Cross-over experiments unveil that the mechanism of the title reaction consists in cleavage of enol esters by cyanide with transient formation of acyl cyanides. These react as ‘soft' C-acylating agents with the enolates freed in the initial step. Use is made of the acyl cyanide derived from pyrrole-2-carboxylic acid to set up the triketide type skeleton 12 of the antibiotic pyoluteorin 16. The aromatization of 12 with mercuric acetate gave the pyrrolo[a]indolone 18.