Naphthofuran-2 -one

Furan derivatives with an aromatic system fused on one of the ring s double bonds, such as benzofuran, naphthofuran etc., can be polymerized cationically through the other ring s double bond. In these polymerizations the complications encountered with furan and alkylfurans [see Section III-A-l-c] are absent because only one unsaturation is available for propagation, the other being tied up in the benzene system... 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

These natural substances range from simple monosubstituted benzo-furans, such as 5-methoxybenzofuran (44) (which has bactericidal propertiesand plays an important part in the biosynthesis of natural benzofurans ) to more complex benzofurans. They include polycyclic compounds with one naphthofuran or phenanthrofuran ring (tanshinones, etc.) and furanic diterpenes [methyl vinhaticoate (45), methyl voucapenate (46), viridine (47)i5o-i52 fungicidal metabolite)]. 

Pd-catalyzed reaction of a-n hthol, isobutyraldehyde, and CO in the presence of CF3CO2H afforded naphthofuran-2(3/7)-one 28 in 79 % yield. The reaction is explained hy acid-catalyzed formation of l-(2-naphthyl)butanol 27, followed by carbonylation of the benzylic alcohol. Although its reactivity is lower, the phenol derivative 29 reacted with acetaldehyde to generate the benzylic alcohol 30, which was carbonylated to provide the benzofiiranone 31 in 54% yield [14]. 

Darzens reported that compounds of type 4 can be dehydrogenated to l-methyl-3-n hthoic acid 22 with the use of sulfur or selenium at elevated temperature. Subsequent decarboxylation using lime yielded 1-methylnaphthalen 23. This sequence of reaction can be carried out in one pot. Through a series of reactions the methyl naphthoic acid 22 can be converted to 4-methyl-2-naphthol 24, and further, with the aid of Vilsmeyer formylation to alkyl naphthofuran 25. ... 

Various other natural extracts are used as feedstocks for the production of terpenoids as shown in Figure 5.35. Two of the most significant ones are clary sage and the citrus oils (obtained as by-products of the fruit juice industry). After distillation of the oil from clary sage, sclareol (203) is extracted from the residue and this serves as a starting material for naphthofuran (204), known... 

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