Furans reaction with imines

Bisketenes can provide different reaction paths in the reaction with imines (Scheme 18). For example, bisketene (74) does not form p-lactam species like (76). Instead, DFT computations [78] indicate that zwitterionic intermediate (75) leads to the furane intermediate (77) whose cyclization yields aziridine (78). 1,2-, 1,3- and 1,4-bisketenyl benzene react normally to yield mainly bis (p-lactams) [78],... 

Indium enolates, prepared conveniently by transmetalation of hfhium enolates with IriCl j, react wifh aldehydes to give fhe corresponding -hydroxy esters [80]. Ultrasound irradiation promotes fhe Reformatsky reaction of aldehydes and ethyl bromoacetate wifh indium [81]. Indium-mediated Reformatsky reaction of phenyl a-bromoalkanoates wifh ketones or aldehydes gives di-, tri-, and tetrasubstituted -lactones (Scheme 8.57) [82]. Indium-mediated reaction of imines with ethyl bromoacetate gives 3-unsubstituted -lactams (Scheme 8.58) [83]. An indium-Refor-matsky reagent prepared from 2-(chlorodifluoroacetyl)furan couples with aldehydes (Scheme 8.59) [84]. 

Table 5.1 Silver catalyzed vinylogous Mannich reaction of 2 silyloxy furan 17a with aromatic imines according to Hoveyda, Snapper and coworkers [7].

Different heterocycles, e.g. furan, pyrrole, N-ben2ylpyrrole, indole, and pyrazole react with methyl a-acetamidoacrylate to give a-amino acid precursors under irradiation with silica-supported Lewis acids as catalysts [135]. Inhomogeneous catalysis, long reaction times were required. The reaction of vmylpyrazoles with imines has also been realized [136]. 

Methylene cyclopropene derivatives undergo reaction with aldehydes and imines that are activated by Lewis acid to generate the indene derivative as the major product in most cases, although occasionally dihydropyrans are obtained. Alternatively, if the reaction is conducted at —25 °C, rather than room temperature, the corresponding furan or p3rrolidine derivative is formed (eq 43). A mechanistic rationale for the divergent reaction pathways was provided. ... 

Besides indole, the AFC reaction of other electron-rich arenes such as furan, pyrrole, naphthol, 4,7-dihydroindole, and substituted benzene with imines have been also well developed over the past years. Some examples are given in this section. Terada and co-workers reported that in the presence of 2 mol% of CPA (R)-13f, the AFC reaction of 2-methoxyfuran with iV-Boc aldi-mines underwent smoothly at -35 °C to afford furan-2-ylamine derivatives in 80-96% yields with 86-97% ee (Scheme 6.16). ... 

A-Vinylic phosphazenes, are useful building blocks, that have been used in Aza-Wittig reactions with unsaturated aldehydes to form 3-azatrienes through a [2 -I- 21-cycloaddition-cycloreversion sequence. " The presence of an alkyl substituent in position 3 of A-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed." Other Aza-Wittig reactions include the reaction of iminophosphorane (60) with aromatic isocyanates to obtain, inter alia, useful carbodiimides for the selective synthesis of pyrimidones." Also the iminophosphorane (61) was reacted with furan-2-carbaldehyde, thiophene-2-carbaldehyde, furan-3-carbaldehyde, and thiophene-3-carbaldehyde to give, depending on temperature and aldehyde, trans imines or mixtures of trans and cis imines." " The... 

Condensation of N-substituted pyrrole-2-carbaldehyde derivatives mainly leads to coloured products, examples of which are compounds 4-220 and 4-221. Derivatives of furan-2-carbaldehyde and many other furans are also significantly involved in non-enzymatic browning reactions. Reactions of furan-2-carbaldehyde with amino acids produce common MaiUard reaction products, such as imines. Subsequent reactions of imines produce more complicated structures, mostly coloured compounds (Figure 4.92). The product illustrated in 4-222 forms... 

Aldehydes and ketones react with ammonia and primary amines to form imines. Reaction with aldehydes yields aldimines, and ketimines arise in reactions with ketones (Figure 8.28). Imines derived from aliphatic carbonyl compounds are generally unstable and are transformed to more stable products, such as amines, diamines and others. Secondary amines react with aldehydes and ketones with the formation of enamines (Figure 8.29). Imines are similarly flavour-active compounds derived from furan-2-carbaldehyde. For example, the aromas of Ar-furfuryl(isobutylidene)amine, N-(furfuryl)isopentyhdeneamine (8-155) and N-(furfurylidene)isobutylamine (8-156) reportedly resemble chocolate. 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

Diphenylbenzo[( ]furan has been observed in a number of other reactions. When benzocyclobutadienequinone (172), monothiophthalic anhydride (173), chlorophthalide (the chloride of o-phthalaldehydic acid, 174),and 2-cyanobenzaldehyde (175) are treated with phenylmagne-sium bromide, 138 could be obtained. This last reaction, which yields an imine in the primary stage, has also been conducted with p-methoxy-phenylmagnesium bromide. ... 

The DFT study of the 3 + 2-cycloaddition between ketene and TV-silyl-, IV-germyl-, and TV-stannyl-imines shows that the TV-germylimine reaction is a two-step process the TV-stannylimine reaction is a competition between two- and three-step processes whereas the TV-silyl process follows a three-step process44 A new and convenient synthesis of functionalized furans and benzofurans based on 3 + 2-cycloaddition/oxidation has been reported. The cyclization of cyclic 1,3-bis-silyl enol ethers (48) with l-chloro-2,2-dimethoxyethane (49), via a dianion, produced 5,6-bicyclic 2-alkylidenetetrahydrofurans (50), which are readily oxidized with DDQ to 2,3-unsubstituted benzofurans (51) (Scheme 13)45 The Evans bis(oxazoline)-Cu(II) complex catalyses the asymmetric 1,3-dipolar cycloaddition of a -hydroxyenones with nitrones to produce isoxazolidines.46 The... 

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