Benzofuran ring 2,3-dihydro

Although alkylbenzofuran derivatives (e.g., 2-methyl-2,3-dihydro-benzofuran) have no vitamin F activity,88 they seem to show some regulating activity on plant growth.89 The 2,2-dimethyl-2,3-dihydro-benzofuran ring is an effective toxophore.90... 

The synthesis of the (i) 4-hydroxy-6-methoxytetrahydrofuro[2,3-6]-benzofuran ring (116), a degradation product of natural substances of the sterigmatocystin series, is similar,318 as is the synthesis of the dihydro derivative (117) from the o-acetylated acetaldehyde (118), a nonisolated intermediate which is ring-closed to 119 and then heated in toluene to give 117. The last is the starting point for the synthesis of aflatoxin M4 (49).153... 

Bcnzofuran-4,7-dioncs have been synthesized regioselectively by [3 + 2] photoaddition of 2-hydroxy-1,4-benzoquinones with a range of alkenes (equation 185)664. The reaction occurs in 30-60% yield and is a useful method for the synthesis of the benzofuran ring system, which is important in natural products like acamelin665. Substituted naphthoquinones may also be used in this reaction666,667 and this has lead to a very simple two-step synthesis of maturinone. In a similar reaction, a [3 + 2] photoaddition reaction of 2-amino-1,4-naphthoquinones with electron-rich alkenes gave 13-82% yields of 2,3-dihydro-177-bcn/ /]indole-4,9-diones in a single-step process which involved photolysis followed by oxidation (equation 186)668,669. 

The furan ring can be made to polymerize through one or both of its double bonds and the polymers obtained will therefore have dihydro- and tetrahydrofuran rings in their backbone. This situation occurs when furan, the alkylfurans, benzofuran and some dihydrofurans are treated with suitable initiators and is discussed in the first section of this chapter. 

This review of furan chemistry is meant to continue the earlier survey by Bosshard and Eugster1 and concentrates upon the period 1968 to the end of 1979. Like the earlier review, this one is limited to the chemistry of the monocyclic furan nucleus and does not deal, except incidentally, with fused rings such as benzofuran or its quinones. Nor does it deal in detail with dihydro- or tetrahydrofurans, nor with compounds like furylpyridine that contain some other heterocyclic nucleus as well. Some butenolides and tetronic acids are admitted to consideration since they are the carbonyl equivalents of hydroxyfurans regarded as enols, but side-chain reactions are wholly excluded unless the furan nucleus clearly affects them in some important way. 

Benzofuranic acids iraws-meranzinic acid (56).162 Euparinoid benzofurans the important class of euparinoids (extracts of plants of the Compositae family), contains benzofurans, dihydro-benzofurans, and also includes compounds with a hydronaphthofuran ring. We mention euparin (the earliest known) (5-acetyl-6-hydroxy-2-isopropenylbenzofuran) (57, R6=OH, R =H) 163 166, dehydro-tremetone (5-acetyl-2-isopropenylbenzofuran) (57, R6—R —H),167,168... 

The unsaturated compound first formed from the o-alkylphenol then undergoes ring closure. Some phenoxylated alcohols, looked upon as forerunners of compounds (104), such as l-phenoxy-2-propanol, also undergo ring closure with dehydrogenation (400°, A1203) to benzofuran derivatives, viz., to 2-methylbenzofuran (mixed with the 2,3-dihydro derivative).290... 

Iodine, ring closure of o-allylphenols. Treatment with iodine chloride (in ethanol or CCI4) of o-allylphenols gives 2-iodomethyl-2,3-dihydro-benzofurans (196).455-457 The latter compounds can lead either to the... 

While the synthesis of benzofuran derivatives from suitable furan derivatives has been comparatively little investigated, the synthesis of fused two-ring and three-ring compounds containing a pyridine nucleus from furan derivatives has been more successful furo[3,2-c]pyridines (345),752>753 2,3-dihydrofuro[2,3-a ]quinolines (from 4,5-dihydro-3-furoic acid),754 acrophyllin (346), and 7-hydroxydictamnine (347) (from ethyl 2-ethoxy-4-oxo-4,5-dihydrofuran-3-carboxylate),755 are examples of compounds in this class which have been prepared. 

Reductions in aqueous THF-TBABF4 of benzo[b]thiophene [55], benzofurans, and 1-methylindole [56] have been shown to yield the 2,3-dihydro compounds this is probably a direct electrochemical reduction, whereas the further reduction involving ring opening of the hetero ring seems to be mediated by TBA(Hg). 

Insertion of arylcarbenes (236) into ortho side chains giving dihydro-benzofurans and related compounds (237) has also been described.282,285 The formation of five-membered rings (237) predominates in such reactions.285 The required carbenes 236 can be generated either directly from... 

Hydrodesulfnrisation of benzothiophenes is conveniently achieved using Raney nickel. Reduction of the hetero-rings of both benzofuran and benzothiophene giving 2,3-dihydro derivatives, notably with retention of the snlfur in the latter case, can be achieved using triethylsilane in acidic solution, or with hydrogen over colloidal rhodium. Reductive cleavage of benzofuran to 2-hydroxystyrene is caused by lithium with 4,4 -di-f-butylbiphenyl (DTBB). ... 

Dihydroxylation of benzofuran and benzothiophene giving d5 -2,3-dihydro-2,3-dihydroxy derivatives can be achieved nsing Pseudomonas putida. Benzofurans can be epoxidised at the hetero-ring double bond with dimethyl dioxirane, or alternatively converted into dioxetanes by reaction of that double bond with singlet oxygen. Both oxidised species are unstable and undergo a variety of complex further processes. ... 

Im Falle des 2-Carboxy-l-benzofurans wird der Furan-Ring so aufgespalten, daB nach Hydrierung 3,4-Dihydro-cumarin vorliegt nach dem Ansauern der Reduktionslosung wird 3-(2-Hydroxy-phenyl)-propansaure4 erhalten ... 

Reduction of the hetero-rings of both benzofuran and benzothiophene, in the latter case with retention of the sulfur, can be achieved using triethylsilane in acidic solution giving 2,3-dihydro-derivatives. ... 

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