A Dipolar Cycloaddition Approach to Pyrroloindoles Using N--Substituted Indoles

Article abstract of DOI:10.1021/jo00264a027

The 1,3-dipolar cyloaddition of a number of N-<(trimethylsilyl)methyl>-substituted indoles with several dipolarophiles has been investigated.The reaction requires the use of an equivalent amount of silver fluoride and provides a direct and efficient synthesis of the pyrrolo<1,2-a>indole ring system.The structures of the cycloadducts were established by high-field NMR spectroscopy as well as by several X-ray crystal structures.The cycloaddition reactions show all the characteristics of a concerted reaction, including complete stereospecificity and regioselectivity.The results are consistent with a mechanism that involves the intermediacy of an azomethine ylide.Formation of the dipole is rationalized by assuming that silver ion behaves as a very specific Lewis acid that attacks the indole ring to give a silver bonded carbonium ion.This is followed by a rapid desilylation reaction to give the azomethine ylide.After the cycloaddition step, the resulting silver-bonded intermediate undergoes consecutive loss of silver and a hydrogen to give the observed product.Attempts to extend the cycloaddition methodology to N-<(trimethysilyl)methyl>-substituted enamines and pyrroles are also described.