Furan reaction with

While (Z)-l,2-bis(phenylsulfonyl)ethylene (140) does not add to dienes such as furan, cyclopentadiene, cyclo-octatetraene, indene and /f-naphthol, ( )-l,2-bis(phenylsulfonyl)ethylene (141) is more reactive and the reaction with furan proceeds at room temperature for 2 h to give the adduct in 95% yield. The reactivity of dienophiles having sulfonyl group in the [4 + 2]cycloaddition is shown in equation 10393,101. 

Bromine, reaction with furan in methanol to yield 2,5-dimethoxy-2,5-dihydrofuran, 40, 29... 

A review on fliran and its derivatives in the synthesis of other heterocycles was published <95CHE1034>. Furan decomposes on Pd(lll) at 300 K to form H, CO and CjH, which can dimerize to benzene at 350 K <96JA907>. Again, a considerable number of Diels-Alder reactions with furan and fiiran derivatives was reported. The synthesis of 2-pyridinyl-7-oxabicyclo[2.2.1]heptanes (e.g., 3, 4) was accomphshed via zinc chloride-mediated Diels-Alder reaction of furan with 2-vinylpyridines <96SL703>. 

Radialenes 86 were obtained when bis(l-diazo-2-oxoalkyl)silanes 84 were decomposed with copper or palladium catalysts60,61. The assumption, that the heterocyclic [3]cumulene 85 is the immediate precursor of 86, is corroborated by its trapping in a Diels-Alder reaction with furan. 

Scheme 13. Possible pathways for the reaction with furan.

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

On the other hand, we also planned alternative completely new approach (Route B) to Nakadomarin A, which involves the spirolactam followed by coupling reaction with furan derivative and subsequent intramolecular electrophilic substitution reaction of an iminium cation generated from an aminal to give highly functionalized tetracyclic core system (Scheme 10.3). 

Bicyclopropylidene (1) does not undergo an intermolecular Diels-Alder reaction with furan and 2-methoxyfuran even under high pressure. Intramolecular cycloadditions of compounds 160 with a furan tethered to bicyclopropylidene, however, were easily brought about under high pressure (10 kbar) and gave cycloadducts 161 stereoselectively in yields ranging from 32 to 95% (Scheme 35) [58]. 

The nature of the dipolar intermediate is crucial in the reaction with furans, since the product distribution is greatly influenced by the substituents present on the furan and vinylcarbenoid (Tab. 14.10) [84—86]. Electron-rich 2-methoxyfuran leads to exclusive formation of triene 89 with vinyldiazoacetate 42, whereas the corresponding reaction with 2,5-dimethylfuran affords only the [3+4] cycloaddition product 88 in 70% yield. 

The reaction with furan forms a mixture of the two products with a preference for the [3+4] cycloadduct 88. Similarly, the reaction of furan with the vinylcarbenoid of si-loxy-substitilted vinyldiazoacetate 87 yields 90% of the [3+4] cycloaddition product with no triene formation. 

Diketone 829 closely parallels 820 in its chemical reactivity. Introduction of a functionalized bridging carbon can be achieved with ethyl formate and base . The acetate group in 8JO 6 is remarkably easily replaced with retention by simple nucleophiles, e.g, the conversion to 8JI. The implicated peristyl-3-ene-2,6-dione (8J2) can in fact be obtained as a colorless crystalline compound. Also, 8J0a spontaneously dehydrates during ketalization to produce 8JJ. The strained double bond in 832 enters readily into Diels-Alder reaction with furan to furnish a 3 1 mixture of 834a and 834b. 

When the chiral catalyst tetrakis(binaphtholphosphate)dirhodium was used, moderate enantiomeric excess (50%) was obtained in cycloaddition reactions with furans and dihydrofurans (359). This method has been applied to the synthesis of several natural products that contain fused furan rings, e.g., the human platelet... 

Diels-Alder reactions with furans constitute a major class of cycloaddition reactions, which have been used for the preparation of a variety of new compounds. Some examples will be given in this section. A one-pot stereoselective synthesis of tricyclic -y-lactones was achieved via Diels-Alder reactions of 2-methoxyfuran with masked o-benzoquinones, which are in turn available from the corresponding 2-methoxyphenols <99CC713>. 

This effect cannot be explained by simply considering differences in frontier orbital energies. A useful monofluorinated dienophile has been prepared [364] using metallated difluoroenol carbamate chemistry (Eq. 143) cycloaddition occurred smoothly with a range of dienes, and desulfination could be achieved under mild conditions without loss of the fluorine atom. Wakselman and co-workers [365] synthesised a rare competent difluorinated dienophile. Lewis acid catalysed Diels-Alder reaction with furan afforded an acceptable yield of (unfortunately) unstable cycloadduct which decomposed to a phenolic product via a dehydrofluorination reaction, circumscribing its utility (Eq. 144). 

