2- Substituted-2,5-dimethoxy-2,5-dihydrofurans

Anodic oxidations of heteroaromatic cycles (furans, pyrroles, benzofurans) in the presence of methanol have been extensively studied [148-165]. The electromethoxyla-tion of differently substituted furans gives 2,5-dimethoxy-2,5-dihydrofurans in moderate to good yields (Scheme 96) [148-159, 166-170]. 

Bromination of pyrrole with bromine gives mainly the thermodynamically more stable 3-bromo derivative, which is produced from the 2-isomer, formed initially under kinetic control, through isomerization catalyzed by the reaction product HBr. If N-bromosuccinimide is used for bromination, no HBr is produced and the 2-isomer dominates (8IJOC222I). Bromination of furan gave cis- and frans-2,5,-dibromo-2,5-dihydrofurans (isolated as the dimethoxy derivatives) in a ratio of 3 I (67BCJ130). A spectroscopic study carried out at low temperature (- 50°C) confirmed this ratio and also showed that 20% of trans-2,3-dibro-mo-2,3-dihydrofuran was formed (75CC875). It is not known whether these adducts (which demonstrate the competition between 1,4- and 1,2-addition) are intermediates along the pathway for bromine substitution, or whether they are produced in a separate equilibrium as is the case for bromination of benzo[6]furan. 

The replacement of the fluorine atoms by chlorine atoms led to the formation of 2,2-dimethoxy-2,3-dihydrofurans 18 as the result of a ring enlargement. As in the difluorocyclopropyl series, the cyclopropane carbon with the geminal halogen substituents reached the oxidation state of a carboxylic acid by a sequence of elimination, addition and substitution reactions of unknown order. 

Disubstituted pyrrolidinonesJ 1-Substituted 5-(l-benzotriazolyl)pyirolidinones are available in one step from primary amines, benzotriazole, and 2,5-dimethoxy-2,5-dihydrofuran. The benzotriazolyl group is readily replaced by nucleophiles. 

Methoxy- and 2-acetoxyfurans are available from 2,5-dimethoxy- and 2,5-diacetoxy-2,5-dihydrofurans (section 15.1.4) via acid-catalysed elimination. They undergo Diels-Alder cycloadditions the adducts can be further transformed into benzenoid compounds by acid-catalysed opening. 3,4-Dihydroxyfuran is undetectable in tautomeric equilibria between mono-enol and dicarbonyl forms the dimethyl ether behaves as a normal furan, undergoing easy a-electrophilic substitution, mono-or dilithiation at the o-position(s), and Diels-Alder cycloadditions.2,5-Bis(trimethylsilyloxy)furan is synthesised from succinic anhydride it too undergoes Diels Alder additions readily.Both 2- and 3-thiols can be obtained by reaction of lithiated furans with sulfur in each case the predominant tautomer is the thiol form. ... 

Nonetheless, furan does react with bromine in methanol to produce the corresponding addition compounds, (E)- and (Z)-2,5-dimethoxy-2,5-dihydrofuran (Equation 8.55) and aromaticity can also be lost through a Diels-Alder reaction of furan and substituted furans with reactive dienophiles such as the diethyl ester of acetylenedicarboxylic acid (Equation 8.56)... 

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