Cyclodehydration benzofurans

A synthesis of the [l]benzothieno[3,2- ][l]benzofuran 63 based on the formation of the furan ring in the key step was elaborated. Alkylation of methyl 2-sulfanylbenzoate 349 with 2-hydroxybenzylalcohol 348 was effected with freshly prepared Znl2 in CH2CI2 and 350 was obtained. Cyclization with lithium diisopropylamide (LDA) in THF afforded 351, which on subsequent cyclodehydration with polyphosphoric acid (PPA) gave 63 in moderate yield (Scheme 36) <2000CCC1939>. 

Cyclodehydration of l-[4-chloro(or bromo)phenoxy]-l-phenylacetones (87) gives benzofurans (83)256 with rearrangement, both with chemical... 

Ozone or performic acid readily converts 106 into the aldehyde 107, which then undergoes cyclodehydration to 108 with PPA.201 Thus, 4-allyloxy-2-hydroxy-cu-methoxyacetophenone gives (Claisen) 3-allyl-2,4-dihydroxy-w-methoxyacetophenone, then (with ozone) aldehyde (109) and 4-hydroxy-5(methoxyacetyl)benzofuran (110). 

Haelters, J.P., Corbel, B., and Sturtz, G., Synthesis of 1- or 3-aryloxy-2-oxopropylphosphonates and their cyclodehydration to benzofuran phosphonates. Phosphorus, Sulfur Silicon Relat. Elem., 42, 85, 1989. 

A procedure by Burgstahler and Worden2 for the preparation of benzofurane (6) from o-formylphenoxyacetic acid (5) calls for refluxing for 8 hrs. a mixture of 0.5 mole of (5), 2.2 moles of sodium acetate. 450 ml. of acetic anhydride, and 450 ml. of acetic acid benzofurane (6), the product of cyclodehydration and decarboxyla-... 

Reaction of 4-f-butyl-l -benzoquinone 145 with silyl enol ethers, in the presence of catalytic trityl perchlorate, gives the benzofuran derivatives 147 (R=Ph, PhCH2) (Scheme 36), presumably via cyclodehydration of the intermediate 1,6-adducts 146 (8 L1105). Use of less-hindered reactants, such as 1,2-benzoquinone and 4-methyl-l, 2-benzoquinone, gives 1,4-adducts, which cannot cyclise. 

Benzofuran is a colorless oil with a boiling point of 173 °C. It occurs naturally in coal tar and is believed to form during carbonization by cyclodehydration of 2-ethylphenol. In contrast, benzothiophene is a white solid with a melting point of 32 °C. It is considered an aquatic toxin with acute and long-lasting effects. 

Dehydration and Cydization. Amberlyst 15 has been used in the cyclodehydration of pentane 1,5 diol to tetrahydropyran and in the synthesis of a library of substituted benzofurans (eq 7)... 

Structure of condidymic acid (461), isolated from Cladonia squamosula 48), was confirmed by total synthesis 48) (Scheme 57). Ullmann reaction of 2-iodo-di-O-methylolivetol and subsequent cyclodehydration/demethylation of the product with hydrobromic acid gave the symmetrical dibenzofuran intermediate (462). Methylation followed by formylation yielded a mixture of the 4-formyl and 2-formyl dibenzofurans (463) and (464), which were separated chromatographically. 3,7-Dimethoxy-l, 9-dipentyl-2-formyldi-benzofuran (464) was then oxidized and selectively demethylated to yield condidymic acid (461). 

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