2 -Hydroxychalcones intramolecular cyclization

Also, Bids adsorbed on silica gel, catalyzes the intramolecular cyclization of 2 -hydroxychalcones under dry conditions. The reaction proceeds efficiently giving... 

With the structurally similar 2 -hydroxychalcones, Lier et al. reported that either InCl3 or InBr3 supported on silica gel catalyze the intramolecular cyclizations in the absence of solvent at elevated temperatures [62]. However, the expected flavanones were not isolated, but the corresponding flavones were obtained instead. The supported In(III) catalyst had not only catalyzed the 1,4-addition, but had proceeded to catalyze the oxidation. This was proven from the authors oxidation of flavanones to flavones under the same reaction condition (Figure 8.35). 

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]. 

A phenolic oxygen participates in facile oxypalladation. 2-Allylphenol (55) undergoes clean cyclization to 2H-l-benzopyran (56) in DMSO under air with a catalytic amount of Pd(OAc)2 without a reoxidant. 2-Methylbenzofuran (57) is obtained when PdCl2 is used [50]. But different chemoselectivity with the Pd(II) salts was also reported [51]. Catalytic asymmetric cyclization of the tetrasubstituted 2-allylic phenol 58 using the binaphthyl-type chiral ligand 60, called (.S, .S )-ip-borax, afforded the furan 59 with 96% ee. Use of Pd(CF3 002)2 as a catalyst is essential in the presence of benzoquinone [52]. Formation of the benzofuran 62 from 61 has been utilized in the synthesis of aklavinione [53]. The intramolecular reaction of 2-hydroxychalcone (63) produces the flavone 64 [54]. 

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