Regioselective Annulation of 6-Carboxy-Substituted Pyrones as a Two-Carbon Unit in Formal [4 + 2] Cycloaddition
Zachary A Kohanov1,2, Suzzudul Islam Shuvo1,2, Andrew N Lowell1,2,3
1Department of Chemistry, Virginia Polytechnic Institute and State University (Virginia Tech), Blacksburg, Virginia 24061, United States.
Abstract:
Heterocycles serve as a critical motif in chemistry, but despite being present in more than 85% of pharmaceuticals, there are limited methods for their construction. Here, we describe the incorporation of intact pyrone (2H-pyran-2-one) into larger ring systems via annulation. In a formal [4 + 2] cycloaddition, the pyrone regioselectively accepts a benzylic anion as a nucleophile in a conjugate addition fashion, with the subsequent pyrone-derived enolate attaching to a pendant ester on the initial nucleophile. Subsequent base-driven enolate formation and elimination establish aromaticity of the newly formed ring. After optimization of this process using an NMR-based assessment to overcome solubility and separation challenges, the reaction was successfully applied to a library of 6-ester and -amide-substituted pyrones and using a phenyl ester and other substituted sulfoxides. This technology enables the incorporation of intact pyrone rings into more complex systems, such as for the total synthesis of the natural product thermorubin.
Related Concept Videos
Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation
Cycloaddition Reactions: Overview
Cycloaddition Reactions: MO Requirements for Thermal Activation
Regioselectivity of Electrophilic Additions-Peroxide Effect
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn...
Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration

![Solid-phase Synthesis of [4.4] Spirocyclic Oximes](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F58508.jpg&w=3840&q=50)
