Decarboxylation of 4-Hydroxy-2H-1,2-Benzothiazine-3-carboxylic Acid 1,1-Dioxide ¹³C NMR Detection of an Enolic Intermediate

Abstract

An intermediate compound, most likely enolic (A), was detected by ¹³C-NMR spectroscopy during decarboxylation of 4-hydroxy-2H-1,2-benzothiazine-3-carboxylic acid 1,1-dioxide (I).

Key Words:

• Decarboxylation
• 4-Hydroxy-2H-1,2-benzothiazine-3-car-boxylic Acid 1,1-Dioxide
• β-Ketoacids
• enolic intermediate

Notes

A freshly prepared DMSO-d₆ solution of a pure I sample conclusively indicated the enolic structure of the compound. However, in our previous paper⁶, the ¹H-NMR spectrum obtained was consistent with the ketonic structure. Most likely, solution deterioration on standing, solvent effects ^8,9 and rapid proton exchanges, among others, may justify our original interpretation.

The suspicion that decarboxylation involves the ketonic form is also strengthened by the fact that carboxylic acid III (R = H) has not been isolated, due to the ease of decarboxylation, although the enol ether III (R = i-C₃H₇) is stable¹¹Consistently, the later compound is incapable of keto-enol tautomerism.

Steinberger and Westheimer⁵ suggested that conjugation of the olefinic double bond with the carboxyl group may lead to an increased stability of the enol formed as the first product in the decarboxylation of dimethyloxaloacetic acid.