Furochromones

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Initial observations on khellin (32), a furochromone-type compound from Ammi visnaga L., showed a mast-cell stabilizing effect. This led to the synthesis of many derivatives of this lead for the potential treatment of allergy and asthma, most notably the approved preventive antiasthma drug, cromolyn sodium (33). The precise molecular mechanism of this class of drugs is not yet clearly imderstood. ... 

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. 

Catalytic dehydrogenation over (AI203, Fe203) at G00°-630° in the presence of moist, air converts 2,3-dihydrobenzofurans (78% yield) into benzofurans, with formation of phenolic compounds as by-products.768,769 Excellent yields are also obtained with Pd/C (reflux diphenyl oxide), Pt or Rh/C at 250°-3500,770 or Pd on norite.761 The method can be applied to the preparation of naphtho[2,l-6]furans,43e furocou-marins515,539,777 (e.g., psoralene781), furochromones,442,534 and 5H-benzofuro[4,5-c]benzopyran-5-one.441... 

Alkaline degradation of furochromones [visnagin (415, R7 = H), khcllin (415, R7 = OCH3)] leads to the corresponding ketones, visna-ginone and khellinone (416).452... 

Oxidizing degradation (H202) of the same furochromones leads to the corresponding benzofuran-5-carboxylic acids.857,858 Similarly, alkaline degradation of 418, a dehydration product of visamminol (417) (a natural coronary dilator) leads to benzofuran (419) and acetone.859... 

Table 19b 13C- h Coupling Constants for the Furan Nucleus in Furocoumarins and Furochromones (Benzofuran... 

Furo[3,4-6]chromone-3-acrolein, 1-aryl-synthesis, 4, 995 Furochromones... 

Hydroxycoumarin can react with alkenes in the presence of cerium(iv) ammonium nitrate (CAN) to form a mixture of furochromones and furocoumarins in modest yield (Equation 316) <1999H(51)2881>. 

Of the x-ray crystal structure determinations made of 6-4-5, 6-5-5, 6-6-5, and 6-7-5 fused heterocycles, many are of furocoumarin/furochromone, ellipticine, and PQQ derivatives because of their importance to biomedical investigations. Mention is made of these in their respective sections (7.22.12.2-7.22.12.4). A bibliography which lists the remaining structures, arranged according to central ring size and ring system formula, is presented in Table 1. 

Reductive desulfurization of 9-(phenylsulfonyl)furo[2,3- ]quinoline and its 2-methyl derivative can be effected by treatment with LAH in THF solution at reflux, but proceeded poorly with aluminum amalgam, the reagent of choice for the reductive cleavage of vinyl sulfones <83JOC774>. Raney nickel was used to remove a thiomethyl substituent from the central ring of a furochromone direct precursor to visnagan <89JOC448l>. 

Synthesis of furochromone (55) was accomplished in 56% overall yield from a 5-acetyl-6-ethoxy-benzofuran via BF3 Et20-mediated deethylation (70%) followed directly by a Claisen-type condensation of the resultant hydroxyketone and EtOAc (Equation (27)) (89JOC4481). 

As regards natural substances, we shall survey only those syntheses starting from nonfused benzofuran derivatives furocoumarins (lactones of Bz-o-hydroxybenzofurylacrylic acids) furochromones (from o-hydroxylated Bz-benzofuranic jS-diketones) furoquinolinea ° (from Bz-aminobenzofurans). 

The synthesis of the rare furochromone ammiol was achieved by R.B. Gammill starting from (methylthio)furochromone in four steps.The last step was the selective conversion of the aldehyde moiety of a six-membered 1,4-dicarbonyl compound using the MVP reduction. 

Gammiii, R. B. The synthesis and chemistry of functionaiized furochromones. 2. The synthesis, Sommeiet-Hauser rearrangement, and conversion of 4,9-dimethoxy-7-[(methyithio)methyi]-5H-furo(3,2-g)benzopyran-5-one to ammioi. J. Org. Chem. 1984, 49, 5035-5041. 

Furochromones are group of coumarins, derived from y- benzopyrone. They are related to furanocoumarins and are present in plants of family Apiaeeae and Rutaceae. Ammi visnaga is a potential souree of furoehromones like khellin and visnagin. Khellin was onee used as coronary vasodilator drug. 

A detailed study of the photochemical addition of kbellin, a photobiologically active furochromone, to dimethylfumarate has been investigated. The (2+2)-eyeloaddition encountered in this... 

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