Δ α, β-butenolide molecules are found in various biological, synthetic, and pharmacologically active natural products. There has been much effort to synthesize such butanolide structures; however, the most facile and cost-efficient process has proven to be the use of 5-alkoxy furans under trifluoroacetic acid and acetonitrile at room temperature. Experiment results reveal the possibility of two different butenolide structures having their pi bond situated in positions, α and β relative to the ester group in the ring. Experiment suggests that the α position is energetically preferred over the β position. To explain this preference density functional theory (DFT) and Moller-Plesset second-order perturbation theory (MP2) calculations are carried out. While the total energies of the two molecules are generally relatively close, the energy difference between them varies depending on the substrates. It is noted that the small energy difference between the two indicates that either molecule can be a product. It is found that the more electronegative the substrate, the larger the energy difference between the two molecules. In addition, when the substrate in the α position is more electronegative than in the β position a smaller energy difference is calculated. The ester group prefers to be parallel or co-planar with the pi bond with the one in the α position energetically favored. The ester group at that position is more stable compared to the one in the β position. In this regard, the experimental and theoretical data agree. The bonding characteristics of the isomeric forms are discussed.

Theoretical investigation of the pi bond position in butenolides

ACS Fall 2020 Virtual Meeting

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