Epoxides furans from

The mechanism of epoxide formation (Scheme 7) has not been established but the intermediacy of nickel enolates and ensuing aldol type reactions are suspected28 (cf. Zn-mediated formation of furans from a-bromoketones29). A limitation on the synthesis is that R cannot be aryl for these cases, the products are 2,4-diarylfurans (see Section IV,B,1).30... 

In contrast, substrates 149 all furnished [4 + 3]-cycloadducts 150 and 151 in yields ranging from 10-79% (Scheme 34)68. In all cases, exclusive approach of the furan from the zwitterion face opposite the epoxide ring was seen. In most cases, the exo diastereomer 151 was the major product or was formed to the exclusion of the endo diastereomer 150. The contrasting diastereoselectivity seen in inter- and intramolecular cycloadditions may result from unfavorable nonbonding interactions in the endo transition state between the tether atoms and the alkyl groups at C-2 and C-5. 

The Ti,Al-Beta shows both acidic and oxidative properties which is reflected in unwanted side-reaction. The group of Corma used the bifunctionality in the epoxidation/rearrangement of cx-terpineol to cineol alcohol and in the formation of furans from linalool.81,82 Similarly van Klaveren et al. applied Ti,Al-Beta in the one-pot conversion of styrene to phenyl acetaldehyde.83 Sato et alM solved the unwanted acid-catalyzed side reaction by neutralizing the acid site by ion exchange with alkali metals. Nevertheless the bifunctionality restricts the use of this catalyst to a limited number of reactions. 

Limonoids are C2(, nortriterpenoids deriving from a C30 triterpene precursor. The best known limonoids are the Azadirachta indica (neem tree) antifeedant azadirachtin (C50L C40 C6 -C60(epoxide methylene cross-link) furan) and the Citrus species (Rutaceae) bitter antifeedant limonin (G50L G40 G6 G6 C50L(epoxide)-furan). Limonin gives a delayed bitter taste to Citrus fruit. The limonoids are typically bitter compounds with insect antifeedant activity... 

The role of one of the vinyl groups can be played by a double bond of furan ring <89T302l>, the rearrangement results in the formation of a dihydrofuro[3,2-c]oxepin. Sulfur dioxide extrusion from tricyclic epoxide obtained from 3-thiabicyclo[3.2.0]hept-6-ene 3,3-dioxide (Scheme 22) affords 4,5-dihydrooxepin in 55% yield <82CC1164>. 

Epoxy fatty acids occur either as 1,2-epoxy compounds derived from ethylene oxide or 1,4-epoxides derived from furan (fiiran acids). 

There is considerable evidence for the formation of the bicycHc oxyaUyl zwitterion prior to the epoxy-cyclopentenone. Pavlik and Barltrop showed that irradiation in trifluoroethanol (TFE) or methanol furnished solvent adducts as primary photoproducts, and Barltrop and co-workers excluded the possible formation of the TFE adduct 6e from epoxycyclopentenone 3e. Barltrop demonstrated that irradiation of 3,5-dimethyl-4-pyrone le in furan/trifluoroethanol mixtures furnished the [4-l-3]-cycloadduct 9e as one of the major photoproducts (Scheme 3). A similar result was reported by West and co-workers using tetramethylpyrone Id. Interestingly, in both cases, a single diastereomer was observed, corresponding to approach of furan from the face opposite the epoxide oxygen and reaction via an endo... 

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

Because of their favourable price, polyesters are preferred to epoxide and furane resins for general purpose laminates and account for at least 95% of the low-pressure laminates produced. The epoxide resins find specialised uses for chemical, electrical and heat-resistant applications and for optimum mechanical properties. The furane resins have a limited use in chemical plant. The use of high-pressure laminates from phenolic, aminoplastic and silicone resins is discussed elsewhere in this book. 

Epoxides (oxirans) and 1,2-diols can also be looked upon as disguised ketones capable of being unmasked by acids. Since its development by Spencer et al49 the idea has been utilized by several other groups who used protic acids in work aimed at syntheses of methyl lambertianate,50 a rare furanoid fatty acid from an Exocarpus species,51 and a terpenoid furan,... 

We owe to Kato and his colleagues a considerable advance in furan copper reagents. They have demonstrated the formation of the lithium di(3-furyl) cuprate species 87 which is highly reactive and possesses hard properties that suit it to reaction at hard centers, mainly carbonyl carbon.223 The reagent is easily prepared in situ from 3-furyllithium and Cu2I2. Simple copper derivatives do not react with ketones, but this cuprate reacts well and quantitatively with acid chlorides. It also reacts well with some epoxides (oxirans). Moreover, there is another form prepared in the presence of... 

As noted above, formation of a furan [4 + 3]-cycloadduct during irradiation of a 4-pyrone was advanced as evidence for the zwitterionic intermediate. This process can be moderately efficient (equation 4)68, and can be envisioned as an approach to substituted cyclooctanoids. Besides the formation of three new carbon-carbon bonds, an additional attractive feature is the complete diastereoselectivity, arising from a compact [4 + 3]-cycloaddition transition state with approach from the face opposite the epoxide. However, the generality of the intermolecular reaction is limited, as competing [2 + 21-photodimerization, solvent trapping and rearrangement often predominate58. 

Racemic analogs of showdomycin have been synthesized from adducts (352 and 353) of methyl 2-nitroacrylate with furan. Synthesis of DL-2-ept-showdomycin involved treatment of the adducts with m-chloroperoxybenzoic acid, followed by nitrous acid elimination, to give the alkenic epoxide 379. Opening of the oxirane ring, and a sub-... 

Oxepane (159) was obtained in high yield as an unexpected product of rearrangement from the peroxyacid oxidation of the substituted furan shown in equation (45) (81JOC2589). The isomerization of a bis-epoxide intermediate to the ring expansion product (159) occurs in the final step of the latter reaction. ... 

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