Abstract
3, 5-Bis (methoxycarbonyl)-4-phenyl-2-isoxazoline-2-oxides (1) were readily transformed into 2-methoxycarbonyl-1-oxido-3H-indole-3-acetates (2) in the presence of Lewis acids such as titanium tetrachloride in dichloromethane. The reaction may occur via initial N-O bond fission to form an ionic intermediate (B), which cyclizes to 3H-indole-1-oxide (2) through an intramolecular aromatic substitution, on the basis of deuterium incorporation experiments as well as stereochemical considerations. With regard to the substituent effect, the reaction of meta-substituted phenylisoxazolines (1f-h) having an o, p-orientating group such as halogen was facilitated to provide 5-substituted 3H-indole-1-oxides (2f-h) in good yield. In contrast, the reaction of para-substituted compounds (1b-e) having the same substituents gave benzofuro-[3, 3a-d] isoxazoles (7b-e) preferentially, rather than 6-substituted 3H-indole-1-oxides (2b-e).
