The synthesis of dibenzofurans from diphenyl ethers by non-phenolic oxidative coupling, U.V. irradiation, and by reaction with palladium(ll) acetate has been investigated. 9-lsopropyl-6-methyl-3-pentyldibenzofuran-l-ol (cannabifuran) (35), a minor cannabis constituent, has been synthesized by annulation of 4-isopropyl-7-methylbenzofuran-2-carbaldehyde (28) by Wittig reaction with 2-carboxy-1 -methoxycarbonylethyltriphenylphosphorane and ring closure and functional-group modification of the resultant (€)-4-(4-isopropyl-7-methyIbenzofuran-2-yl)-3-methoxycarbonylbut-3-enoic acid (29).DIBENZOFURANS are comparatively rare natural products: nine are known to occur in lichens,132 and five have been isolated from higher plant^.^-^ We are interested in developing syntheses of some of these compounds since there is a dearth of good general synthetic methods available for dibenzofurans. The classical met hods of synthesis involve Pschorr cyclization of 2-phenoxyanilines,6 or cyclization of 2,2'-dihydroxybiphenyls or their methyl ethers under acidic condition^.^,^ Owing to the difficulty of preparation of many of these intermediates these methods are not easily adapted to the synthesis of the natural products. Some success has been achieved by Diels-Alder annulation of 2-vinylbenzofurans and by rearrangement of 3-(2-methylbenzofuran-3-yl)prop-l ,%dienones but these methods are of limited use.l0Non-phenolic oxidative coupling of suitably substituted diphenyl ethers appeared to be an attractive method for the synthesis of dibenzofurans. Consequently we prepared the diphenyl ethers (1) and (3) by conventional methods and attempted to convert them into dibenzofurans by treating them with vanadium trifluoride oxide l1 or thallium( 111) trifluoroacet ate l2 under a variety of conditions. In no case was any dibenzofuran detected. The diphenyl ether (4) was recovered unchanged from an attempted reaction with vanadium trifluoride oxide under conditions whereby the methyl phosphinate analogue undergoes smooth cyclization to the dibenzophosphole.13~tThe preparation of a number of alkyldibenzofurans by U.V. irradiation of diphenyl ethers in the presence of iodine has been reported.l* We investigated this method using the diphenyl ethers (1) and (3). In the first example poor yields of the isomeric dibenzofurans (5) and (7) were isolated. Their structures were deduced from their lH n.m.r. spectra. Similarly the diphenyl ether (3) gave the three possible products (9), (ll), and (12) in poor yield. Again their structures were deduced from their lH n.m.r. spectra. An attempt was made to convert the diphenyl ether (2), which contains an ester function as a blocking group, into the dibenzofuran (8). The only product which was isolated from this irradiation was the xanthone (13).It has been reported that diphenyl ether is converted t We thank Dr. A. Sierakowski for these experimental details.into dibenzofuran on treatment with palladium(I1) acetate in boiling acetic acid.15 We investigated this reaction for a number of diphenyl ethers but the ...

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The structure of schizopeltic acid as 1,7-dimethoxy-8-methoxycarbonyl 3,9-dimethyldibenzofuran-4-carboxylic acid (1 ) has been confirmed by rational synthesis. 6-Methoxy-4-methylbenzofuran-2-carbaldehyde (1 0), available from 3,5-dimethoxytoluene (4) in six steps, was converted into methyl 1 -acetoxy-7-methoxy-9-methyldibenzofuran-3-carboxylate (1 2) by Wittig reaction with 2-carboxy-l-methoxycarbonylethyltriphenylphosphorane and subsequent ring-closure of the resultant (€)-3-methoxycarbony1-4-(6-methoxy-4-methylbenzofuran-2-y1)but-3-enoic acid (1 1 ) with acetic anhydride. Reduction of compound (1 2) gave 7-methoxy-3,9-dimethyldibenzofuran-1 -01 (1 4) which was converted into schizopeltic acid (1 ) by sequential formylations and oxidations.

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