Pyran derivatives, formation

These reactions are related to the formation of pyrroles and quinolines from aminocarbonyl compounds and acetylenes (582,583) and may be contrasted with the formation of pyran derivatives by electrophilic attack on an enamine, followed by addition of an oxygen function to the imonium carbon (584-590). 

Scheme 150).225 227 The pyran products predominate when the ratio of triphenylphosphine to palladium catalyst exceeds two whereas the linear oligomers are the major products when this ratio is close to unity. The suggested227 mechanism (Scheme 151) includes a step of insertion of C=0 into a C—Pd palladium-catalyzed reactions leading to the formation of pyranones (see Scheme 152)228 and piperidones (see Scheme 139 in Section V,A,2).211 It is useful to note that the 2,5-divinyltetrahydropyran derivative can be transformed catalytically by ruthenium trichloride into synthetically useful 3,4-dihydro-2//-pyran derivatives (Scheme 153).229... 

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

Phosphorus ylides can be generated from triphenylphosphine, 3-chloro-(3//, 5//)-furan-2,4-dione, and alkynyl esters. Additional alkynyl ester, acting as a Michael acceptor, reacts with the ylides in a [4+2] cycloaddition reaction that results in the formation of furo[2,3-3]pyran derivatives (Equation 30) <2000T5221>. 

A solid-phase synthesis of furo[3,2-3]pyran derivatives utilizing highly functionalized sugar templates has been reported <2003JOC9406>. After incorporation of alkenes within the sugar template, such as compound 95, the solid support is introduced via formation of the acid amide. This immobilized system then allows a ruthenium-catalyzed ring-closing metathesis that leads to the formation of the fused oxacycles. 

The Heck coupling is also efficient in the formation of pyrane derivatives. By starting from the appropriately substituted 1,3,5-tribromobenzene derivative, three fused pyrane rings were constructed in the same reaction. The triple Heck reaction in the presence of palladium acetate and triphenylphosphine led to the formation of the tetracyclic product in good yield accompanied by a minor amount of the tricyclic intermediate(4.12.).13... 

When the pentamer (66) reacts with alkoxide anions at low temperatures (-30 to -40°C), then the products of kinetic control (102) are isolated, whereas at higher temperatures, thermodynamic control prevails and the products (103) are obtained [131,132] (Scheme 68). Similar observations have been made with sulphur nucleophiles [132], and complex products are obtained with amines, including the formation of heterocycles [132]. Reaction of (66) with ethyl acetoacetate gave a pyran derivative (104) in a reaction that may be rationalised as shown in Scheme 69 [133]. In an analogous way.furan derivatives are formed from perfluoro-2-butene and -cyclohexene in base-induced reaction with 1,3-dicarbonyl derivatives [133]. 

Dehydration Reactions. Detailed analysis of the pyrolysis tar as discussed previously (Figure 12 and Scheme 3) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-p-D-glucofuranose) and their transglycosylation products as the main components. In addition to these compounds, the pyrolyzate contains minor amounts of a variety of products formed from dehydration of the glucose units. The dehydration products detected include 3-deoxy-o-erythrohexo-sulose, 5-hydroxymethyl-2-furaldehyde, 2-furaldehyde (furfural), other furan derivatives, levoglucosenone (l,6-anhydro-3,4-dideoxy-P-D-glycerohex-3-enopyranos-2-ulose), l,5-anhydro-4-deoxy-D-hex-l-ene-3-ulose, and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation. 

The reaction of l-(alkylsulfanyl)-2,3-diphenylcyclopropenylium halides 22 with cyclic 1,3-diketones in the presence of triethylamine resulted in the formation of 2-(alkylsulfanyl)-27/-pyran derivatives 23 in good yields. 

Acylation of 1 with acid anhydrides 408 in the presence of sodium hydroxide gave the expected 2-acyl derivatives 409 as major products and the pyran derivatives 407 as the minor ones. The formation of the pyrans 407 have been explained to proceed by O-acylation of the initially formed 409 followed by an intramolecular base catalyzed aldol condensation and loss of H20 (75TL2491) (Scheme 84). 

Likewise, cyclic sulfate 134 underwent ring opening with tbe anion derived from a-pbenyltbio-substituted pyran derivatives 135, leading to tbe formation of a spiro ketal 137 (Scheme 33). A similar reaction with cyclic sulfate 138 led to spiro ketal 140. In contrast, it has been reported that the reaction carried out with the corresponding epoxides did not succeed. 

