Furans typical reactivity

Typical Reactivity of Indoles, Benzo[ ] thiophenes, Benzo[ ]furans, Isoindoles, Benzo[c]thiophenes and Isobenzofurans... 

Typical reactivity of indoles, benzo[b]thiophenes, benzo[b]furans, isoindoles, benzo[c]thiophenes and isobenzofurans... 

Nevertheless, we can interpret the reactions of furan and thiophene by logical consideration as we did for pyrrole. In electrophilic substitutions, there is again a preference for 2- rather than 3-substitution, and typical electrophilic reactions carried out under acidic conditions are difficult to control. However, because of lower reactivity compared with pyrrole, it is possible to exploit Friedel-Crafts acylations, though using less-reactive anhydrides rather than... 

Amelated furanes. Attempts to apply some typical steroid reactions to wortmannin (1) arc hampered by the high reactivity of the furane unit to nucleophiles. The furane ring can be masked by reaction with an amine to form an... 

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

The third typical reaction of carbenes is combination with a nucleophile. Carbenes are electron-deficient species, so they combine with nucleophiles that have reactive lone pairs. Addition of a carbonyl O to a carbene gives a carbonyl ylide, a reactive compound useful for making furan rings by a 1,3-dipolar cycloaddition reaction (see Chapter 4). 

Here, the parallels with benzenoid counterparts continue, for these compounds have no special properties - their reactivities are those typical of benzenoid aldehydes, ketones, acids and esters. For example, in contrast to the easy decarboxylation of a-acids observed for pyrrole and furan, thiophene-2-acids do not easily lose carbon dioxide nevertheless, high-temperatme decarboxylations are of preparative value (see also 17.12.1.2). "... 

Electron-releasing substituents on the oxazoles increase the rate of reaction 5-alkoxy-oxazoles are comparable in reactivity to typical all-carbon dienes. 4-(Trimethylsilyloxy)-oxazoles react smoothly to produce furans at modest temperatures. ... 

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