5-Propenyl-benzofurans

A project for the synthesis of eupomatenoid-1 involved dehydrogenation of ( )-eupomatenoid-8 (7.1b), available via dehydrodiisoeugenol (7.1a) (see Section VI. 1). It turned out, however, that the double bond in the side chain has an adverse effect on this reaction attempted under a variety of conditions (70d), and the preparation of 5-propenyl-benzofurans by this sequence is satisfactory only if the conjugated ethylenic function is protected, e.g. by bromine addition (6). 

Hydroxy-4-methoxyphenyl)-5-( 1 -propenyl)benzofuran, in H- 214 6-Hydroxy-7,9-octadecadiene-11,13,15,17-tetraynoic acid, H-70195 2-(4-Hydroxyphenyl)- 7-methoxy- 5-( 1 -propenyl)benzofuran, in H-W214... 

Hydroxy-4-methoxyphenyl)-7-methoxy-5-(l-propenyl)benzofuran, in H-60214 Isotanshinone IIB, 1-60138... 

Dihydro-2-(4-hydroxy-3-methoxyphenyl)-3-methyl-5-( 1 -propenyl)benzofuran, in C-60161... 

To a solution of 5.2 g of 2,2-dimethyl-6-formyl-5-methoxy-2,3-dihydro-benzofuran in 20 mL glacial acetic acid there was added 3 mL nitroethane followed by 1.6 g anhydrous ammonium acetate. This mixture was heated for 4 h on the steam bath, and then a small amount of H20 was added to the hot solution. This instigated the formation of a copious deposition of brick-red crystals which were, after cooling, removed by filtration, and recrystallized from 50 mL boiling MeOH. After air drying there was thus obtained 2.7 g of day-glo yum-yum orange crystals of 2,2-dimethyl-5-methoxy-6-(2-nitro- l-propenyl)-2,3-dihydrobenzofuran. An additional 0.6 g of product was obtained by working the mother liquors. 

The benzofuran derivatives 215 and 217 were obtained also by Claisen rearrangement of 2-phenylsulfinyl-2-propenyl phenyl ethers 214 (refluxing in mesitylene in the presence of SiOi, 180 °C, 22 h) (equation 97) as well as of aryl /3-chloroallyl ethers 216 (equation 98). These aryl ethers act here as the synthetic equivalents of aryl propar-gyl ethers. 

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