Furans precursors

The IMDAF (intramolecular Diels-Alder furan) precursors 492 were prepared via Michael addition of nucleophiles possessing an unsaturated tether 491 to furoyl nitroalkene 490. Furyl nitroalkene 490 was prepaperd via the nitroaldol (Flenry) reaction. Compound 492 was heated in appropriate solvent such as toluene, xylene, etc., to provide the IMDAF cycloadducts 65 and 66 (Table 16) <2005JOC2235>. 

Intramolecular C-C bond formation in the furan precursor is the main synthetic method for furobenzazepines. 2-Hydroxybenzonitrile 35c produces the corresponding benzofuran benzazepine dione 36c when reacted with o-carboxymethyl bromoacetophenone in refluxing DMF (Scheme 6, Section 2.1.1.2 (1991JHC379)). Alternatively, benzofurobenzazepinone 91 can be synthesized starting from benzofuran amino ester 90 by intramolecular acylation... 

The synthesis of these rings involves annulation of the furan ring onto the preformed benzoxepine core or intramolecular oxepine C-C bond formation of the furan precursors. Thus, 2-methyldibenzo[ 7,/]furo[2,3-d]oxepines 148 (R = H, Cl)... 

The next two examples illustrate intramolecular oxepine C-C bond formation of the furan precursors. Palladium catalyzed intramolecular arylation of 153... 

Another furan precursor 161 has been used to generate a series of pyrrolo[2,3-rf]pyrimidine triones 162 (Equation 60) the furandiones react with aryl isocyanates, releasing 2 mol of carbon dioxide, to form compounds 162a-f <2004ASJ877>. 

Dimethylene-2,3-dihydrofuran derivatives, which are produced by fluoride-induced 1,4-conjugative elimination of trimethylsilyl acetate from the [(trimethylsilyl)methyl]-3-furan precursor 207, undergo subsequent [4-1-4] dimerization reactions to produce cycloocta[l,2-3 6,5-. ]difuran derivatives as a mixture of isomers (Equation 137) <1995JA841 >. A methyl substituent at the 3-methylene position was found to retard the rate of dimerization, an observation which is consistent with the proposed two-step mechanism involving the initial formation of a diradical intermediate in the rate-determining step (Table 16). 

For an unsuccessful attempt to prepare an isobenzofuran from a furan precursor, see Wulff et al. (88JA7419). 

The reaction of a-bromoacetals with trimethylsilylenolates catalyzed by titanium tetrachloride provides /3-alkoxy-y-bromoketones, which are useful furan precursors (Scheme 33) (75CL527). A new synthesis of acylfurans is exemplified by the formation of the 3-acetyl derivative (146) by heating the brdmoalkene (145) (78JOC4596). 2,2-Dimethyl-3(2//)-furanone (148) has been synthesized from 3-hydroxy-3-methylbutan-2-one treatment with sodium hydride and ethyl formate gave the hydroxymethylene derivative (147), which was cyclized and dehydrated to the furanone (148) with hydrochloric acid (Scheme 34) (71TL4891). O... 

Nowadays, there is a renewed interest for the preparation of fructose, for its food applications as a diet sugar[16] as well as for its nonfood applications as a starting material for the synthesis of furanic precursors of nonpetroleum derived polymeric materials. [17 19]... 

Compound 87 (R = r-Bu, R = Ph, R = H) has been prepared by addition of tert-butylmagnesium chloride to 1-phenylphthalide (102) and subsequent dehydration with p-toluenesulfonic acid. It is described as a yellow oil with brilliant fluorescence under UV light, which on oxidation with sodium dichromate yields diketone 103. Compound 87 (R = /-Bu, R = Ph, R = H) with dimethyl acetylenedicarboxylategives 104(mp 128-129 C, 80%) on reduction 105 is obtained 105 is also accessible from 87 (R = <-Bu, R = Ph, R = H) and dimethyl maleate (mp 151-152°C, endo) a maleic anhydride adduct has also been described (mp 147.5-148°C, 83%). Diels-Alder adducts have been also prepared from the benzo[c]furan precursors 106 (R = CHjPh, R = H, Ph) in the presence of catalytic amounts of acid 107 (mp 141-142.5X, 70%) and 108 (mp 140-141.5°C, 49%) were obtained, which on catalytic hydrogenation (Pd/C) gave the exo isomers 109(110-1 irC) and 110(90-92°C, 125.5-127°C, dimorphism). The basis for these stereochemical assignments remains unclear, however. Compounds... 

Among the monosaccharide syntheses in which a furan precursor is amenable to ring opening, the one based on transformation of 2-furylmethanols into methyl... 

Several butenolide natural products of terpenoid origin have been synthesized from furan precursors. On the other hand furan natural products are synthesized from butenolide precursors, e.g. ancistrofuran (133) (Scheme 75). The synthesis of two members of the drimane class of sesquiterpenes containing a butenolide ring have been reported. The first synthesis provides a route to racemic cinnamodial, and the second describes the synthesis of optically active confertifolin (134) from the natural product manool. A rather doubtful mechanism is proposed for the most important reaction in this sequence (Scheme 76). ... 

Furan precursors are oxidation products of polyunsaturated fatty acids, in particular of a-linolenic acid and linoleic acid. In the case of linolenic acid, the C-H bond cleavage at C-17 of (9Z,12Z,15Z)-17-hydroperoxyoctadeca-9,12,15-trienoic acid yields (9Z,12Z,15Z)-17-oxoheptadeca-9,12,15-trienoic acid. Subsequent autoxidation (primarily cleaved is the C-H bond at C-11 and the free radical isomerises) produces (9Z,11E,15Z)-13-hydroperoxy-17-oxoheptadeca-9,ll,15-trienoic acid, which subsequently decays to form ( )-but-2-enal, which can isomerise to (Z)-but-2-enal (crotonaldehyde. Figure 12.6). In model experiments, the amount of furan increased by several orders of magnitude in the presence of iron ions. 

Four-carbon furan precursors are formed from hexoses in essentially three ways via l-deoxyhexo-2,3-diulose and aldotetrose, respectively), from 2-deoxytetros-3-ulose and 3-deoxyhexos-2-ulose. The most common l-deoxyhexo-2,3-diulose is 1-deoxy-D-erythro-hexo-2,3-diulose, which is produced by 2,3-enoUsation of ketosamines derived from glucose as the major intermediate and furan is also produced in a small amount from fructose in... 

Another natural furan precursor is L-ascorbic acid (the potential of dehydroascorbic and isoascorbic adds found in model experiments were still one order of magnitude higher). The reaction mechanism of furan formation from ascorbic add is not yet fully understood. The expected precursor is 3-deoxypentos-2-ulose,... 

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