Metal-Free Synthetic Shortcut to Octahydro-Dipyrroloquinoline Skeletons from 2,5-Cyclohexadienone Derivatives and l -Proline

The tandem decarboxylative condensation-dimerization reaction of l-proline with 2,5-cyclohexadienones including p-quinone monoacetals, p-quinol ethers, and p-quinols is reported to provide a concise and rapid synthesis of octahydro-dipyrroloquinoline compounds. The reaction features the use of cost-effective and readily available starting materials, high efficiency, metal-free and green reaction conditions. The reaction is applied to the synthesis of incargranine B aglycone. The discovery of this reaction may suggest a biosynthetic pathway from 2,5-cyclohexadienones and proline for natural ingredients containing pyrroloquinoline moieties.