An Improved Method for the Preparation of N-Unsubstituted 1,4,5,6-Tetrahydrocyclopenta[b]pyrroles: Synthesis of an Azaprostacyclin Analogue and Its 7-Cyano Derivative

Paal-Knorr cyclization of 2-acetonylcyclopentanone derivatives can be carried out most efficiently by reaction with hexamethyldisilazane (HMDS) when the reagents are previously adsorbed on alumina. By this procedure, the unstable pyrroloprostacyclin derivative rac-6a has been synthesized in 66% yield from racemic 6-oxoprostaglandin E1 (rac)-5b. In order to obtain less sensitive pyrroloprostacyclin derivatives, rac-6a has been transformed into the stable nitriles rac-7a and rac-7b by cyanation of the pyrrole ring with chlorosulfonyl isocyanate in the presence of triethylamine and subsequent cleavage of the protecting groups, respectively.