ABSTRACT
Two examples of ring-closure to give derivatives of cyclopentene have been reported; a suitably placed alkyl substituent that can be deprotonated is a requirement, as is a 1,3-diene that undergoes iminoalkylation, directed by an electron-rich alkoxy or amino group. Although fulvenes can be obtained from cyclopentadiene by a Vilsmeier formylation, they have also been obtained from 1,4-diaryl- butadienes which undergo iminoalkylation with Vilsmeier reagents to give, after hydrolysis, fulvenecarboxaldehydes. The electron-rich five-membered ring of azulene undergoes reaction with Vilsmeier reagents to give iminium salts. Homoallylic benzylic tertiary alcohols undergo terminal imino- alkylation as well as elimination. Electrocyclic ring closure of a presumed aminohexatriene intermediate, followed by elimination with aromatization, provides an elegant route to biphenyl and derivatives of biphenyls with substituents in a variety of positions. The ring-closure of certain allylbenzenes to formyldihydronaphthalenes under the Vilsmeier conditions has been reported.
