Natural Benzofuran Derivatives

Extraction, determination of structures, partial or total synthesis of the simple or complex heterocyclic derivatives constituting the natural benzofuran derivatives of vegetable origin. 

This method is suited to the synthesis of natural benzofuran derivatives karanjin (111, R = R = H) (a furoflavone) and pongapin (111, RR = 0-CH2-0) have been prepared in this way,201 as well as khellin (26).315 From the chromone acetaldehyde (112), visnagin (113) (4-methoxy-7-methyl-5//-furo[3,2-g][l]benzopyran-5-one) was synthesized.310... 

When the alkynyl substrate possesses a nucleophilic functionality, intramolecular solvomercuration reactions can sometimes be observed. These reactions, which result in the formation of a heterocycle, have been used in the total synthesis of natural products. For example, reaction of 114 with Hg(OAc)2 affords the benzofuran derivative 115, which is an important intermediate for the synthesis of various neolignans (Equation (42)).154 155... 

Members of the retinoid natural product class, containing a quaternary benzylic stereocenter, were prepared by the ring closure of allyl-arylethers. The palladium catalysed process in the presence of sodium formate led to the formation of a partially reduced benzofurane derivative through an insertion-hydride capture reaction sequence (3.24.), The coupling, which was run in the presence of a silver-zeolite, showed acceptable enantioselectivity when... 

As regards natural substances, we shall survey only those syntheses starting from nonfused benzofuran derivatives furocoumarins18 (lactones of Bz-o-hydroxybenzofurylacrylic acids) furochromones10 (from o-hydroxylated Bz-benzofuranic /J-diketones) furoquinolines20 (from Bz-aminobenzofurans). 

Bactericidal, Antifungal, and Antibiotic Benzofuran Derivatives Natural griseofulvin and its synthetic derivatives,44 as well as natural and synthetic furoquinolines,45 show such properties, as do some pterocarpans, such as phaseolidin,46 which is antifungal and lipophilic (21). 

Only one class of benzofuran derivative, natural or synthetic, shows this property—the furocoumarins-, these are important for plant biosynthesis, since they take part in the elaboration of chlorophyll.308 13 Among linear furocoumarins, psoralene (27a) is particularly active. Angelicin (28), another fundamental furocoumarin heterocycle with an angular structure, has only about 12% of the photosensitizing activity of 27a.74-75 On the other hand, it has antibiotic activity. Furochromones have no photosensitizing properties. 

A number of benzofuran derivatives are also formed by degradation of complex natural substances. These miscellaneous examples form the third group. 

The benzofuran-derived natural product caleprunin A was synthesized by R. Stevenson et al. using the Gattermann formylation as the key step. " The starting 3,4,5-trimethoxyphenol was suspended with Zn(CN)2 in ether and dry HCI gas was bubbled through the reaction mixture at room temperature for 2h. The solvent was decanted, water was added and the mixture was heated for 15 minutes. The natural product was obtained by reacting the benzaldehyde derivative with chloroacetone in DMF in the presence of anhydrous K2CO3. 

This chapter surveys the structure of furans and their benzofuran derivatives in order to complement the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) with materials published between 1983 and 1993. The determination of the structural features of the furan ring has received remarkable attention, since this five-membered heterocyclic ring is encountered as a building unit in a variety of natural and synthetic products. Besides terpenes, it suffices to mention ribose and deoxyribose, which play an important part in the vital function of nucleic acids. 

Benzofuran derivatives are one of the most important oxygen-containing heterocycles. Many natural or synthetic compounds containing benzofuran skeletons have been foxmd to possess remarkable activity as agrochemicals and pharmaceuticals [66]. Many methods have been developed for the synthesis of benzofuran in homogeneous and heterogeneous catalytic pathway. However, only the heterogeneously catalyzed reactions will be discussed here. 

Heterocyclic acetylenes also worked very well. Alkynes bearing on the backbones of quinoline, isoquinoline, benzothiophene, benzofuran, dibenzofiuan, and pyrazole were converted to heterocyclic nitriles (109-114) in 63-86 % yields. Interestingly, this nitrile synthesis could be applied to natural product derivatives. For example, the alkynyl ester of Vitamin E gave nitrile 115 in 45 % yield (in 80 % yield based on recovered starting material) the alkynyl ester of oleic acid gave nitrile in 68 % isolated yield. 

The chemistry of fiiians and benzofurans was a Held of lively research in the last year. There are a number of reasons for this activity. The furan ring system - both in its native as well as in its reduced form - occurs in a great number of natural products and a wide variety of these compounds has been isolated from natural sources. This subject is treated regularly with care and accuracy in Heterocycles and will not be repeated here. Therefore only a few examples will be given in this chapter. Several new furan derivatives were isolated from natural sources (-)-wistarin 1 from the marine sponge Ircinia sp. <99TA3869>,... 

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