Pyrrole chemistry and the synthesis of some pyrrole thiolesters

Abstract

The biogenisis of porphyrins and the problems involved in the chemical synthesis of porphyrins and their biogenetic intermediates are briefly discussed. The direct synthesis from readily available pyrroles is not often possible, because substitution reactions are sometimes blocked by groups already present on the pyrrole ring. -- The properties of some pyrrole anilides, thioanilides and esters were investigated, and some unusual reactions were encountered. Several pyrrole thiolesters were successfully synthesised from the thioanilides and from a modified Knorr synthesis with ethyl acetothiolacetate. -- The Raney nickel desulphurisation of pyrrole thiolesters was investigated, and it was found that the thiolester group could be converted to a formyl or in some cases a methyl group in this way. The desulphurisation reactions were found to give very good yields and the method represents a useful new route to pyrrole aldehydes. -- The acylation of pyrrole esters and thiolesters by perchloric acid/acetic anhydride; silver borofluoride/acetyl chloride; and trifluoroacetic anhydride/carboxylic acid mixtures were investigated. Trifluoroacetic anhydride alone produced good yields of trifluoroacetylated pyrroles all of which were new compounds. -- Some examples were found in which carbon disulphide, used as a solvent in Friedel and Crafts reactions with pyrrole esters, sometimes reacted with the pyrrole to produce derivatives of dithioacids. -- Nuclear magnetic resonance spectra are given for many of the compounds prepared in this work.