Chemical valorization of aldaric acids: synthesis of pyrroles from di-unsaturated aldaric lactones

Chemical building blocks from biomass are gaining increasing attention for the production of high added-value chemicals.[1] Aldaric acids are sugar dicarboxylic acids, produced by oxidative hydrolysis of biomasses, like lemon peel, apple peel, and in general pectins, with relevant potentiality to high value polyfunctional fine chemicals, pharmaceuticals, cosmetics, and polymeric materials.[2] Alternative oxidative processes are also known from simple hexose sugars both by chemical or biochemical methods [2][3] via the corresponding aldonic and alduronic acids (Fig. a). However, the just recognized potentialities of aldaric acid is mainly related to simple chemical transformations (esterification, amidation, etc.) and this induced us to a deep analysis of the chemistry of this class of compounds in new directions, focusing mainly on high value compounds of the pyrrole series. The approach involves the selective dehydration of galactaric acid (as representative aldaric acid) to 2,5- dihydroxy-muconic acid (and related keto and internal lactone forms) identifying the possible use of these compounds in a Paal-Knorr reaction with amines (Fig. b). This allows us to better envisage the mechanistic details of previous reports of pyrrole synthesis in low yield from aldaric acid [4] and to obtain a large variety of 2,5-disubstituted, 2-substituted and unsubstituted N-alkyl and N-aryl pyrroles, useful as intermediates for fine chemicals and as monomer for polymers.