Synthesis and reactions of 7-methylpyrano[4,3-b]pyran-2,5-dione

Abstract

7-Methylpyrano[4,3-b]pyran-2,5-dione has been synthesised by condensation of triacetic lactone with malic acid in sulphuric acid. Among the by-products isolated are cis- and trans-β-(4-hydroxy-6-methyl-2-oxopyran-3-yl)acrylic acids. The former is believed to be an intermediate leading to the formation of the pyranopyrandione. This pyranopyrandione is converted by hot aqueous sodium hydroxide into α-coccinic acid. Its aromaticity has been demonstrated by the electrophilic substitutions which it undergoes, its high resonance energy, and by its n.m.r. spectrum.