Furans 1 -methyl-1 - allene

The irradiation of the thiophene in gas phase yields ethylene, allene, methyl-acetylene, carbon disulfide, and vinylacetylene. No Dewar thiophene or cyclo-propene derivatives were isolated (69CJC2965). The irradiation in liquid phase gave the Dewar thiophene which can be trapped as a Diels-Alder adduct with furan (85JA723). The Dewar thiophene and cyclopropene-3-thiocarbaldehyde can be obtained by irradiation in argon matrices at 10 K (86JA1691). 

Attempts to liberate l-methyl-l-aza-2,3-cyclohexadiene (329) from 3-bromo-l-methyl-l,2,5,6-tetrahydropyridine (326) by KOtBu in the presence of [18]crown-6 and furan or styrene did not lead to products that could have been ascribed to the intermediacy of 329 (Scheme 6.70) [156], Even if there is no doubt as to the allene nature of 329 on the basis of the calculations on the isopyridine 179 and 3d2-lH-quinoline (257), it is conceivable that the zwitterion 329-Za is only a few kcal mol-1 less stable than 329. This relationship could foster the reactivity of 329 towards the tert-butoxide ion to an extent that cycloadditions to activated alkenes would be too slow to compete. On the other hand, the ultimate product of the trapping of 329 by KOtBu could have been an N,0-acetal or a vinylogous N,0-acetal, which might not have survived the workup (see, for example, the sensitivity of the N,0-acetal 262 [14], Scheme 6.57). 

Both uncatalyzed and catalyzed [4+2]-cycloaddition reactions of furans with the allenic esters have been reported (Table 12.6) [93]. The allene adds from the less hindered C1-C2 Jt-face. The unfavorable steric interaction between the a-hydrogen atom of the furan and the methyl group at C4 of the allene is responsible for this selectivity. The more reactive 2-methylfuran adds to the allenic ester also in a regio-selective manner. The C2 carbon atom of 2-methylfuran was exclusively attached to the Cl carbon atom of the allenic ester, providing a mixture of endo- and exoadducts. 

Furan has served as a starting material for the synthesis of methyl rt-tetradeca-frans-2,4,5-trienoate (177), a compound presumed to be a sex attractant of the male dried bean beetle (72TL3777). Conversion of furan to 4,4-dimethoxy-frans-but-2-en-l-al (174) was accomplished in two steps by a previously reported procedure. Intermediate (174) was then ethynylated, acetylated and reacted with lithium dioctylcuprate to yield allene (176). Hydrolysis of the acetal and Mn02-NaCN oxidation of the aldehyde afforded the methyl ester (177 Scheme 37). 

As shown below, allene derivatives have been adopted as starting materials in the synthesis of furans. Palladium(II)-catalyzed cyclization of a-allenic ketones afforded 2,3-disubstituted furans in good yields <07EJO2844>. The substituent at the a-position of a carbonyl group influences the reaction pathway. Cyclization products were provided when the substituent was a methyl group or a phenyl group, while dimerization products were produced when the substituent was a hydrogen atom. 

Other methods were also used for the synthesis of trifluoromethylfurans. Thus, flash vacuum thermolysis of silyl enol ether of 1,3-diketone 148 at 800 °C afforded furan 69 (70 %) via intermediate allenic ketone 149 [110], Another synthesis of trifluoromethylated furan from 1,3-diketone comprised reaction of diazomethane with 2-(trifluoroacetyl)dimedone 150. This approach produced dihydrofuran 152 in a mixture with methylated product 151. Compound 151 underwent aromatiza-tion into 153 under heating withp-toluenesulfonic acid [111]. 

A 20%-soln. of butyllithium in hexane added drop wise with toluene-liq. Ng-cool-ing at -95° to a soln. of 9,9-dibromo-c/5 -bicyclo[6.1.0]nonane in dry tetrahydro-furan, stirred 10 min., methyl iodide added in 1 portion, and after 1 hr. gradually warmed to 0° 9-cj o-bromo-9-endo-methyl-c/ -bicyclo[6.1.0]nonane. Y 70%. F. e., also formation of cyclic allenes (cf. Synth. Meth. 17, 959 21, 734) s. H. J. J. Loozen et al., J. Org. Chem. 41, 2965 (1976) cf. Synth. Meth. 24, 664 suppl. 31. 

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