Cycloocta difuran

Cyclohexyl[l 5]crown-5, potassium complex equilibrium constant, 7, 742 (73MI52101) Cyclohexyl[15]crown-5, sodium complex equilibrium constant, 7, 742 (73MI52101) Cycloocta[ 1,2-c 5,6-c ]difuran H NMR. 4. 564 (70JA973) Cyclopenta[h][2.2.4]cyclazine... 

Dimethylene-2,3-dihydrofuran derivatives, which are produced by fluoride-induced 1,4-conjugative elimination of trimethylsilyl acetate from the [(trimethylsilyl)methyl]-3-furan precursor 207, undergo subsequent [4-1-4] dimerization reactions to produce cycloocta[l,2-3 6,5-. ]difuran derivatives as a mixture of isomers (Equation 137) <1995JA841 >. A methyl substituent at the 3-methylene position was found to retard the rate of dimerization, an observation which is consistent with the proposed two-step mechanism involving the initial formation of a diradical intermediate in the rate-determining step (Table 16). 

Cycloocta[l,2-c 5,6- ]difuran (291), a 16 n-electron analog of benzo[c]-furan, has been obtained by a Wittig reaction as a colorless crystalline solid (mp 131-133°C), rapidly becoming orange in air. In the absence of air the compound is stable. Thus, treatment of 291 with an excess of dimethyl fumarate in boiling benzene for 96 hr gave 76% of adduct... 

A further route to the cycloocta[l,2-c 5,6-c ]difuran system makes use of a Perkin-type reaction. Condensation of furan-3,4-dicarboxaldehyde with furan-3,4-diacetic acid in acetic anhydride and triethylamine at room temperature, followed by esterification with methanol and sulfuric acid, gave... 

SimUar [4 + 4] dimers have been reported during the flash vacuum pyrolysis of 2-methyl-3-furylme-thyl benzoate in which 4//,5//,9//,l(iF/-cycloocta[l,2-b 6,5-b difuran (117) is produced in virtually quantative yield.A stepwise mechanism has been forwarded to rationalize the high-yield formation of the corresponding head-to-head [4 + 4] dimer.Other examples of the [4 -h 4] dimerization of 2,3-di-methylene furan and 2,3-dimeAylene thiophene have been reported as well. 

A 2.0 g 3-methylfurfuryl benzoate (9.25 mmol) was flash vacuum pyrolyzed at 630°C in the normal manner. A quantitative NMR analysis of the pyrolysate indicated the presence of 3-methyl-4-methylenecyclobutenone in a 21% yield and its dimmer in a 24% yield. Thick layer chromatography on silica gel plates (2000 /j.) using two elutions with 20% EtOAc in hexanes afforded two major bands. The upper band consisted of 0.313 g 4H,5H,9H, 10H-cycloocta[l,2-b 6,5-t> ]difuran, in a yield of 18%, m.p. 49-52°C. The lower band was concentrated to give 0.139 g 3-methyl-4-methylenecyclobutenone, in a yield of 16%. 

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