1.3- Dihydrobenzo furan derivatives

Pd/C in AcOH515 539 or on Pd/BaS04 in AcOH,534 the reaction proceeds through to the 2,3-dihydrobenzofuran stage. Some catalysts (Raney nickel at 60° 50 kg/cm2) lead directly from 237 to 2,3-dihydrobenzo-furan derivatives, without going through the acetoxy derivatives.516 The corresponding benzofuran derivative is obtained under normal pressure.516... 

As depicted in Equation (68), anodic fluorination of ethyl 3-benzo[4]furanyl acetate was applied to the synthesis of a 2,3-difluoro-2,3-dihydrobenzo[/ ]furan derivative. A 2-fluoro-3-hydroxyl derivative was also obtained as a minor product <2003SL1631>. 

This methodology was applied to a two-step sequence for the preparation of enantio-merically enriched dihydrobenzo[h]furans (Scheme 10.11) [46]. Rhodium-catalyzed allylic etherification of (S)-47 (>99% ee), with the sodium anion of 2-iodo-6-methyl-phenol, furnished the corresponding aryl allyl ethers (S)-48/49 as a 28 1 mixture of regioisomers favoring (S)-48 (92% cee). Treatment of the aryl iodide (S)-48 with tris(trimethylsilyl)silane and triethylborane furnished the dihydrobenzo[h]furan derivatives 50a/50b as a 29 1 mixture of diastereomers [43]. 

As was reported by Adams and Wearn, °° 4,5-diaroylcyclohexenes react with bromine in chloroform to give dibromo compounds which, on treatment with acetyl chloride and catalytic amounts of sulfuric acid, give furan derivatives (stereochemistry probably as in Scheme 3) these latter compounds could also be obtained from 4,7-dihydrobenzo[c]furans with bromine. oo.ioi examples have been described where these dibromo-... 

Iodomethyl-2,3-dihydrobenzofurans (196) and the corresponding 2-iodomercurimethyl derivatives can be obtained from o-allylphenols and mercuric acetate, via the 2-acetoxymercuri-2,3-dihydrobenzo-furans.420... 

Table 21 1 NMR Chemical Shifts for 2,3-Dihydrofuran and 2,3-Dihydrobenzo[6]furan Derivatives...

Benzo[6]furans and 2,3-dihydrobenzo[6]furans have been obtained by condensation of quinones with compounds having an active methylene group (B-74MI31201) and 2-aminobenzo[6 ]furan derivatives are obtained with malononitrile (68M2359). The reaction of p-benzoquinone with enamines also affords benzo[6]furans. 

Two model p-quinone methide ring systems of kendomycin were obtained by oxidation with 2,2-dimethyldioxirane (DMDO) and NalO, respectively. The demonstrated chemistry paves the way for the to synthesis of kendomycin <04OL3131>. Anodic oxidation of 2,3-dihydrobenzol )]furan derivatives was also utilized to synthesize 2-fluoro-and 2,3-difluoro-2,3-dihydrobenzo[fc]furan derivatives <04JOC5302>. 

A number of 2,3-dihydrobenzo[ ]furans can be made by the Ru-catalyzed olefin-metathesis approach in the presence of trimethylsilyl vinyl ether <04AG(E)4063>. The isovanillin derived benzofi ]furan was also made by the olefin metathesis approach <04H(63)I771>. 

Enantiomerically pure l,3-dihydrobenzo[c]furan derivatives were recently obtained from o-phthaldehyde and 1,2-0-isopropylidene-a-D-xylofuranose <01TA4995>. An oxonium ion was proposed as an intermediate in the Amberlyst 15E promoted reaction of the cyclic allylic lactol ether shown in the following scheme. A (2,5)oxonium-ene process was believed to be the subsequent route from which aldehydes were generated <01TL6859>. 

A silver-catalyzed system promotes the C-C bond forming reaction of the o-alkynylacetophenone derivatives and carbon dioxide effectively to afford the dihydrobenzo[c]furan derivatives bearing a carboxyl... 

Canesi and coworkers [73] recently reported an elegant oxidative dearomatizing formal cycloaddition process between sulfonamides 105 and naphthalene to afford highly fused tetracyclic compounds 106 (Scheme 25). The reaction is postulated to occur via an intramolecular nucleophilic addition of the A -moiety to Wheland intermediate 107 generated during the oxidative umpolung activation. The above reaction with slightly modified conditions was successfully applied to the synthesis of dihydrobenzo[A,d]furan derivatives [73]. 

The oxidation of a ( )-flavanone with Tl(ni) nitrate, Pb tetracetate, phenyliodonium diacetate (PIDA), or [hydroxyl(tosyloxy)iodo]benzene in trimethyl orthofonnate affords the corresponding ( )-2,3-dihydrobenzo[h]furan derivative as a major product. The structures, including the relative stereochemistry, and a plausible mechanism of formation are reported. The preferred formation of a flavone from the ( )-flavanone by PIDA is explained by quantum-chemical calculations on the intermediate formed by the addition of this reagent to the enol ether derivative of the ( )-flavanone." Formation of mixed anhydrides by rapid oxidation of aldehydes, activated by pivalic acid, Bu OCl in presence of pyridine and MeCN is catalysed by TEMPO (2,2,6,6-tetramethylpiperidin-l-oxyl). The anhydrides can be converted in situ to esters, secondary, tertiary or Weinreb amides in high yield. Oxidation of the aldehyde to 2-propyl esters is also possible using only catalytic amounts of pivalic acid." ... 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

Ring contraction was also noted on acetolysis of 3-tosyltetra-O- methyl-(+)-catechin the 2,3-dihydrobenzo[Z> ]furan formed can be oxidized to the benzo[f>]furan (Scheme 105) (66JCS(C)634). The 2-phenylbenzo[6]furan and its 2,3-dihydro derivative are degradation products of lignin and as such have been investigated extensively. 

From benzofurans. Russian workers have reported several examples of the isomerization of 2,3-dihydrobenzo[6]furans into chromans. Triphenylmethyl perchlorate in acidic media converts the 2-ethyl derivative into 2-methylchroman, whilst the same chroman results when the 2,3-dimethylbenzofuran is treated with aluminosilicates at high temperature (72DOK(204)879, 75CHE278). 

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

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