Bromination, nuclear, aluminum chloride as catalyst for, 40, 7 Bromine, reaction with furan in methanol to yield 2,5-dimethoxy-2,5-dihydrofuran, 40, 29 3-Bromoacetophenone, 40, 7... 

Diastereoselective Diels-Alder reactions The (S)- and (R)-3-(2-pyridylsul-finyl)acrylates (1) are obtained by addition of 2-mercaptopyridine to ( + )-menthyl propiolate followed by oxidation and fractional crystallization of the diastereoiso-meric sulfoxides. In the presence of (QH AIG, both undergo highly diastereoselective Diels-Alder reactions with furan to give mainly endo-adducts without epimerization of the sulfoxide group. The products are converted as shown into ( + )- and ( - )-ewdo-2-hydroxymethyl-7-oxabicyclo[2.2.1]heptane (2). 

Diels-Alder reactions with furans.2 This sulfonate ester undergoes cycloaddition to furans at room temperature. 

A similar unravelling of the heterocyclic ring occurred in intramolecular reactions with furans,79b,159 161 leading to cyclic 1,4-diacyl-1,3-butadienes (204 equation 42)159 and (205 equation 43).79b The ideal size for the tether was five, but less efficient capture of the carbenoid was also possible with six- and seven-membered rings. With the 3-furanyl derivative (206), the acid labile structure (207) was formed, which readily rearranged to p-hydroxyphenylacetaldehyde (208 Scheme 43).161... 

Cycloaddition reactions with furans remain a source of new compounds. On photochemical excitation of benzofuro-annulated oxanorbomadiene 42 in the presence of dipolarophiles Ill-addition compounds are obtained. The formation of 43 constitutes one of the rare examples of the cycloaddition of a nitrile as a dipolarophile <95LA1503>. 

For fiirther reactions of this type see <95CC2025>. The synthesis of l,4-dihydrofurano[3,4-d]-3,2-oxathiine-2-oxides, precursors for nonclassical heteroaromatic o-quinodimethanes, and their application in the Diels-Alder reaction were reported <95CC2537>. The intramolecular Diels-Alder reaction with furans (e. g., 47) offers a rapid access to highly functionalized isoquinolines <95 JCS(P 1)2393>. 

Wine, P.H., Thompson, R.J. (1984) Kinetics of OH reactions with furan, thiophene, and tetrahydrothiophene. Int. J. Chem. Kinet. 16, 867-878. 

In order to introduce functionality at key positions, an intramolecular Diels-Alder reaction with furan as the diene component was planned. Suzuki coupling of the previously mentioned 105 with furan-3-boronic acid 106 gave compound 107, which was N-tosylated to 108. Construction of the dienophile portion was performed as in the previous case, by addition of vinylmagne-sium bromide to give 109, followed by MnC>2 oxidation to give the cyclization precursor, which was immediately heated at 120 °C in toluene and afforded compound 110, which incorporates an oxygen atom at both the ketone and vinyl chloride positions of welwistatin (Scheme 24). 

The only paper concerning catalysis by Lewis acids of the Diels-Alder reactions of these simple sulfinyl ethylenes was due to Ronan and Kagan [20], who studied the influence of TMSOTf in the reaction of compound (S)-l with cyclo-pentadiene and furane. In the first case, the reaction occurs at 0°C in 3 h, giving an 89 11 mixture of endo and exo adducts (overall yield 60%) with very high n-facial selectivity (de> 92%). The high efficiency of the catalyst increasing the reactivity of 1 also made possible its reaction with furan, which evolved with low endo/exo selectivity (55 45) and lower 7r-facial selectivity (de 70%) than that observed with cyclopentadiene. These excellent results were nevertheless, eclipsed by those reported in the same paper [20] concerning the activation of... 

The most recent report in this field is related to the use of optically pure 3-(2-exo-hydroxy-10-bornyl)propenamide 56 [43]. This compound reacted with cyclopentadiene under both atmospheric and high pressure conditions in CH2Cl2/methanol to give similar mixtures of endo and exo cycloadducts (de 80-88%). Reactions with furan and 2-methoxyfuran took place only under high pressure (12.6 kbar). With furan, the endo-adduct was obtained as a single diastereoisomer (82% yield), whereas with 2-methoxyfuran a 92 8 mixture of endo-57 and exo-57 adducts was obtained (Scheme 30). 

Nevertheless, the authors do not comment on the different endo/exo selectivity observed in reactions with furan (the endo adduct is predominant, Scheme 55) and cyclopentadiene (only exo adducts are obtained, Scheme 54) under kinetic control conditions. They explain the changes in the facial diastereoselec-tivity, observed in the absence of ZnCl2, as a consequence of the increase in population of conformation B (Fig. 9), attributed to dipole-dipole repulsion. However they pay no attention to the role of the diene, which could be mainly responsible for the observed differences. With furan, the de decreased, but the major adduct remains the same, whereas with cyclopentadiene, the 7r-facial... 

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