Synthesis and Isomerization of Tetrahydro-4<i>H</i>-chromene Derivatives

One-pot multicomponent reactions (MCRs) are commonly used to synthesize heterocyclic compounds including tetrahydro-4<i>H</i>-chromene derivatives. Despite extensive research on 4<i>H</i>-chromene synthesis via MCR, the formation and characterization of structural isomers arising from diketones that lack a vertical plane of symmetry have been neglected. MCR routes to several 4<i>H</i>-chromene derivatives were examined with an emphasis on characterizing the isomeric products that have likely been overlooked in previous studies. We report the synthesis and characterization of chiral tetrahydro-4<i>H</i>-chromene derivatives as racemic mixtures of constitutional isomers. Experiments conducted under varying reaction conditions consistently produced comparable quantities of isomeric product pairs. Effective characterization required heteronuclear multiple bond correlation (HMBC) NMR and reaction monitoring by high performance liquid chromatography (HPLC). Additionally, it is demonstrated that these product pairs isomerize in aqueous environments, suggesting that similar isomerization may occur under physiological conditions. A kinetic analysis of the isomerization and a putative conversion mechanism are reported. Because isomeric pairs were consistently produced regardless of reaction conditions, previous studies likely generated similar pairs. The confirmation of isomeric counterparts of known 4<i>H</i>-chromene derivatives suggests more careful analysis is necessary in related MCRs where multiple reaction pathways exist.