Substituent effects on partition coefficients of barbituric acids

Area of Science:

• Medicinal Chemistry
• Physical Chemistry
• Drug Discovery

Background:

• Partition coefficients (log P) are crucial for predicting drug absorption, distribution, metabolism, and excretion (ADME).
• Understanding substituent effects on log P is vital for designing molecules with desired pharmacokinetic properties.

Purpose of the Study:

• To accurately determine partition coefficients for various barbituric and 2-thiobarbituric acids.
• To correlate experimental log P values with molecular descriptors, specifically carbon number and C5 substituent effects.
• To evaluate the contribution of polar effects and determine hydrophobic constants for specific substituents.

Main Methods:

• Experimental determination of partition coefficients in 1-octanol-water at 25°C.
• Linear regression analysis correlating experimental log P (log Pexp) with carbon number and C5 substituent branching.
• Determination of hydrophobic constants (pi) for allyl, phenyl, and chloro substituents.

Main Results:

• Precise log P values were obtained for 14 barbituric acids and 3 2-thiobarbituric acids.
• Carbon number was identified as the primary determinant of partition coefficients.
• The polar effect of C5 substituents showed an insignificant contribution, contrary to prior studies.
• Empirical hydrophobic constants (pi) for allyl, phenyl, and chloro groups improved the prediction accuracy of calculated log P (log Pcalc).

Conclusions:

• Molecular size (carbon number) is the dominant factor influencing barbituric acid lipophilicity.
• The established hydrophobic constants provide a more accurate method for predicting log P values of substituted barbituric acids.
• These findings aid in the rational design of pharmaceuticals with optimized lipophilicity for improved ADME profiles.