Pyrrole and oligopyrrole synthesis by 1,3-dipolar cycloaddition of azomethine ylides with sulfonyl dipolarophiles.

A procedure for the synthesis of functionalized, substituted pyrroles by 1,3-dipolar cycloaddition of azomethine ylides has been developed. This protocol is based on the metal-catalyzed cycloaddition of alpha-iminoesters with sulfonyl dipolarophiles, followed by the base-promoted elimination of the sulfonyl groups. A wide variety of 2,5-disubstituted and 2,3,5- and 2,4,5-trisubstituted pyrroles have been prepared in satisfactory yields from 1,2-bis(sulfonyl ethylene), beta-sulfonylenones, and beta-sulfonylacrylates. This method can be applied in an iterative and straightforward manner to the construction of oligopyrroles, from bipyrroles to pentapyrroles. Iterative [n+1] and [n+2] approaches have been devised, the latter involves double 1,3-dipolar cycloaddition from pyrrolyl-based bis(iminoesters).