The halopalladation of MCPs gives CP-Pd and CPC-Pd intermediates depending on the reaction conditions. Thus, the isomerization of alkylidene cyclopropyl ketones to 4ff-pyran derivatives takes place in the presence of a palladium chloride catalyst via chloropalladation to form a CPC-Pd and the successive P-carbon elimination (Eq. 10) [25]. In contrast, the addition of Nal changes the reaction pathway dramatically. Under the conditions, the reaction proceeds through a CP-Pd intermediate and results in the formation of furan derivatives. 

Six-membered ring formation. Reactions of orf/io-quinones with l-(dimethylamino)-but-3-ene-l-)me (80MI29) and ethyl diethoxyphosphor-ylacetate (92PS241) have been reported to give benzo[6]pyran derivatives. 

A number of reports concern the attack of nucleophiles on perfluoro-olefins to give cumulenes or heterocumulenes, which in turn imdergo nucleophilic attack to give heterocyclic compounds with perfluoroalkyi substituents. o-Phenylenedi-amine reacts with hexafluoropropene to give 2-(o j8j8>tetrafluoroethyl)benzimidazole (301) ketenimine intermediates have also been utilized in the formation of quinolines (302) (see p. lll) 89.59o j,y intramolecular cyclization, and (303) (see p. 112) by intermolecular cyclization, naphthyridines (304), pyridopyrimidines (305), and isoquinolines (306). The pyran derivative (307) is formed via isomerization of an allene intermediate (see p. 100). 3-Fluoro-2-isopropyl-l,2,4-thiadiazolin-... 

Several years ago, we initiated a synthetic project leading from sugars to 1-oxabicyclic -lactams (Scheme 3). This has prompted us to investigate the [2+2]cycloaddition of isocyanates to glycals and to the related dihydro-2H-pyran derivatives. The direct formation of a p-lactam ring was the crucial step in the planned synthesis. 

Magnetic FCjO nanoparticles have been reported by Kidwai et al. to efficiently catalyze the formation of 4H-pyran derivatives 25 in ethanol at40 C (Scheme 13.9) [16]. 

SCHEME 13.8 Mixed metal oxides (MMO) catalyze the formation of 4//-pyran derivatives 22. 

Pyran derivatives with extended cyclic systems have been reported. Penimal et al. used 20mol% of InClj to catalyze the pyrano-ring formation between 3-cyanoacetylindole 26, malonodinitrile 21, and various aromatic aldehydes 27 (Scheme 13.10) [6]. This method led to the formation of the desired products 28 in short reaction times in refluxing ethanol, which could be isolated in good to high yields by a filtration/recrystallization procedure. 

A similar reaction was published by Song et al. in 2013 (Scheme 13.29) [47]. 2-Hydroxynaphthoquinone 75 was reacted with aromatic aldehydes 97 and ethyl 4,4,4-lrifluoro-3-oxobutanoate 98 catalyzed by a mixture of ammonium acetate and acetic acid (25mol% each). A Knoevenagel-Michael addition sequence was followed by hemiketal formation to give the desired product 99 in moderate to good yields. Dehydration of the product yielded the 4 f-pyran derivatives. 

Recently reported rearrangements of linear triquinanes (244) to their angularly fused isomers (247) have been envisaged to involve the formation and equilibration of the enolates (245) and (246) followed by an intramolecular Michael addition of the enolate ion in (246) to the cyclopentenone moiety see Scheme 62. It has been reported that treatment of l,10-dibromobicyclo[8.1.0]undecane-3,8-dione (248) with triethylamine in dichloromethane results in the formation of a novel cyclopenta[(>]pyran derivative (249). The mechanism outlined in Scheme 63 has been postulated to account for this unusual transformation. 

Allyl cyclohexanols (e.g. 64) undergo regiospecific, but not always stereoselective, intramolecular cyclization when treated with mercuric acetate or mercuric chloride, giving salts (65) reduction of the latter in some cases occurs with ring enlargement thereby allowing the formation of furan and pyran derivatives from the same substrate (Scheme 25). ... 

Hong reported an unexpected formation of a cyclopenta[c]pyran derivative 173 by the [6+3]-cycloaddition of 6-dimethylaminofulvene 109 andp-benzoquinone 174 (Scheme 6.44a) usually, fulvenes react with 174 to give [4+2] cycloadducts... 

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