“…The explanation proposed by Erlenmeyer was based on the theory of resonance and concluded that exchange at the 4-position was possible because of the existence of tautomeric molecules such as 247 (507,508). Pullman and Metzger discussed these conclusions in terms of HMO theory and suggested that the 5-carboxylic group contributes to the lability of the 4-methyl protons (6).…”