Enhanced Solubility and Deprotection of Pyrene-4,5,9,10-tetraones through Propylene Glycol and Propanediol Protection

Pyrene-4,5,9,10-tetraone (PTO) is a building block of significant interest for functional organic materials. Due to the sensitivity of the vicinal diones toward bases and metal-ion chelation, and low solubility, PTO is typically protected as a ketal with ethylene glycol (EtG). Herein, we report the use of propylene glycol (PrG), propane-1,3-diol (PD) or 2,2-dimethylpropane-1,3-diol (DMPD) for its protection. The PrG-protected PTO has a 44-fold increased solubility relative to EtG-protected PTO due the introduced regio- and stereoisomerism. Protection with PD and DMPD surprisingly produced "half" protected PTOs, with maintained protective capabilities, and milder deprotection conditions for PD. All protected PTOs underwent successful functionalization reactions with high yields. Due to its stereocenter, use of PrG-protecting groups gives entry to protected PTOs with chiral-optical properties. This new series of ketal-protecting groups should considerably facilitate the future synthesis of PTO-based materials.