Benzofuranols, synthesis

The benzofuran-3-carbaldehyde (398) is the key intermediate in the synthesis of rus-codibenzofuran. Condensation of (398) with Meldrum s acid in the presence of piperidine acetate afforded the arylmethylene derivative (399), which on flash vacuum pyrolysis gave the benzofuranol (400) (Scheme 107) (80AJC1817). 

The synthesis of benzofuranols has been accomplished by reaction of/ -benzoquinones with enamines110,117. For example, when silylenamine 217 was allowed to react with /7-benzoquinone 216, a 1,3-cycloaddition took place to give dihydrofuranol 218, which after treatment with acid was transformed to benzofuranol 219110 (equation 47). 

Indole derivatives have been prepared by reaction of 2-bromoanilines with enamines in the presence of palladium(II) acetate127, reductive cyclization onto a nitro group128, or by the same type of cycloaddition that allows the synthesis of benzofuranols (see above) but using / -quinone diimides129. An example of an application of this latter method is the regioselective nucleophilic addition of 1-piperidino-l-propene (245) to the selectively activated AT4-(phenylsulfonyl)-p-quinone diimide 244 (equation 51)130. Fur-... 

The synthesis of 2,3-dihydro-2,2-dimethyl-7-benzofuranol (16), required for the preparation of carbofuran, is carried out according to the following route ... 

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