Structure correlations among arylpyran pseudoacids and derivatives

Many 4- and 5-oxocarboxylic acids show open-cyclic solution racemization/tautomerism including the dehydration-resistant arylpyran pseudoacid 3-hydroxy-4,4-dimethylisobenzopyran-1-one. The crystal and molecular structures of nine α-anomeric derivatives (including 11 distinct molecular structures) of this pseudoacid (or close analogs) have been determined, including pseudoacyl chlorides, normal, pseudoanhydrides, a dipseudoanhydride, and a tertiary pseudoamide. The basicity of the implied exocyclic leaving groups span 40 pK units. Together with seven closely related pseudoacid molecular structures previously determined, a data set containing 18 compounds was generated. Exocyclic C–O bonds shortened as endocyclic C–O bonds lengthened in linear functions of exocyclic leaving group basicity with l(xC-O, Å) = 1.4426 − 0.00400 * pK_a, N = 13, R² = 0.969; l(nC-O, Å) = 1.4204 + 0.00205 * pK_a, N = 18, R² = 0.951. These correlations were compared with results for other α-anomeric pyranoid systems, tetrahydropyran and dihydropyran, and an arylfuran pseudoacyl system. Exocyclic substituents effectively modified endocyclic C–O bond polarities over a considerable range.