-2,3-dihydro-furan. Compounds of this structure can also be obtained by the photocyclization of the vinyloxy ketones (104a) and (104b),. which yields the adducts (105a) and (105b), respectively." Intramolecular cycloaddition is also encountered in the photoreactions of the cyclic alkanones (106)." However the reaction favours the formation of the crossed adduct (107) in all the cases studied. The accompanying products are the alternative oxetan (108) and the products from Norrish Type I fission. 

Alonso et al. (1983) also studied the regio- and stereoselectivity of these reactions. Interesting reagents are 1- and 2-methoxybuta-l,3-diene. As shown in (8-50), l-methoxybuta-l,3-diene (8.113) reacts with high selectivity at the unsubstituted double bond, whereas, with the 2-methoxy isomer (8.114), cycloaddition at the more electron-rich double bond is the main reaction (8-51). There are also cases known from the series of reactions with substituted alkoxyethenes (8.109), which show that these cycloadditions proceed stereospecifically the configuration of the dihydro-furan corresponds to that of the alkoxyethene. 

The presence of five-membered rings such as cyclopentanes, cyclopentenes, and dihydro-furans in a wide range of target molecules has led to a variety of methods for their preparation. One of the most successful of these is the use of trimethylenemethane [3 -1- 2] cycloaddition, catalysed by palladiumfO) complexes. The trimethylenemethane unit in these reactions is derived from 2-[(trimethylsilyl)methyl]-2-propen-l-yl acetate, which is at the same time an allyl silane and an allylic acetate. This makes it both a weak nucleophile and an electrophile in the presence of palladiumfO). Formation of the palladium c-allyl complex is followed by removal of the trimethylsilyl group by nucleophilic attack of the... 

While the cycloaddition of oxazole with acetylene is a well-established method for furan synthesis [36], the corresponding reaction of 5-amino oxazole was unknown at the outset of this work. As shown in Eq. (3) of Scheme 15.19, the reaction of 24b with 3-phenyl-2-propynoyl chloridephenylpropioloyl chloride (55a) proceeded smoothly to provide the 5,6-dihydro-furo[2,3-c]pyrrol-4-one (56) in greater than 95% yield. A triple domino sequence involving acylation/intramolecular DA cycloaddition/retro DA could explain the reaction outcome. That the domino process was initiated by acylation was evidenced by the fact that 24b failed to react with dimethyl acetylenedicarboxylate (DMAD) under the identical conditions. 

---