Benzofurans, formation

Phenolic oxygen participates in facile oxypalladation. The intramolecular reaction of 2-hydroxychalcone (105) produces the flavone 106[127]. The ben-zofuran 107 is formed from 2-allyIphenol by exo cyclization with Pd(OAc)2, but benzopyran 108 is obtained by endo cyclization with PdChf S], Normal cyclization takes place to form the furan 109 from 2-(l-phenylethenyl)phe-nol[129]. Benzofuran formation by this method has been utilized in the synthesis of aklavinione (110)[130]. 

Thiolate anions37 38 and oxime alkoxides38 react under phase transfer conditions to give aryl sulfides and O-aryl oximes, respectively the o-dichloro benzene complex can be converted selectively to the monosubstitution product (equation 9). The arylation of oximes leads to a simple process for benzofuran formation (equation 10). Simple primary and secondary amine nucleophiles react smoothly in the absence of added base, in a very general and efficient process for aniline derivatives.49... 

From a mechanistic point of view two possible hypotheses were discussed (Scheme 2). Since radical cations are intrinsically very acidic [29-32] one would expect enol radical cation 27 to deprotonate efficiently and rapidly thus providing an a-carbonyl radical (mechanism 1). In a further one-electron oxidation step the a-carbonyl cation is formed, that cyclizes to an intermediate cyclohexadienyl cation 28. After a 1,2-methyl shift and deprotonation the benzofuran 29 is obtained. The mechanistic proposal is in line with benzofuran formation from a-carbonyl cations as demonstrated by Okamoto [120]. Interestingly, the above mechanism was first proposed by Bailey to explain the formation of 3% of benzofuran 24 in the ozonization of enol 8 [121], Years later, however, the mechanistic hypothesis was proven to be untenable [122] under ozonization conditions. A priori, it cannot be excluded that intramolecular cyclization of the enol radical cation 27 " is faster than deprotonation (mechanism 2). The distonic radical cation formed is expected to lose a proton readily and after a second one-electron oxidation the same cyclohexadienyl cation intermediate as in mechanism 1 is formed. 

Even in the presence of nucleophiles like methanol, these sterically hindered enols only afforded benzofurans under oxidative conditions. Importantly, when stable enols of the Fuson type, which are less sterically hindered in the P-position, were oxidized with one-electron oxidants a-substituted carbonyl compounds 31 and 34 were obtained with water and various alcohols [64]. With acetonitrile, formation of the oxazole 32 was observed. A related example of this chemistry can be found in the context of Fuson s oxidation studies using Pb(OAc)4 [123] that provided the a-acetoxy aldehyde 31c from 30. To explain the products 31 and 34, three different mechanistic hypotheses were advanced (Scheme 3) [64] similar to the ones rationalizing the benzofuran formation. 

The following general mechanism for benzofuran formation and a-substitu-tion was proposed for the mesityl substituted enols [64]. Indeed, this mechanistic scheme parallels the one for phenol oxidation, where phenoxyl radicals and... 

As shown in figure 2, the best activity for 2-acetyl-benzofuran formation was obtained with ZF 520. If we compare this zeolite to LZY 82, we can assume that its higher activity is due to a higher strength of the acid sites because of dealumination [12] or to a lower level of deactivation by coke because of a lower density of acid sites [13],... 

As can be seen in the figure 4, the mol percent conversion of benzofuran into 2-acetyl-benzofuran decreases when the initial concentration of benzofuran increases. However, the specific rate of 2-acetyI-benzofuran formation increases and the kinetic order with respect to the substrate is of about 0.5. 

Hydroxylamines deprotonated by KOH substitute for chloride in (chloro-arene)Cr(C0)3 in high yields [24]. Reaction of a (fluoroarene)Cr(CO)3 complex with dialkyl phosphite 12 resulted in formation of a phosphate 13 [25]. The ar-ylation of oximes under phase transfer conditions leads to a simple process for benzofuran formation [26]. 

Furans and Benzofurans.—Formation. The acid-catalysed condensation of 2-chlorocyclopentanone with dimethyl jS-oxoglutarate leads to the furan ester (100). Compound (101) is obtained by the action of nitrotrichloroethylene on acetylacetone in the presence of bases. 1-Diethylaminopropyne, MeC=CNEt2, and benzoin undergo three competing reactions to give a mixture of the furans (102) and (103) and the butenolide (104). The sulphonium ylide (105) and... 

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