Dihydrofuran systems

Acylation with imidazolides of a cyclopropane-CH activated by a phenylsulfonyl group proceeded under ring-enlargement to give a dihydrofuran system [103a]... 

Substituted dihydrofuran system Substituted furan system... 

The monoanions of 1,3-dicarbonyl compounds react smoothly with the cis-oriented epoxy triflate 1 to give the intermediate I which, after further base treatment, leads to a dihydrofuran system [20] (Scheme 7). After treatment of 1 with dianions of 1,3-dicarbonyl compounds, tetrahydrofurylidine formation is observed under kinetic conditions [20] (Scheme 7). 

For the second ABX-spin system the sum of the vicinal coupling constants is 10.8 Hz for 242, 12.0 Hz for 243, 17.1 Hz for 244 and 17.4 Hz for 245. For an ABX-spin system in a substituted dihydropyran ring the sum of the vicinal coupling constants is smaller than or equal to 12 Hz, while the value is higher than 17 Hz in a dihydrofuran system (43,44). Taking this into account, it follows that compounds 242 and 243 contain a dihydropyran ring, while compounds 244 and 245 have a dihydrofuran ring. Further indications for the proposed structures are the chemical shifts for the... 

Larhed et al. investigated enantioselective Heck reactions with 2,3-dihydrofuran as alkene [86]. In the coupling with phenyl triflate, conditions previously reported by Pfaltz [87] were attempted under microwave irradiation. Interestingly, the catalytic system Pd2(dba)3/(4S)-4-t-butyl-2-[2-(diphenylphosphanyl)phenyl]-4,5-dihydro-l,3-oxazole, identified by the Swiss team, was found suitable for high-temperature microwave-assisted enantioselective Heck reactions (Scheme 76). Using a proton sponge as a base and benzene as a solvent gave the best conversions (Scheme 76). At tempera-... 

Dicyanopyridazine 4-145 has been used for the synthesis of carbo- and het-ero-cage systems employing nonconjugated dienes such as cyclooctadiene 4-146, or 4-148 or 4-150 to give 4-147, 4-149 and 4-151, respectively (Scheme 4.31) [50]. In a similar way, dihydrofurans, dihydropyrans, pyrrolines, and enol ethers have also been used [51],... 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

Similar cross-coupling reaction of another derivative of the same ring system 141 with dihydrofuran has also been described. In this case, palladium diacetate, tributylamine, triphenylphosphine, and tetrabutylammonium chloride were used to afford the product 142 in high yield (70%) <1995NN105>. 

Abstract This chapter presents the latest achievements reported in the asymmetric hydroformylation of olefins. It focuses on rhodium systems containing diphosphites and phosphine-phosphite ligands, because of their significance in the subject. Particular attention is paid to the mechanistic aspects and the characterization of intermediates in the hydroformylation of vinyl arenes because these are the most important breakthroughs in the area. The chapter also presents the application of this catalytic reaction to vinyl acetate, dihydrofurans and unsaturated nitriles because of its industrial relevance. 

Although aldehydes obtained through the hydroformylation of dihydrofurans are interesting building blocks for organic synthesis, few studies have been reported on the subject. In 1998, previous work on the control of the regio-selectivity in the hydroformylation of dihydrofurans has been reported with rhodium systems modified with different ligands [77,78]. In the hydroformylation of 2,5-dihydrofuran 46 the expected product is the tetrahydrofuran 3-carbaldehyde 49 (Scheme 7). 

The Mo(CO)6-TBHP system promoted autoxidation of 5-alkylidenene-4,5-dihydrofurans (168) under mild conditions, allowing the preparation of primary, secondary and tertiary furyl hydroperoxides. A radical mechanism has been proposed and was supported by the experimental data. 

Synthesis of dihydrofuran derivatives by cyclization of oxime derivatives has been described. Thus, reduction of 2-quinolineacetaldoxime (26) with H2/Pt02 afforded fura-noquinoline 27 as a single product (equation 1. 4-Formy l-3-hydroxy-5-hydroxymethyl-2-methylpyridine oxime (28) in the system NaN02/HCl/H20 cyclized to furopyridine 29 (equation 13). ... 

