3- Chromenes 1-benzoxepins

However, the presence of a fused benzene ring was found to hmit the scope of this type of ring expansion severely. Thus the dichloro-carbene adducts of both 2H- (33) and 4H-chromen (34) failed to rearrange to 2,3-benzoxepines (35) on heating. Although some... 

When the analogous starting material without the hydroxy group was used, 2,3-dihydro-l-benzoxepin was obtained in low yield (8%). The main product (87%) was 2-methyl-2//-chromene.97... 

The formation of a benzoxepin derivative during the synthesis of chromenes from phosphonium salts has been postulated (see p. 749). In keeping with this idea, chromenes are produced by ring opening of benzoxepin (121) with methoxide ion in DMF (69C108). The initial product, a butadienylphenol (122), undergoes a [l,7]-hydrogen shift to the quinoneallide. 

On reflux in dimethylformamide solution, the dichlorocarbene adducts 4,6 and 8 of chromenes rearrange to give 3-(or 4-)chloro-l-benzoxepines 5, 7 and 9, respectively, in good yields, provided that the cyclopropane ring carries electron-donating substituents such as alkyl, alkoxy, phenyl, 4-methoxyphenyl or a 7c-excessive heterocycle such as 2-thienyl. 

The reagent reacts with the sodium salt of salicylaldehyde (6) to give 2,3-dihydro-7-benzoxepine (7) and 2-methyl-3-chromene (8). 

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