Dihydrofuran formation

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

Feldman, K. S., Wrobleski, M. L. Alkynyliodonium salts in organic synthesis. Dihydrofuran formation via a formal stevens shift of a carbon substituent within a disubstituted-carbon oxonium ylide. J. Org. Chem. 2000, 65, 8659-8668. 

Dihydrofurans—see also Dihydrobenzofurans, 5-Hydroxy-6,7-dimethyl-2,3-dihydrofurans formation of 1039... 

Stereoselectivity of cyclopropanation with 22 alkenes and regioselectivity of monosubstituted buta-l,3-dienes are highest for a copper(i) catalyst, intermediate for two Rh catalysts and lowest for a PdCl2 complex. On the basis of these comparisons, Doyle et al. were able to define and determine simple linear relationships, namely an index S of relative stereoselectivity (1984 a), and R of relative regioselectivity (1982 b). More data on stereo- and regioselectivities were summarized by Doyle (1986). We shall return to the mechanism of stereoselectivity of cyclopropanation later in this section in the context of dihydrofuran formation. 

There is no doubt that such a ketocarbene is expected to be a 1,3-dipole, as discussed in Section 6.2, but the process 8-52 is not a carbeno/rf reaction, as shown in Doyle s general scheme 8-46. The dihydrofuran syntheses are, therefore, only apparently dipolar cycloadditions. Doyle et al. (1984 b) suggested a mechanism for these cycloadditions that is closely related to his explanation of the preferential trans-stereoselectivity in cyclopropanation by a-carbonylcarbenes. One argument of Doyle for this conclusion is the close analogy between the results of dihydrofuran formation of 1- and 2-methoxybuta-l,3-diene with ethyl 3-diazo-2-oxopropionate (8.110) and the cyclopropanation of these butadiene derivatives with ethyl diazoacetate (Doyle et al., 1981, and other papers see Maas, 1986, p. 97). We return, therefore, to the transition state of cyclopropanation (8.105) here in order to investigate whether it is consistent with the mechanism of formation of dihydrofuran. 

Muscarine derivatives were the target of another synthesis, which starts off with a simple aldol condensation between (65) and (66). A novel method of dihydrofuran formation was achieved by reaction of the dienone derivative (67) with ethylene glycol and p-toluenesulphonic acid to give (68). ° The success of the reaction, which would appear to contravene Baldwin s Rules, was attributed to the formation of the delocalized carbonium ion (69). 

Ph2CHC02-2-tetrahydrofuranyl, 1% TsOH, CCI4, 20°, 30 min, 90-99% yield. The authors report that formation of the THF ether by reaction with 2-chlorotetrahydrofuran avoids a laborious proce ure that is required when dihydrofuran is used. In addition, the use of dihydrofuran to protect the 2 -OH of a nucleotide gives low yields (24-42%)." The tetrahydrofuranyl ester is reported to be a readily available, stable solid. A tetrahydrofuranyl ether can be cleaved in the presence of a THP ether. ... 

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

However, the 2 3-dihydrofuran units in these chains are readily susceptible to further attack by the acid or a growing species at the 4,5-unsaturation (see Section III-4) and this produces branching and the formation of tetrahydrofuran rings ... 

In 1996, the first examples of intermolecular microwave-assisted Heck reactions were published [85]. Among these, the successful coupling of iodoben-zene with 2,3-dihydrofuran in only 6 min was reported (Scheme 75). Interestingly, thermal heating procedures (125-150 °C) resulted in the formation of complex product mixtures affording less than 20% of the expected 2-phenyl-2,3-dihydrofuran. The authors hypothesize that this difference is the result of well-known advantages of microwave irradiation, e.g., elimination of wall effects and low thermal gradients in the reaction mixture. 

There are only a few examples of intramolecular ring opening of azoniaspiroalkanes. These are mostly mediated by base. The formation of dihydrofurans 46 has been reported using this approach (Equation 13) these were obtained... 

A new domino lithium acetylide addition/rearrangement procedure on trans-1,2-dibenzoyl-3,5-cyclohexadiene furnished 3-alkylidene-2,3-dihydrofurans via an intriguing mechanism involving three bond formations and two bond cleavages in one single operation <06SL1230>. The reaction of dimedone with meso-diacetoxycyclohexene in the presence of a palladium catalyst led to the formation of the tricyclic product as depicted below <06S865>. 

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