A novel homogeneous process for the catalytic rearrangement of 3,4-epoxy-l-butene to 2,5-dihydrofuran has been successfully developed and scaled-up to production scale. A tri(n-alkyl)tin iodide and tetra-(n-alkyl)phosphonium iodide co-catalyst system was developed which met the many requirements for process operation. The production of a minor, non-volatile side product (oligomer) was the dominating factor in the design of catalysts. Liquid-liquid extraction provided the needed catalyst-oligomer separation process. 

Imines derived from aniline and glyoxylic acid esters can be regarded as electron-poor 2-azadienes, in which an aromatic carbon—carbon double bond takes part of the diene system. In this context, Prato and Scorrano et al. were able to achieve the [4 + 2] cycloaddition of ethyl N-phenyl glyoxylate imines with dihydrofuran and indene leading to hexahydrof-uro[3,2-c]- and tetrahydro-7//-indeno[2,l-c]quinolines, respectively, in moderate to good yields (88JHC1831). Similarly, tetrahydroquinoline derivatives were formed by [4 + 2] cycloaddition of 1,2-bis(trimethylsily-... 

As mentioned previously, the partially reduced forms of five membered heteroaromatic systems might act as olefins in insertion reactions. This behaviour is characteristic particularly of dihydrofuranes. The olefin insertion and the following / hydride elimination should in principle lead to a trisubstituted olefin, which is rarely observed, however. Typical products of this reaction are 2-aryl-2,3-dihydrofuranes. A characteristic example of such a reaction is presented in 6.54. The coupling of 4-iodoanisole and dihydrofurane led to the formation of the chiral 2-anisyl-2,3-dihydrofurane in excellent yield.83 The shift of the double bond, which leads to the creation of a new centre of chirality in the molecule, opens up the way for enantioselective transformations. Both intermolecular and intramolecular variants of the asymmetric Heck reaction have been studied extensively.84... 

The reaction of 5-bromo-2-furoic acid with the sulfur tetrafluoridc/hydrogen fluoride system proceeds under mild conditions and results not only in conversion of the carboxylic acid group but also in addition of two fluorines and replacement or migration of bromine a 1 3.5 mixture of 2,2,5-trifluoro-5-(trifluoromethyl)-2.5-dihydrofuran (10) and 2-(bromodifluoromethyl)-2,5.5-trifluoro-2,5-dihydrofuran (11) is obtained.153... 

The oxolane (tetrahydrofuran) ring system can be incorporated into polymers either by polymerization of the suitably substituted heterocycle itself or by addition polymerization of a dihydrofuran derivative. A prime example of the former case is found in the utilization, as a component of adhesives and coatings for example, of the acrylate (38) and methacrylate (39) esters of tetrahydrofurfuryl alcohol. Although the bulk of the investigations concerning these monomers is recorded in the patent literature, a detailed study of the polymerization behavior of ester (39) has appeared (74MI11101) that indicates it is a fairly typical methacrylate monomer. 

Dihydrofurans are known to polymerize readily by cationic means but not by free radical initiation (77MI11102). 2,3-Dihydro-5-methylfuran (40), for example, has been polymerized to a stereoregular, optically active polymer (Scheme 9) by the appropriate choice of a catalyst system. Interestingly, the double bond isomer 2-methylenetetrahydrofuran (41) can be cationically polymerized (74CL499) to the isomeric polymer (42 Scheme 10). Under the polymerization conditions, no isomerization of (41) to (40) occurred, and the resultant polymer was shown to be isotactic and crystalline by NMR and X-ray analysis, respectively. 

For 2,3-dihydrofuran itself no molecular dimensions have been recorded as yet, though a few natural products containing the system have been studied by X-ray diffraction. These include prerotundifolin (73CL1035) and also nepetaefolin (75JA6236), some relevant dimensions of which are shown (68). A c/s-fused furofuran system is characteristic of several... 

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